Critical crops' alternative way to succeed in heat and drought

Scientists have discovered that certain plants can survive stressful, dry conditions by controlling water loss through their leaves without relying on their usual mechanism -- tiny pores known as 'stomata'.

Nonstomatal control of transpiration in maize, sorghum, and proso millet -- all C4 crops which are critical for global food security -- gives these plants an advantage in maintaining a beneficial microclimate for photosynthesis within their leaves.

This allows the plants to absorb carbon dioxide as part of the photosynthesis and growth process, despite raised temperatures and increased atmospheric demand for water without increasing the water expenditure.

Publishing their findings in PNAS, researchers from the University of Birmingham, Australian National University, Canberra, and James Cook University, Cairns, challenge traditional understanding of plant transpiration and photosynthesis under stressful and dry growing conditions -- namely that stomata alone control leaf water loss.

Co-author Dr Diego Márquez, from the University of Birmingham, commented: "This revolutionised our understanding of plant-water relations by showing that nonstomatal control of transpiration limits water loss without compromising carbon gain -- challenging what is typically accepted as an unavoidable trade-off.

"Our findings have significant implications for plant adaptation to climate change and how crops might be grown in arid environments. Understanding this mechanism could open new avenues for improving water-use efficiency in C4 crops, which are vital for global food security."

The study confirms that C4 plants maintain reduced relative humidities in the substomatal cavity, down to 80% under vapour pressure deficit (VPD) stress, reducing water loss and highlighting a critical role of nonstomatal control in water-use efficiency.

This mechanism helps plants sustain photosynthesis by reducing water loss without significantly lowering intercellular CO2 levels for photosynthesis. This is crucial for maintaining growth and ensuring that the crops thrive.

The findings also suggest that nonstomatal control mechanisms may have evolved before the divergence of C3 and C4 photosynthetic pathways, indicating a shared evolutionary trait.

"Our research reframes understanding of water-use efficiency in C4 plants and reveals that this alternative mechanism helps plants continue to grow and capture carbon dioxide, even when atmospheric water demand is high, challenging traditional assumptions about how these plants survive droughts," added Dr Márquez.

Photosynthesis is how plants use light and carbon dioxide to make sugars for growth, using an enzyme called Rubisco. Plants use the carbon dioxide that enters through open stomata to produce sugar, whilst open stomata also let water vapour out.

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Detecting climate change using aerosols

Researchers analyzed long-term aerosol satellite observation big data focusing on the Pacific Ocean downwind of China. Using a newly developed metric that considered aerosols as tracers, they detected altered atmospheric transport patterns associated with climate change. They observed that the distance of transboundary air pollution moving east from China had shortened. Thus, long-term satellite-based Earth observations are crucial for early climate change detection and accurate evaluation of this trend.

Climate change is one of the most significant environmental challenges of present times, leading to extreme weather events, including droughts, forest fires, and floods. The primary driver for climate change is the release of greenhouse gases into the atmosphere due to human activities, which trap heat and raise Earth's temperature. Aerosols (such as particulate matter, PM2.5) not only affect public health but also influence the Earth's climate by absorbing and scattering sunlight and altering cloud properties. Although future climate change predictions are being reported, it is possible that the impacts of climate change could be more severe than predicted. Therefore, it is necessary to detect climate change accurately and as early as possible.

Building on these insights, a research team from Japan, led by Professor Hitoshi Irie from the Center for Environmental Remote Sensing at Chiba University, utilized long-term observational data to study the effect of climate change on transboundary air pollution in the downwind area of China by using aerosols. They utilized a completely unique perspective on how aerosols impact climate and developed a new metric to detect climate change by considering aerosols as tracers.

"The significance of this study lies in the fact that most of its results are derived from observational data. In natural sciences focused on Earth studies, the ultimate goal is to piece together highly accurate data obtained from observations to quantitatively understand the processes occurring on Earth and to pursue immutable truths. Therefore, the more observational data we have, the better. With the continued Earth observations by Japan's major Earth observation satellites (such as the GCOM series, GOSAT series, Himawari series, and ALOS series), we aim to complement these efforts with numerical simulations and data science methodologies to achieve a safe and secure global environment that mitigates the impacts of the climate crisis." explains Prof. Irie.

The research team included Ms. Ying Cai from the Graduate School of Science and Engineering, Chiba University, Dr. Alessandro Damiani from the Center for Climate Change Adaptation, National Institute for Environmental Studies, Dr. Syuichi Itahashi and Professor Toshihiko Takemura from the Research Institute for Applied Mechanics, Kyushu University, and Dr. Pradeep Khatri from Faculty of Science and Engineering, Soka University. Their study was made available online on May 23, 2024, and published in Science of The Total Environment on August 20, 2024.

China is a major contributor to air pollution in East Asia. The downwind area of China analyzed in this study is a unique open ocean area with minimal human interference yet an important zone of transboundary air pollution pathways, making it an ideal location for studying meteorological variations due to climate change.

In their study, the researchers analyzed aerosol optical depth (AOD) datasets derived from satellites, reanalysis datasets, and numerical simulations focused on the Pacific Ocean in the downwind area of China, over 19 years from 2003 to 2021. AOD, a measure of the amount of sunlight blocked by aerosols, is a key factor is analyzing aerosols and their impact on climate change.

The researchers developed a new metric called RAOD which utilized the potential of aerosols as tracers to evaluate the impact of climate change on transboundary air pollution pathways. Using RAOD the researchers were able to quantify significant temporal variations in aerosol transport. They discovered that long-term changes in RAOD due to climate change were outweighed by larger year-to-year variations in the meteorological field. Moreover, seasonal trends showed that aerosols moved west to east during spring and winter, and northward in summer. They concluded that the probability of aerosols from China to be transported far eastward was low, highlighting a shift in transboundary pollution pathways due to global warming. In this study the authors successfully detected climate change using long-term satellite observational data, in contrast to most existing studies that tracked transboundary air pollution using model simulations.

"These results suggest that RAOD is a valuable metric for quantifying the long-term changes in transboundary air pollution pathways due to climate change. These results are particularly significant because most of them are derived from observational data," says Prof. Irie, highlighting the importance of the study. Sharing the future implications of their study he concludes, "The effects of climate change could be more severe than currently predicted. This study will help verify climate change predictions from an unconventional perspective of 'aerosol observation,' enabling a more accurate understanding of climate change progression and implementation of rational countermeasures."

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Agriculture: Less productive yet more stable pastures

Climate change will have a considerable influence on the biodiversity and productivity of meadows and pastures. However, according to the results of the large-scale climate and land use experiment, GCEF, which has been conducted at the Helmholtz Centre for Environmental Research (UFZ) for 10 years, the extent of these changes depends on the land use. Grassland optimised for high yield responds much more sensitively to periods of drought than less intensively used meadows and pastures. According to an article recently published in Global Change Biology, this can certainly have economic consequences for the farmers affected.

Grassland is one of the most important and most widespread ecosystems on earth. Such open landscapes with grasses and herbs not only cover more than one quarter of the entire land surface but also store at least one third of the terrestrial carbon, are crucial for food production, and can be extremely species-rich in a relatively small area. But what is the future of these habitats? The study provides new insights into this question.

It has long been clear that two environmental changes are threatening the world's grasslands. Particularly in Europe, grasslands are now fertilised much more heavily, mowed more frequently, and grazed more intensively. In addition, farmers often sow only a handful of grass varieties that promise a particularly high yield. This intensification of land use is fundamentally changing the species composition and functionality of meadows and pastures. The same applies to climate change. For Germany, climate change will result in a shift in the seasonal distribution of precipitation as well as an increase in hydrological extremes (e.g. heavy rainfall and droughts), among other things. It is considered the second largest threat for these ecosystems.

When both changes come together, they can reinforce each other. However, nobody yet knows exactly what will happen. Most experiments on this topic have so far focussed on either the climate or land use. "What makes our study unique is that we investigated the interaction of both factors," explains Dr Lotte Korell, biologist at the UFZ and first author of the publication.

This was made possible by the large-scale and long-term experiment of the UFZ in Bad Lauchstädt near Halle, the Global Change Experimental Facility (GCEF). It consists of 50 plots, each measuring 16 × 24 m; these are used with varying degrees of land use intensity. Temperatures and precipitation levels can also be manipulated with the help of mobile roof systems. For example, some plots receive 10% more precipitation in spring and autumn and 20% less in summer than the untreated control plots. This roughly corresponds to the conditions that climate models project for central Germany.

An eight-year data series from this experiment has now been compiled for the new study. The researchers analysed the biodiversity and productivity of the plants on the differently used plots between 2015 and 2022. "This period includes three of the driest years this region has experienced since beginning of records," recalls Korell. These droughts apparently had a much stronger effect on the plants than the experimentally simulated climate change.

However, in both cases, the trend pointed in the same direction: species-rich grassland that is only rarely mown or sparsely grazed withstood the heat and drought much better than the intensively used high-performance meadows. "Among other factors, this is probably related to the diversity of species," says Korell. This varied greatly depending on the land use of the grasslands.

A diverse mixture of more than 50 native grasses and herbs grew on the less intensively used meadows and pastures of the GCEF. However, on the intensively used grassland, the UFZ team had sown only the five grass varieties recommended to farmers by the Saxony-Anhalt State Institute for Agriculture and Horticulture for drier sites at the start of the experiment. These included varieties of meadow grass (Dactylis glomerata) and perennial ryegrass (Lolium perenne).

Because such grasses are bred for maximum yield and were also heavily fertilised -- as is common in agricultural practice -- the intensive meadows were initially much more productive than the more diverse grasslands. However, they were able to make use of this advantage only in favourable climatic conditions and were not able to withstand the drought as well as the plants in the low-intensity meadows and pastures. In times of drought, the grasses in the intensively used meadows increasingly died back and were replaced by other species such as chickweed (Stellaria media), shepherd's purse (Capsella bursa-pastoris), dandelion (Taraxacum officinale), and small-flowered cranesbill (Geranium pusillum). "These are mostly short-lived species that survive as seeds," explains Dr Harald Auge, also a biologist at the UFZ and senior author of the study. When the more competitive plants succumb to drought, these species take the opportunity to invade their habitats: they either migrate from the low-intensity grassland or germinate from the seed stock in the soil.

This shift in species composition is not particularly welcomed by farmers, especially because most of the new arrivals have a lower fodder quality than the grasses originally sown. The common ragwort (Senecio vulgaris), which was frequently represented among the immigrating species in the experiment, is in fact poisonous. All of this reduces the productivity of the land.

Farmers have long been aware of this kind of degradation of high-performance grassland by immigrating species. They therefore expect to have to plough up and reseed their land every few years. "However, climate change may accelerate this need and lead to additional costs," says Korell. Perhaps everything will go well for a few years and it will rain enough. However, it is also possible that several dry summers will follow one another. Climate change is making conditions even more unpredictable.

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Drought, soil desiccation cracking, and carbon dioxide emissions: an overlooked feedback loop exacerbating climate change

The accuracy of climate models depends on many factors -- greenhouse gas emissions from industrial and transportation activity, farm animal "emissions," urban growth and loss of forests, and solar reflections off snow and ground cover. Natural phenomena like volcanic eruptions also contribute and are incorporated into models.

However, some other natural processes have been overlooked. Farshid Vahedifard, professor and Louis Berger Chair in civil and environmental engineering, points to an important one that lies directly beneath our feet and covers most of our planet above water.

In a study published in Environmental Research Letters, Vahedifard notes that soil stores 80 percent of carbon on Earth, and with increasing cycles and severity of droughts in several regions, that crucial reservoir is cracking and breaking down, releasing even more carbon dioxide and other greenhouse gases into the atmosphere.

In fact, it may be creating an amplified feedback loop that could accelerate climate change well beyond current predictions.

"This process has not been sufficiently evaluated in the existing literature or incorporated into models," said Vahedifard.

"If we don't consider the interplay of drought, soil desiccation cracking, and CO2 emissions, that could result in significant inaccuracies when modeling and predicting climate change. There are other repercussions as well. Poorer soil health can lead to reduced photosynthesis and lower carbon dioxide uptake, and it can compromise the structural integrity of earthen dams that protect against floods."

There are also other amplifying feedback loops that may not have been fully accounted for in climate change models, he said.

These include melting of sea ice and exposure of darker ocean surfaces that absorb more heat from the sun.

The increase of wildfires due to warm, dry conditions releases a lot of carbon dioxide into the atmosphere, which in turn creates hotter, drier weather more conducive to fires.

Another amplified feedback loop is the thawing of Arctic and sub-Arctic permafrost, which also releases carbon dioxide into the atmosphere and raises climate temperature, leading to more melted permafrost.

But soil changes caused by drought could be as significant, if not more significant, than any of those factors.

Drought, manifested by long periods of low soil moisture content and high temperature, leads to cracking in fine-grained soils, sometimes extending meters below the surface.

The cracks result in more exposure to the air, increased microbial activity and breakdown of organic matter, released carbon dioxide, and loss of nutrients and ability to support plant growth, reducing carbon dioxide sequestering.

The deep cracks expose much older reserves of carbon that had previously been stable and protected.

The permeation of air into the soil accelerates the release of not only carbon dioxide from organic matter but also other greenhouse gases like nitrous oxide.

Small animals like earthworms and millipedes that help turn the soil over are also affected by the reduced moisture and increased air exposure, being less able to play active roles in nutrient cycling and soil structure maintenance.

That, in turn, increases the likelihood of soil cracking and aeration.

"The amplifying effect of soil carbon feedback loops and its interactions with other loops could carry us across tipping points and lead to even more severe and permanent shifts in climate," said Vahedifard.

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Paper provides a clearer picture of severe hydro hazards

Over the last two decades an estimated three billion people have been affected by water-related natural disasters such as droughts and floods. Climate change is expected to increase the frequency of these hydro hazards, with some prognosticators estimating there will be upwards of $3.7 trillion in water-related damage over the next 30 years in the U.S. alone. Beyond damaging homes and infrastructure, severe wet and dry spells will also devastate crops and deplete water reservoirs.

An increasing area of interest to researchers is the frequency of compound drought and pluvial flooding (caused by quick, heavy rainfall or sustained rainfall beyond the norm), which is when both occur in succession in the same area within a year of each other.

Historically, this level of coincidence has been under-examined.

Of similar interest is when the reverse happens: extreme rainfall followed by a meteorological drought.

Meteorological drought is when dry weather patterns prevail, which can eventually trigger hydrological drought, leading to dry streams and plunging reservoir levels, such as what happened at Lake Mead in 2022.

A new study co-authored by researchers in the University of Arkansas Department of Geosciences, as well as colleagues in China, now provides a global examination of drought-pluvial volatility -- or the tendency to shift from one extreme to another (from dry to wet or wet to dry) in a short period of time.

Yichan Li, a Ph.D. candidate at the U of A, was the first of four authors on the paper, "Observational Uncertainty for Global Drought-Pluvial Volatility," published in Water Resources Research, while Linyin Cheng, an assistant professor of geosciences, was second author.

The study looks at extreme dry-to-wet and wet-to-dry transitions over the past seven decades through event coincidence analysis, a method of quantifying the number of consecutive extreme events that also considers instantaneous or lagged responses within an uncertain period between them.

The study used three widely used climate data sets to provide evidence of increased drought-pluvial volatility on time scales of less than a year.

The team also evaluated the accuracy of these data sets, finding varying strengths and weaknesses of each due to observational uncertainties in data collection.

For instance, the remoteness of a region may play a role in collecting accurate data.

Averaged out at the global scale, the team found that 15.46% of all meteorological droughts were succeeded by a pluvial the following season.

The wet-to-dry transition percentage proved remarkably similar: 15.49%. However, prominent differences exist when looking at particular regions.

Toward that end, the study provides a map demonstrating how incidents of these two phenomena are distributed globally.

Overall, the spatial pattern of extreme dry-to-wet and wet-to-dry events' coincidence rates is largely in agreement among the three data sets, though there is prominent regional variability.

For instance, in Eurasia since the mid-20th century, there is a relatively low probability for meteorological droughts transitioning to pluvials, but a higher chance for the opposite scenario, rapid shifts from wet to dry events.

A similar pattern also exists over western North America, which sees severe wet to dry transitions at a frequency greater than 17% on average.

Conversely, South Asia and Australia are more prone to immediate transitions from meteorological droughts to pluvials.

The authors noted: "Our findings indicate that differences associated with drought-pluvial volatility among the considered observations are in many regions larger than that of their single events [droughts or pluvials alone], highlighting a need of to use multiple independent observation-based data sets for more robust examinations when studying such compound extreme events."

Ultimately, the authors stress the need to use multiple independent observation-based data sets when analyzing extreme, compound dry-to-wet events.

This will provide clearer guidelines for climate-related decision making, especially water resources planning, as well as ensure better accuracy when modeling future weather events.

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Study quantifies how aquifer depletion threatens crop yields

Three decades of data have informed a new Nebraska-led study that shows how the depletion of groundwater -- the same that many farmers rely on for irrigation -- can threaten food production amid drought and drier climes.

The study found that, due in part to the challenges of extracting groundwater, an aquifer's depletion can curb crop yields even when it appears saturated enough to continue meeting the demands of irrigation. Those agricultural losses escalate as an aquifer dwindles, the researchers reported, so that its depletion exerts a greater toll on corn and soybean yields when waning from, say, 100 feet thick to 50 than from 200 feet to 150.

That reality should encourage policymakers, resource managers and growers to reconsider the volume of crop-quenching groundwater they have at their disposal, the team said, especially in the face of fiercer, more frequent drought.

"As you draw down an aquifer to the point that it's quite thin, very small changes in the aquifer thickness will then have progressively larger and larger impacts on your crop production and resilience," said Nick Brozović, director of policy at the Daugherty Water for Food Global Institute. "And that's a thing that we don't predict well, because we tend to predict based on the past. So if we base what's going to happen on our past experience, we're always going to underpredict. We're always going to be surprised by how bad things get."

The team came to its conclusions after analyzing yields, weather and groundwater data from the High Plains Aquifer, which, as the largest in the United States, underlies portions of eight states -- including nearly all of Nebraska. Some areas of the aquifer, especially those beneath Texas and Kansas but also the Cornhusker State, have diminished considerably over the past several decades, pumped for the sake of irrigating land that would otherwise stand little chance of sustaining crops.

"In terms of things that let you address food security under extreme conditions -- in particular, drought and climate change -- we really can't do without irrigation," said Brozović, professor of agricultural economics at the University of Nebraska-Lincoln. "If we want to feed the world with high-quality, nutritious food and a stable food supply, we need to irrigate."

Brozović and Husker colleague Taro Mieno had already constructed plenty of models, and run plenty of simulations, on how the High Plains Aquifer responds to drought and dry conditions. But talking with farmers revealed that the models were not addressing their primary concern: well yield, or the amount of groundwater that growers can expect to continuously draw when trying to buffer their crops against drought.

"Everybody's interested in how aquifer depletion affects the resiliency of irrigated agriculture in the region," said Mieno, an associate professor of agricultural economics and lead author of the study, which was published in the journal Nature Water.

So the researchers consulted annual estimates of the High Plains Aquifer's thickness, which date back to 1935, along with county-level yields of corn and soybean from 1985 through 2016. Meteorological data, meanwhile, allowed the team to calculate seasonal water deficits, or the difference between the water gained from precipitation and the amount that crops lost via evaporation and transpiration.

When the latter exceeds the former, farmers often turn to aquifers for help in making up the difference, the researchers knew. What they didn't know: Under what conditions, and to what extent, would an aquifer's depletion make pumping its water too difficult or expensive to undertake? And how much would the resulting decisions -- to reduce the amount of irrigation per acre, to cease irrigating certain plots all together -- influence corn and soybean yields?

Farmers fortunate enough to be growing corn and soybean above the most saturated swaths of the High Plains Aquifer -- roughly 220 to 700 feet thick -- continued to enjoy high irrigated yields even in times of extreme water deficits, the team found. By contrast, those depending on the least saturated areas -- between 30 and 100 feet -- saw their irrigated yields begin trending downward when water deficits reached just 400 millimeters, a common occurrence in Nebraska and other Midwestern states.

In years when the deficit approached or exceeded 700 millimeters, irrigated fields residing above the thickest groundwater yielded markedly more corn than those sitting above the thinnest. The results were starker during a 950-millimeter water deficit, which corresponds with extreme drought: Fields atop the least saturated stretches of aquifer yielded roughly 19.5 fewer bushels per acre.

"Because of the way that aquifers work, even if there's a lot of water there, as they deplete, you actually lose the ability to meet those crop water needs during the driest periods, because well yield tends to decline as you deplete an aquifer," Brozović said. "That has an economic consequence and a resilience consequence."

The study captured another telling link between the water residing underground and that applied at the surface. When atop groundwater roughly 330 feet thick, farmers irrigated 89% of their acres dedicated to growing corn. Where the aquifer was a mere 30 feet thick? Just 70% of those acres received irrigation. That's likely a result of lower well yield driving farmers to irrigate only some of their fields, Taro said, or even give up on irrigation.

To better understand how that reduced irrigation was contributing to agricultural losses amid dry conditions, the researchers then factored in yields from both irrigated and non-irrigated fields, the latter of which rely on precipitation alone. That analysis pegged yields as even more sensitive to even smaller water deficits, suggesting that the decline in irrigated land was compounding the losses endured on still-irrigated plots.

And it illustrated the runaway threat posed when an aquifer's average thickness drops below certain thresholds. At a water deficit of 950 millimeters, reducing an aquifer's thickness from roughly 330 to 230 feet was estimated to initiate an average loss of about 2.5 corn bushels per acre, what the authors called a "negligible difference." The same absolute decrease, but from 230 to 130 feet, led to an estimated loss of 15 bushels per acre.

"As a consequence, your resilience to climate decreases rapidly," Mieno said. "So when you're operating on an aquifer that is very thick right now, you're relatively safe. But you want to manage it in a way that you don't go past that threshold, because from there, it's all downhill.

"And the importance of aquifers is going to increase as climate change progresses in the future, for sure. As it gets hotter, you typically need more water. That means you need more irrigation, and you're going to deplete the aquifer even faster, and things can get worse and worse."

Nebraska is lucky, Brozović said, in that it sits above such a massive reservoir and has established a governance system designed to conserve it at a local scale. But most regulations focus on mandating how much and when groundwater gets pumped, not safeguarding the aquifer's saturation level or the corresponding ability to extract water from it.

Brozović conceded that convincing policymakers to consider revising those parameters now, when much of the state still boasts sufficient groundwater, is "perhaps a tough sell." He's hopeful that the new study can at least help put that conversation on the table.

"Once you have a problem -- once well yields are already declining and the aquifer's really thin -- even if you put in policies, you still get a lot of the (negative) impacts," he said. "So the time to really put in meaningful policies is before things have gone off the cliff.

"First, you have to understand, you have to measure, you have to educate. You have to understand what you're preserving, and why. The more you can provide the quantitative evidence for why it's worth going to the trouble of doing all of this, and what's at stake," he said, "the easier that conversation is."

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Measuring the extent of global droughts in unprecedented detail

While some parts of the world suffer extreme heat and persistent drought, others are being flooded. Overall, continental water volumes vary so much over time that global sea levels fluctuate significantly too. By combining the hydrological model WaterGAP with GRACE satellite data, a team of geodesists at the University of Bonn have come up with a new set of data that shows how the total distribution of water over the Earth's land surfaces has changed over the past 20 years more accurately than ever before. Their findings are now being published in the Journal of Geodesy.

"The new method allows us to test out model calculations on the future effects of climate change, particularly how rising temperatures and changes in precipitation patterns will impact the water balance in different parts of the world," says Prof. Dr.-Ing. Jürgen Kusche from the Institute of Geodesy and Geoinformation at the University of Bonn. The process involves comparing climate models, which invariably cover a certain period of time in the past, with the results of actual measurements, and Kusche and his team are planning several such studies over the coming months.

The improved resolution that the team has achieved shows that droughts are significantly more common across the world than the GRACE satellite data would suggest in isolation. "What we're seeing is that even extensive droughts like the massive one that struck the whole of the Amazon in 2010 are spread across much wider areas than the satellite data indicates on its own," Kusche says. "This means that the satellites aren't picking up many of the more localized droughts."

Working together with counterparts from Goethe University Frankfurt and the Polish capital Warsaw, a team of researchers from the University of Bonn has now combined satellite measurements with high-resolution meteorological data for the first time. "What's special about this method is that it's enabled us to improve the resolution of the water distribution maps that are generated from around 300 kilometers to 50 kilometers," explains Kusche, who is a member of the Modelling and Sustainable Futures Transdisciplinary Research Areas and the Regional Climate Change Collaborative Research Center at the University of Bonn. To do so, the researchers used the "WaterGAP" hydrological model developed at Goethe University Frankfurt plus a mathematical technique borrowed from weather forecasting.

Masses of water causing changes in the gravitational field

Between 2002 and 2017, the GRACE (Gravity Recovery and Climate Experiment) twin satellites measured changes in the Earth's gravitational force. Its successor project, "GRACE-FO," launched in 2018, and it was this data that the researchers from the University of Bonn used. Since the Earth's gravitational force is dependent on changes in mass, this allows conclusions to be drawn about the water cycle close to its surface. Gravity is affected by changes in groundwater and surface reservoirs and by melting glaciers.

"One unique advantage of the GRACE measurements is that they cover all kinds of reservoir, i.e. including changes in groundwater reserves that are hidden deep below the Earth's surface and in tens of thousands of artificial lakes and wetlands," says Kusche's colleague Helena Gerdener. The disadvantage, she says, is that the spatial resolution of the data on the gravitational field is relatively inexact at about 300 to 350 kilometers as a result of the measurement principle applied. This means that reliable statements can only be made for areas around 100,000 square kilometers in size. To give some idea of scale, this minimum area is still larger than Bavaria, Germany's largest federal state at "only" 70,000 or so square kilometers.

By contrast, global hydrological models permit a resolution of 50 kilometers or even less. These use meteorological measurements of precipitation, temperature and radiation as well as maps of land use and soil composition and data on how water is being used by industry, agriculture and other consumers. Hydrological models simulate evaporation as well as changes to water levels in the soil and groundwater-bearing strata, lakes, rivers and reservoirs. "However, the drawbacks of these models are that they can only reflect reality to a limited extent and meteorological measurements often contain systematic errors," Kusche says, for example if no data on the extraction of groundwater is made available.

For the first time, the researchers have now combined measurements from the GRACE and GRACE-FO satellites with the WaterGAP hydrological model, which itself integrates high-resolution meteorological data. This has enabled the resolution of the water distribution maps thus generated to be improved to 50 kilometers. To do so, the researchers used a mathematical technique known as data assimilation, which is more usually to be found in weather forecasting. However, the scientists did not simply take the results of the hydrological model and the satellite data and calculate the average values. As Kusche explains: "The calculations from the hydrological model are adjusted so that you get close to the satellite data while modifying the physics that the hydrological model draws on as little as possible."

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Increased droughts are disrupting carbon-capturing soil microbes, concerning ecologists

Soil stores more carbon than plants and the atmosphere combined, and soil microbes are largely responsible for putting it there. However, the increasing frequency and severity of drought, such as those that have been impacting California, could disrupt this delicate ecosystem. In a perspective publishing in the journal Trends in Microbiology on April 12, microbial ecologist Steven Allison warns that soil health and future greenhouse gas levels could be impacted if soil microbes adapt to drought faster than plants do. He argues that we need to better understand how microbes respond to drought so that we can manage the situation in both agricultural and natural settings.

"Soil microbes are beneficial, and we couldn't live without their cycling of carbon and nutrients, but climate change and drought can tweak that balance, and we have to be aware of how it's changing," says Allison of the University of California, Irvine.

Some soil microbes take carbon from decomposing plants and store it in the soil, while others release plant carbon back into the atmosphere. The carbon that ends up in the soil is beneficial in multiple ways. "The carbon in the soil has these reverberating effects out to the rest of the world in terms of the infrastructure in our natural and managed ecosystems," says Allison. "Carbon-rich soils hold more nutrients, so plants growing in those soils tend to be more productive, and the carbon changes the physical properties of the soil, which prevents erosion."

"In California now, we have this system where the droughts are more intense, and then the rainfall is more intense," he says. "So, if you're losing your soil carbon, when it rains really hard it could carry away your soil and cause erosion, landslides, mudslides, sediments, and all kinds of problems that we're actually seeing right now."

The carbon that is released back into the atmosphere is another story. "From a climate mitigation standpoint, what we want is for more carbon to be in plants and soils and less carbon to be in the atmosphere, so the more carbon we can absorb into plants through photosynthesis and the more we can transfer and keep in the soil, the better off we're going to be in terms of climate change," says Allison. "That's why it's really important to know how the balance of incoming versus outflowing carbon changes with drought, or warming, or any other climate factor."

Plants and microbes will both be impacted by the increasing frequency of drought, but Allison suspects that microbes will be able to bounce back faster. "Microbes are really adaptable -- they can change their physiology, they can change their abundances so that more drought-adapted microbes take over, and they can potentially evolve -- so we expect that they are going to resist or bounce back from drought," says Allison. "All those different processes can happen pretty quickly with microbes, and much more quickly than with plants."

If more carbon-releasing microbes survive than carbon-sequestering microbes, we could end up with carbon-depleted soils, which would have serious negative implications for plant productivity and future greenhouse gas levels.

We may be able to nudge the balance in the right direction, Allison says, but more research is needed first. "There's still a lot to be done. Right now, we have data that suggests that when we have drought, something changes that results in carbon loss, but we don't understand exactly how or why that's happening, whether drought's changing the abundance of beneficial plant associated microbes versus the carbon releasing microbes, or if it's causing the evolution of one of the microbe groups, or if it's more determined by changes to their immediate physiology," says Allison.

Some microbes could actually help plants cope with drought. If we knew which microbes were most beneficial to plants, and most likely to retain carbon in soil, we could try to tip the balance in their favor.

"There's a lot of potential for us to manage or engineer soil microbes," says Allison. "In agricultural systems, we can look into manipulating the soil or adding beneficial microbes back in. In more natural systems, management would probably be on the plant side: soil microbes are often closely intertwined with plants, so managing the plants can also benefit the microbial part of the ecosystem."

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Drought, heat waves worsen West Coast air pollution inequality

A new study led by North Carolina State University researchers found drought and heat waves could make air pollution worse for communities that already have a high pollution burden in California, and deepen pollution inequalities along racial and ethnic lines.

Published in Nature Communications, the study also found financial penalties for power plants can significantly reduce people's pollution exposure, except during severe heat waves.

"We have known that air pollution disproportionally impacts communities of color, the poor and communities that are already more likely to be impacted by other sources of environmental pollution," said the study's lead author Jordan Kern, assistant professor of forestry and environmental resources at NC State. "What we know now is that drought and heat waves makes things worse."

For the study, researchers estimated emissions of sulfur dioxide, nitrogen oxides and fine particulate matter from power plants in California across 500 different scenarios for what the weather could look like in future years, which they called "synthetic weather years." These years simulated conditions that could occur based on historical wind, air, temperature and solar radiation values on the West Coast between 1953 and 2008. Then by using information about the location of power plants in California and how much electricity they would be generating under different weather conditions, they estimated air pollution within individual counties.

They saw the worst air pollution in the hottest, driest years, which Kern said is due to the demand for more air conditioning during hot years. In addition, drought can impact the availability of hydropower. The excess electricity has to come from somewhere else, which is where fossil fuel plants come in.

"One of the things we were interested in was teasing apart the relative roles of drought, which can be chronic, lasting for months or years, versus heat waves, which can happen like a flash in a pan," Kern said. "We found drought is a driver of chronic pollution exposure, but heat waves are responsible for these incredible spikes in emissions in a short period of time."

They also saw that counties with a higher existing pollution burden were disproportionately impacted by pollution during drought and heat waves. Counties that were more diverse by race and ethnicity were also far more likely to be impacted by increased emissions from power plants during droughts and heat waves.

"The more diverse your county is by race and ethnicity, the more likely you are to be impacted by air pollution on an annual basis," Kern said. "During a drought, the relationship is more pronounced."

When they simulated the impact of three different policies that taxed power generators for emitting air pollution locally, overall, or both, they found that penalties helped reduce pollution health damages in more than 99% of days. However, during extreme heat waves, penalties failed to reduce emissions.

"Penalties make the more damaging power plants more expensive to operate, while it makes clean power plants comparatively less expensive," Kern said. "It incentivizes the system to switch to rely on more clean power plants, but that stops happening during really massive heat waves. The power operators have no choice but to turn on every power plant. They can't switch from the dirty power plants to the clean ones."

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With rapidly increasing heat and drought, can plants adapt?

At a time when climate change is making many areas of the planet hotter and drier, it's sobering to think that deserts are relatively new biomes that have grown considerably over the past 30 million years. Widespread arid regions, like the deserts that today cover much of western North America, began to emerge only within the past 5 to 7 million years.

Understanding how plants that invaded these harsh deserts biomes were able to survive could help predict how ecosystems will fare in a drier future.

An intensive study of a group of plants that first invaded emerging deserts millions of years ago concludes that these pioneers -- rock daisies -- did not come unequipped to deal with heat, scorching sun and lack of water. They had developed adaptations to such stresses while living on dry, exposed rock outcroppings within older, more moist areas and even tropical forests, all of which made it easier for them to invade expanding arid areas.

The study by University of California, Berkeley, researcher Isaac Lichter-Marck is the first to provide evidence to resolve a long-standing evolutionary debate: Did iconic desert plants, like the stately saguaro cacti, the flaming ocotillos and the Seussian agaves, adapt to arid conditions only after they invaded deserts. Or did they come preadapted to the stresses of desert living?

The question has relevance today, Lichter-Marck said, because accelerating aridity due to climate change is challenging plants to adapt much more quickly than they have in the past. Already, about one-fifth of Earth's land surface is desert. If adaptation to arid conditions was only possible for plants that had already evolved to deal with such stresses, then many today may not be equipped with an adequate genetic tool kit to survive.

"If you think about aridity only as a stimulus to plant evolution, then in many cases people could say these plants are survivors, they are adaptable, and they will be fine. They will take advantage of these new conditions, and they will thrive," said Lichter-Marck, who is also a National Science Foundation postdoctoral research fellow at UCLA.

But the history of rock daisies suggests that "when the deserts emerged, those plants that had the necessary preadaptations to take advantage of new conditions were the ones that thrived," he said. "Adding more aridification to the system doesn't necessarily mean more rapid adaptive evolution will occur. There's a limited source of lineages that can take advantage of new levels of aridity, and that is important for understanding the effect of climate change on biodiversity."

Lichter-Marck and Bruce Baldwin, UC Berkeley professor of integrative biology, curator of the Jepson Herbarium and chief editor of The Jepson Desert Manual: Vascular Plants of Southeastern California (2002), published their study about the evolution of rock daisies in North American deserts this week in the journal Proceedings of the National Academy of Sciences.

Seven years roaming the desert

Botanists realized long ago that when plants invaded desert areas, they quickly diversified to fill the many niches created by this new type of habitat.

"Even as recently as 1 million to 1.5 million years ago, it would have been difficult to find widespread desert habitats like we see today in North America, which is kind of surprising because now deserts and arid habitats are the most widespread biome on earth," Lichter-Marck said. "But during the late Miocene Epoch, dry habitats spread, and the world's lineages of desert plants, especially the succulent lineages like the cacti, the agaves and the ice plants -- as well as many other drought tolerant lineages -- underwent a synchronous rapid diversification."

Paleontologists pointed out, however, that fossilized plants that thrived tens of millions of years before the proliferation of deserts had characteristics similar to those of desert plants today. Some scientists, like the late paleoecologist Daniel Axelrod of UCLA and UC Davis, argued that this meant the plants that thrived in the desert today evolved earlier and were preadapted -- or exapted -- to survive desert conditions by growing in dry microsites, such as rock outcrops, rain shadows or mountaintops. Others, like UC Berkeley's Ledyard Stebbins, an evolutionary biologist who helped found the UC Davis Department of Genetics, argued that aridity itself spurred plants to diversify and develop traits to withstand dryness, heat, intense sunlight and strong winds.

Despite the similarities between rocky outcrops and deserts, it has been hard to prove that desert plants descended from plants already adapted to the stresses of aridity, in part because fossils rarely form in dry habitats and cannot tell us much about the habitat in which these ancient plants were growing.

To Lichter-Marck and Baldwin, rock daisies, which are classified in the tribe Perityleae in the sunflower family, seemed like a good group in which to explore the connection. Some species live on dry, exposed rock in tropical areas of Mexico -- what might be considered "micro-deserts" -- while others have fully adapted to desert areas, such as the Mojave in California and the Great Basin, Chihuahuan and Sonoran deserts that cover most of western North America.

"Plants that live on rock outcrops face many of the same challenges as those living in a dry, desert habitat," Lichter-Marck said. "Rock outcrops tend to be exposed to UV light, wind and dry, desiccating conditions, as well as heat and frost. They also tend to be more exposed to herbivores.

"The ways that plants deal with them are diverse, but they usually involve some kind of specialized root morphology that helps them to anchor in rock outcrops, as well as deal with the heightened arid conditions. And they tend to have smaller leaves, or leaves with a dense covering of hairs that help buffer them against drought and block sunlight, including UV light. They also tend to have heightened chemical defenses against herbivores, because it takes a lot of energy to regenerate after being munched."

For his Ph.D. thesis in the Department of Integrative Biology and at the Jepson Herbarium, Lichter-Marck, a Southern California native, roamed the deserts of Arizona, California, Texas and Mexico for months at a time in a pickup truck, accompanied by his blue heeler, Rio, to collect hundreds of specimens of rock daises. Some rock daisies are among the most dramatic bloomers in spring, carpeting the desert with colorful blossoms. Many, however, are limited to small geographic regions where they grow only on vertical rock faces or sky island mountain ranges, making them hazardous to collect. Lichter-Marck is an experienced mountaineer, an important skillset for field work in rough terrain.

He later sequenced the DNA of these specimens -- 73 of the 84 recognized species of rock daisy -- and catalogued their life histories, such as where they grew, what type of root system they had, and whether they were annual or perennial, an herb or a shrub. He then compared them to fossilized daisies to develop a rough timeline of the evolution of these characteristics and the lineage's eventual shift into deserts.

This allowed him to conclude that most rock daises -- in particular, the genus Laphamia, which was the first to move into deserts and is the largest rock daisy genus -- had adapted to the stress of heat, aridity, wind and sun by virtue of their growth on cliffs before invading deserts.

"This is a clear empirical demonstration of what was originally Axelrod's hypothesis -- of a desert plant group originating in dry microclimates prior to the widespread emergence of desert habitats," said Lichter-Marck. "What this means is that the strategies for drought tolerance that are so characteristic of desert vegetation might not actually represent responses to the dry conditions found in deserts. Instead, they could be traits that evolved earlier in association with much older and more stable dry microclimates, such as rock outcrops in tropical settings."

Preadaptation may be the key to the success of many desert plants, including cacti, which are known to inhabit rock outcrops or grow as epiphytes in the canopies of trees within tropical areas, though these large lineages would require a much more extended analysis, he said.

Rock daisies, many of which live in specialized habitats that make them vulnerable to extinction, highlight the importance of conserving seemingly niche species.

"A lot of the rock daisies are very specialized and tend to be very narrow in their distribution and might be seen as less significant to the survival of the ecosystem as a whole. In evolutionary biology and in conservation biology, specialized organisms with narrow geographic ranges are often considered vulnerable lineages and have sometimes even been called evolutionary dead ends," he said. "An important implication here is that a group of ecological specialists growing on scattered cliffs in tropical habitats started this major radiation in the desert. So, it actually shows that specialists are not just these vulnerable lineages on the edge of extinction. They might actually be really important sources for innovation in evolution."

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Drought encouraged Attila's Huns to attack the Roman empire, tree rings suggest

Hunnic peoples migrated westward across Eurasia, switched between farming and herding, and became violent raiders in response to severe drought in the Danube frontier provinces of the Roman empire, a new study argues.

Hungary has just experienced its driest summer since meteorological measurements began, devastating the country's usually productive farmland. Archaeologists now suggest that similar conditions in the 5th century may have encouraged animal herders to become raiders, with devastating consequences for the Roman empire.

The study, published today in the Journal of Roman Archaeology, argues that extreme drought spells from the 430s -- 450s CE disrupted ways of life in the Danube frontier provinces of the eastern Roman empire, forcing Hunnic peoples to adopt new strategies to 'buffer against severe economic challenges'.

The authors, Associate Professor Susanne Hakenbeck from Cambridge's Department of Archaeology and Professor Ulf Büntgen from the University's Department of Geography, came to their conclusions after assessing a new tree ring-based hydroclimate reconstruction, as well as archaeological and historical evidence.

The Hunnic incursions into eastern and central Europe in the 4th and 5th centuries CE have long been viewed as the initial crisis that triggered the so-called 'Great Migrations' of 'Barbarian Tribes', leading to the fall of the Roman empire. But where the Huns came from and what their impact on the late Roman provinces actually was unclear.

New climate data reconstructed from tree rings by Prof Büntgen and colleagues provides information about yearly changes in climate over the last 2000 years. It shows that Hungary experienced episodes of unusually dry summers in the 4th and 5th centuries. Hakenbeck and Büntgen point out that climatic fluctuations, in particular drought spells from 420 to 450 CE, would have reduced crop yields and pasture for animals beyond the floodplains of the Danube and Tisza.

Büntgen said: "Tree ring data gives us an amazing opportunity to link climatic conditions to human activity on a year-by-year basis. We found that periods of drought recorded in biochemical signals in tree-rings coincided with an intensification of raiding activity in the region."

Recent isotopic analysis of skeletons from the region, including by Dr Hakenbeck, suggests that Hunnic peoples responded to climate stress by migrating and by mixing agricultural and pastoral diets.

Hakenbeck said: "If resource scarcity became too extreme, settled populations may have been forced to move, diversify their subsistence practices and switch between farming and mobile animal herding. These could have been important insurance strategies during a climatic downturn."

But the study also argues that some Hunnic peoples dramatically changed their social and political organization to become violent raiders.

From herders to raiders

Hunnic attacks on the Roman frontier intensified after Attila came to power in the late 430s. The Huns increasingly demanded gold payments and eventually a strip of Roman territory along the Danube. In 451 CE, the Huns invaded Gaul and a year later they invaded northern Italy.

Traditionally, the Huns have been cast as violent barbarians driven by an "infinite thirst for gold." But, as this study points out, the historical sources documenting these events were primary written by elite Romans who had little direct experience of the peoples and events they described.

"Historical sources tell us that Roman and Hun diplomacy was extremely complex," Dr Hakenbeck said. "Initially it involved mutually beneficial arrangements, resulting in Hun elites gaining access to vast amounts of gold. This system of collaboration broke down in the 440s, leading to regular raids of Roman lands and increasing demands for gold."

The study argues that if current dating of events is correct, the most devastating Hunnic incursions of 447, 451 and 452 CE coincided with extremely dry summers in the Carpathian Basin.

Hakenbeck said: "Climate-induced economic disruption may have required Attila and others of high rank to extract gold from the Roman provinces to keep war bands and maintain inter-elite loyalties. Former horse-riding animal herders appear to have become raiders."

Historical sources describe the Huns at this time as a highly stratified group with a military organization that was difficult to counter, even for the Roman armies.

The study suggests that one reason why the Huns attacked the provinces of Thrace and Illyricum in 422, 442, and 447 CE was to acquire food and livestock, rather than gold, but accepts that concrete evidence is needed to confirm this. The authors also suggest that Attila demanded a strip of land 'five days' journey wide' along the Danube because this could have offered better grazing in a time of drought.

Hakenbeck said: "Climate alters what environments can provide and this can lead people to make decisions that affect their economy, and their social and political organization. Such decisions are not straightforwardly rational, nor are their consequences necessarily successful in the long term."

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Old-growth trees more drought tolerant than younger ones, providing a buffer against climate change

A new analysis of more than 20,000 trees on five continents shows that old-growth trees are more drought tolerant than younger trees in the forest canopy and may be better able to withstand future climate extremes.

The findings highlight the importance of preserving the world's remaining old-growth forests, which are biodiversity strongholds that store vast amounts of planet-warming carbon, according to University of Michigan forest ecologist Tsun Fung (Tom) Au, a postdoctoral fellow at the Institute for Global Change Biology.

"The number of old-growth forests on the planet is declining, while drought is predicted to be more frequent and more intense in the future," said Au, lead author of the study published online Dec. 1 in the journal Nature Climate Change.

"Given their high resistance to drought and their exceptional carbon storage capacity, conservation of older trees in the upper canopy should be the top priority from a climate mitigation perspective."

The researchers also found that younger trees in the upper canopy -- if they manage to survive drought -- showed greater resilience, defined as the ability to return to pre-drought growth rates.

While deforestation, selective logging and other threats have led to the global decline of old-growth forests, subsequent reforestation -- either through natural succession or through tree planting -- has led to forests dominated by increasingly younger trees.

For example, the area covered by younger trees (<140 years old) in the upper canopy layer of temperate forests worldwide already far exceeds the area covered by older trees. As forest demographics continue to shift, younger trees are expected to play an increasingly important role in carbon sequestration and ecosystem functioning.

"Our findings -- that older trees in the upper canopy are more drought tolerant, while younger trees in the upper canopy are more drought resilient -- have important implications for future carbon storage in forests," Au said.

"These results imply that in the short term, drought's impact on forests may be severe due to the prevalence of younger trees and their greater sensitivity to drought. But in the long run, those younger trees have a greater ability to recover from drought, which could be beneficial to the carbon stock."

Those implications will require further study, according to Au and colleagues, given that reforestation has been identified by the Intergovernmental Panel on Climate Change as a potential nature-based solution to help mitigate climate change.

The Sharm el-Sheikh Implementation Plan published during the 2022 United Nations Climate Change Conference in Egypt (COP27) also reaffirmed the importance of maintaining intact forest cover and associated carbon storage as a social and environmental safeguard.

"These findings have implications for how we manage our forests. Historically, we have managed forests to promote tree species that have the best wood quality," said Indiana University's Justin Maxwell, a senior author of the study.

"Our findings suggest that managing forests for their ability to store carbon and to be resilient to drought could be an important tool in responding to climate change, and thinking about the age of the forest is an important aspect of how the forest will respond to drought."

The researchers used long-term tree-ring data from the International Tree-Ring Data Bank to analyze the growth response of 21,964 trees from 119 drought-sensitive species, during and after droughts of the past century.

They focused on trees in the uppermost canopy. The forest canopy is a multilayered, structurally complex and ecologically important zone formed by mature, overlapping tree crowns.

The upper canopy trees were separated into three age groups -- young, intermediate and old -- and the researchers examined how age influenced drought response for different species of hardwoods and conifers.

They found that young hardwoods in the upper canopy experienced a 28% growth reduction during drought, compared to a 21% growth reduction for old hardwoods. The 7% difference between young and old hardwoods grew to 17% during extreme drought.

While those age-related differences may appear fairly minor, when applied at the global scale they could have "huge impacts" on regional carbon storage and the global carbon budget, according to the study authors. That's especially true in temperate forests that are among the largest carbon sinks worldwide.

In the study, age-related drought-response differences in conifers were smaller than in hardwoods, likely because needle-bearing trees tend to inhabit more arid environments, the researchers say.

The current study was part of Au's doctoral dissertation at Indiana University, and he continued the work after joining U-M's Institute for Global Change Biology, which is based at the School for Environment and Sustainability.

The new study is a synthesis that represents the net effects of thousands of trees in diverse forests across five continents, rather than focusing on single forest types. In addition, the new study is unique in its focus on trees in the upper forest canopy, which reduces the confounding effects of tree height and size, according to the authors.

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Oft-overlooked grasslands build biodiversity, resilience over centuries

Grasslands' biodiversity and resilience to disturbances such as fire, heat and drought is the result of a slow process over hundreds of years, like that of old growth forests, finds new University of Colorado Boulder-led research.

Publishing in the journal Science on Aug. 5, 2022, as part of a special issue on grasslands, the study contradicts years of assumptions that grasslands' ecological development is quick and their recovery is rapid, posing new challenges to their successful restoration.

"Old growth grasslands have a unique suite of characteristics that develop over a really long time. Recovering grasslands do not have the same species or the same characteristics as they did prior to soil tilling or tree planting, and they take centuries to redevelop," said Katharine Suding, senior author of the paper and Distinguished Professor in the Department of Ecology and Evolutionary Biology and Institute of Arctic and Alpine Research (INSTAAR) at CU Boulder. "It's an important reminder that we need to conserve the ancient grasslands that are still intact."

An expert in the field of North American grasslands, Suding partnered with other experts from around the world to evaluate the current state of global grassland science, conservation and restoration -- from arid, prairie and coastal grasslands, to those in the tropics and savannahs.

Grasslands, which account for nearly 40% of land-based ecosystems, provide habitat for a wide diversity of animals and plants, and contribute to the livelihoods of over 1 billion people worldwide. They also provide significant carbon sequestration and biodiversity benefits, and can be more resilient than forests in the face of a quickly changing climate.

Yet over the past couple of centuries, ancient grasslands around the world have largely been converted into farmland, used to grow trees or been developed as cities expand.

The researchers found that while the destruction of these pristine grasslands can occur very quickly, complete recovery of grassland biodiversity and essential ecosystem functions occurs slowly or not at all. The findings further emphasize the importance of conserving the world's remaining untouched grasslands.

"If you plant trees in an older grassland or till it for agriculture, you will probably never get many of the unique diversity and belowground characteristics back. It is irreversible," said Suding.

Restoration takes time

Grasslands store the bulk of their material underground, in roots that can reach as far as 20 feet deep. This unseen physical presence is how they can store a lot of carbon -- about a third of all carbon stored on land -- and remain resilient to fire and other ecological disturbances. It's also why grasslands are often underappreciated in comparison to forests. If it's out of sight, it's out of mind.

Grassland restoration, however, can take a page out of forests' playbook.

"'Old growth' is not only a term for forests, but one that applies to grasslands as well," said co-author Elise Buisson, who co-authored that finding in a 2015 publication.

Old growth grasslands are unique in their underground structures and biodiversity compared to newer, younger grasslands. And while these old growth ecosystems may never be fully replicated in modern-day landscapes, they provide a model for restoration efforts, said Suding.

Even a decade ago, grassland restoration focused on distributing species' seed onto a landscape, adding grazing or fire, and stepping aside. The new analysis finds that it takes more than a hands-off approach to be successful. Instead of tossing all the ingredients into a crockpot and turning it on high, grasslands may need more of a step-by-step recipe approach to restoration.

"We should think of restoration as more of guiding a trajectory. Some species don't come in right at the start, and the disturbance that maintains the grassland needs time to grow and be tweaked as these species get established and the soil develops," said Suding. "These processes take time."

For example, some plants do well reproducing from seed in, say, the upper Midwest but not in Colorado due to the drier climate. Many tropical grasses don't spread by seed at all, instead by rhizomes and tubers underground, and are much more difficult to reestablish.

Implications for policy

The report comes a year after the start of the United Nations Decade on Ecosystem Restoration, which aims to restore degraded ecosystems around the world to increase biodiversity, help achieve the Sustainable Development Goals and the Paris Climate Agreement. At the same time, planting trees has become a popular "natural solution" around the world to remove large quantities of carbon from the atmosphere.

Yet while the UN initiative explicitly states, "planting trees on natural grassland may destroy more than it creates," as countries make ambitious goals and commitments to ecosystem restoration this decade, Suding worries that for many, this only means planting trees.

"We would lose a huge element of the biodiversity on Earth if we planted trees in old growth grasslands," said Suding. "I think we need to be a little bit more careful about what's best for the globe, in terms of where to restore what."

As climate change threatens the American West through drought, heat and wildfire, grasslands are also a resilient choice to use less water, reduce soil erosion and keep carbon in the ground over time. It's the older, veteran grasslands that are most beneficial in this regard.

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The European drought event from 2018 to 2020 was the most intense in over 250 years

These were days, months and years that many will come to remember: the drought from 2018 to 2020. An international team of researchers led by scientists from the Helmholtz Centre for Environmental Research (UFZ) has succeeded in categorizing the historical dimensions of this event. Based on their findings, no drought covering such a large area for an extended period and coinciding with warmer temperature has occurred in Europe since the middle of the 18th century. The years from 2018 to 2020 thus represent a new benchmark for droughts. Because such an unprecedented event is likely to occur more frequently in the future, the scientists urgently recommend the development and implementation of suitable, regionally adapted drought prevention measures.

Withered meadows and fields, dry stream beds, dead forests, and reduced power plant outputs -- the drought years of 2018, 2019 and 2020 were exceptional and had substantial impacts on nature and the economy. Previously it was not clear where they should be classified in their historical dimension. Now we know: "The 2018 to 2020 drought sets a new benchmark for droughts in Europe," says Dr. Oldrich Rakovec, UFZ modeller and lead author of the article published in the Earth's Future journal of the American Geophysical Union. The scientists documented this with a large compilation of data and modelling techniques which allowed them to reconstruct historical droughts back to 1766 and comparing their extents with the drought of 2018 to 2020.

The drought from 2018 to 2020 thus affected approximately one third of the land area of Europe, especially in central Europe, such as Germany, France and the Czech Republic. "No other drought event over the last 250 years had such a large spatial extent as this one," explains Oldrich Rakovec. The total duration of the drought event in Europe was also unusually long, starting in April 2018 and not ending until December 2020: 33 months. Only the drought between 1857 and 1860 lasted slightly longer for a total of 35 months. What's more: The drought from 2018 to 2020 also continued in 2021 and 2022 in deeper soils (i.e., up to 2m below the ground surface). "Although 2021 was wetter and supplied much needed water in the upper soil important for sustaining agriculture activities, the moisture did not penetrate to greater depths," says the UFZ modeller.

The average drought duration was also unusually long in the 50 x 50 km grid cells in which the scientists subdivided Europe for their modelling activity. Because a drought event develops dynamically in space and time (i.e., it starts at one point, then continues developing and finally ends somewhere else) its mean duration differs from its total one. In this case, the 2018-2020 event exhibited a mean drought duration of 12 months.

In the past, only the drought event from 1857 to 1860 lasted longer, with a mean duration of 13 months. The scientists define drought as the time in which the current soil-water content in top 2-m soil falls below the level that has been reached only 20 percent of the time during the 250 years. To reconstruct these historical droughts, the scientists used the mHM hydrologic model developed at the UFZ. Among other things, this environmental model can be used to estimate soil moisture content based on past temperature and precipitation records.

The rise in air temperature also reached a historical record during the 2018-2020 drought event, with an anomaly of 2.8 degrees Celsius above the long-term average over the past 250 years. "The droughts in the past were colder than recent droughts in which the average temperature hardly changed," says Dr. Rohini Kumar, UFZ modeller and co-author of the article. The effects of a drought event become significantly more severe if, on addition to the precipitation deficit (approximately 20 percent for major drought events in past centuries), the warmer conditions prevail. This combined effect results in greater evaporation losses, leading to declining soil-water levels. The scientists also examined the consequences of the lack of water for agriculture during this drought event. They compared average annual crop yields for wheat, grain maize and barley, between 2018 and 2020 with those between 1961 and 2021. The results indicate that harvests were significantly reduced in countries affected primarily by the 2018-2020 drought. For example, grain maize production decreased between 20 and 40 percent in the Benelux countries, Germany and France; wheat reduced by up to 17.5 percent in Germany; and barley reduced by 10 percent in nearly all of Europe.

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Rewriting the history books: Why the Vikings left Greenland

One of the great mysteries of late medieval history is why did the Norse, who had established successful settlements in southern Greenland in 985, abandon them in the early 15th century? The consensus view has long been that colder temperatures, associated with the Little Ice Age, helped make the colonies unsustainable. However, new research, led by the University of Massachusetts Amherst and published recently in Science Advances, upends that old theory. It wasn't dropping temperatures that helped drive the Norse from Greenland, but drought.

When the Norse settled in Greenland on what they called the Eastern Settlement in 985, they thrived by clearing the land of shrubs and planting grass as pasture for their livestock. The population of the Eastern Settlement peaked at around 2,000 inhabitants, but collapsed fairly quickly about 400 years later. For decades, anthropologists, historians and scientists have thought the Eastern Settlement's demise was due to the onset of the Little Ice Age, a period of exceptionally cold weather, particularly in the North Atlantic, that made agricultural life in Greenland untenable.

However, as Raymond Bradley, University Distinguished Professor of geosciences at UMass Amherst and one of the paper's co- author, points out, "before this study, there was no data from the actual site of the Viking settlements. And that's a problem." Instead, the ice core data that previous studies had used to reconstruct historical temperatures in Greenland was taken from a location that was over 1,000 kilometers to the north and over 2,000 meters higher in elevation. "We wanted to study how climate had varied close to the Norse farms themselves," says Bradley. And when they did, the results were surprising.

Bradley and his colleagues traveled to a lake called Lake 578, which is adjacent to a former Norse farm and close to one of the largest groups of farms in the Eastern Settlement. There, they spent three years gathering sediment samples from the lake, which represented a continuous record for the past 2,000 years. "Nobody has actually studied this location before," says Boyang Zhao, the study's lead author who conducted this research for his Ph.D. in geosciences at UMass Amherst and is currently a postdoctoral research associate at Brown University.

They then analyzed that 2,000 year sample for two different markers: the first, a lipid, known as BrGDGT, can be used to reconstruct temperature. "If you have a complete enough record, you can directly link the changing structures of the lipids to changing temperature," says Isla Castañeda, professor of geosciences at UMass Amherst and one of the paper's co-authors. A second marker, derived from the waxy coating on plant leaves, can be used to determine the rates at which the grasses and other livestock-sustaining plants lost water due to evaporation. It is therefore an indicator of how dry conditions were.

"What we discovered," says Zhao, "is that, while the temperature barely changed over the course of the Norse settlement of southern Greenland, it became steadily drier over time."

Norse farmers had to overwinter their livestock on stored fodder, and even in a good year the animals were often so weak that they had to be carried to the fields once the snow finally melted in the spring. Under conditions like that, the consequences of drought would have been severe. An extended drought, on top of other economic and social pressures, may have tipped the balance just enough to make the Eastern Settlement unsustainable.

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Precipitation trends determine how often droughts and heat waves will occur together

The fact that global warming will increase temperatures over land masses, increasing the frequency of droughts and heat waves, is a certainty -- as is the fact that climate change will alter the average amount of precipitation on land. However, it has remained unclear until now under what conditions both extreme events will occur together, known as 'compound hot-dry-events'. The UFZ researchers have defined these events as summers in which the average temperature was higher than in 90 percent of the summers between 1950 and 1980, and precipitation was simultaneously lower than in 90 percent of those years.

"In the past, periods of drought and heat waves were often considered separately; there is, however, a strong correlation between the two events, which can be seen in the extremes experienced in 2003 and 2018 in Europe. The negative consequences of these compound extremes are often greater than with one single extreme," says UFZ climate researcher Dr Jakob Zscheischler, last author of the study. Until now, however, it was not known what the future simultaneous occurrence of these extremes depends on -- the uncertainties in the occurrences estimated via routinely used climate model simulations were too large to arrive at robust pronouncements.

The researchers have now used a novel model ensemble, comprising seven climate models, to reduce and better understand these uncertainties. Each model simulation was carried out up to 100 times in order to account for natural climate variability. They examined the historical period between 1950 and 1980, comparing the results with those of a potential future climate that is two degrees warmer than preindustrial conditions. "The advantage of these multiple simulations is that we have a much larger volume of data than with conventional model ensembles, enabling us to better estimate compound extremes," explains Dr Emanuele Bevacqua, first author and climate researcher at the UFZ. The researchers were able to confirm the previous assumption that the average frequency of compound hot-dry events will increase with global warming: while the frequency lay at 3 percent between 1950 and 1980, which statistically is an occurrence every 33 years, in a climate that is two degrees warmer, this figure will be around 12 percent. This would be a fourfold increase compared to the historical period studied.

The climate researchers were also able to determine from the simulations that the frequency of compound hot-dry events in the future will be determined not by temperature trends, but by precipitation trends. The reason for this is that, even with a moderate warming of two degrees, local temperature increase will be so great that in the future, every drought anywhere in the world will be accompanied by a heat wave, regardless of the exact number of degrees by which the temperature increases locally. The uncertainty in the warming leads to an uncertainty in the prediction of compound hot-dry event frequencies of only 1.5 percent. This discounts temperature as a decisive factor for uncertainty. For precipitation, however, the researchers calculated an uncertainty of up to 48 percent. "This demonstrates that local precipitation trends determine whether periods of drought and heat waves will occur simultaneously," explains Emanuele Bevacqua. For Central Europe, for example, this implies that in the case of a 'wet storyline' with increasing precipitation, concurrent droughts and heat waves will occur on average every ten years, whereas in the case of a 'dry storyline' with decreasing precipitation, they will occur at least every four years. For Central North America, these events would be expected every nine years ('wet storyline') and six years ('dry storyline'). These regional storylines for precipitation trends can be used as a basis for decisions on adaptation, for example to evaluate best and worst case-scenarios.

However, even if we know that precipitation trends are decisive for the occurrence of concurrent droughts and heat waves, it is still difficult to predict them any more reliably: "Climate change may shift the distribution of precipitation in certain regions. The pattern of precipitation depends on atmospheric circulation, which determines regional weather dynamics through numerous interactions over large parts of the globe," says Emanuele Bevacqua. Since the dynamic of many of these processes is not yet fully understood, it is difficult to reduce these uncertainties any further. 

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Co-occurring droughts could threaten global food security

Droughts occurring at the same time across different regions of the planet could place an unprecedented strain on the global agricultural system and threaten the water security of millions of people, according to a new study in Nature Climate Change.

A Washington State University-led research team analyzed climate, agricultural and population growth data to show continuing fossil fuel dependence will increase the probability of co-occurring droughts 40% by the mid-21st century and 60% by the late 21st century, relative to the late-20th century. That comes out to an approximately ninefold increase in agricultural and human population exposure to severe co-occurring droughts unless steps are taken to lower carbon emissions.

"There could be around 120 million people across the globe simultaneously exposed to severe compound droughts each year by the end of the century," said lead author Jitendra Singh, a former postdoctoral researcher at the WSU School of the Environment now at ETH Zurich, Switzerland. "Many of the regions our analysis shows will be most affected are already vulnerable and so the potential for droughts to become disasters is high."

The elevated risk of compound droughts estimated by Singh and colleagues is a result of a warming climate coupled with a projected 22% increase in the frequency of El Niño and La Niña events, the two opposite phases of the El Niño Southern Oscillation (ENSO).

The researchers' projections show that nearly 75% of compound droughts in the future will coincide with these irregular but recurring periods of climatic variation in the world's oceans, which have played a large role in some of the greatest environmental disasters in world history.

For example, El Nino-fueled droughts that concurrently occurred across Asia, Brazil and Africa during 1876-1878 led to synchronous crop failures, followed by famines that killed more than 50 million people.

"While technology and other circumstances today are a lot different than they were in the late 19th century, crop failures in multiple breadbasket regions still have the potential to affect global food availability," said study coauthor Deepti Singh, an assistant professor in the WSU School of the Environment. "This could in turn increase volatility in global food prices, affecting food access and exacerbating food insecurity, particularly in regions that are already vulnerable to environmental shocks such as droughts."

The researchers' analysis specifically focused on ten regions of the planet that receive most of their rainfall during June-September, have high variability in monthly summer precipitation and are affected by ENSO variations, factors that lead to an increased potential for co-occurring drought. Several of the regions analyzed include important agricultural regions and countries that are currently facing food and water insecurity.

Their results indicate areas of North and South America are more likely to experience compound droughts in a future, warmer climate than regions of Asia, where much of the agricultural land is projected to become wetter.

Food produced in the Americas could therefore be more susceptible to climatic hazards. For instance, the United States is a major exporter of staple grains and currently ships maize to countries across the globe. Even a modest increase in the risk of compound droughts in the future climate could lead to regional supply shortfalls that could in turn cascade into the global market, affecting global prices and amplifying food insecurity.

"The potential for a food security crisis increases even if these droughts aren't affecting major food producing regions but rather many regions that are already vulnerable to food insecurity," said coauthor Weston Anderson, an assistant research scientist at the Earth System Science Interdisciplinary Center at the University of Maryland. "Simultaneous droughts in food insecure regions could in turn amplify stresses on international agencies responsible for disaster relief by requiring the provision of humanitarian aid to a greater number of people simultaneously."

There is some good news, Anderson said. The researchers' work is based on a high fossil fuel emissions scenario, and in recent years, the global community has made progress toward lowering carbon emissions which would greatly mitigate the frequency and intensity of co-occurring droughts by the end of the 21st century.

Also, the occurrence of nearly 75% of compound droughts alongside ENSO events in the future climate highlights the potential to predict where these droughts may occur with a lead time of up to nine months.

"This means that co-occurring droughts during ENSO events will likely affect the same geographical regions they do today albeit with greater severity," said Deepti Singh. "Being able to predict where these droughts will occur and their potential impacts can help society develop plans and efforts to minimize economic losses and reduce human suffering from such climate-driven disasters."

Moving forward the researchers plan to take a closer look at how co-occurring droughts will affect various aspects of the global food network, how vulnerable communities are affected by and adapting to such climate extremes, as well as how society can be better prepared to manage the risk of increasing simultaneous disasters.

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Ancient Maya lessons on surviving drought

A new study casts doubt on drought as the driver of ancient Mayan civilization collapse.

There is no dispute that a series of droughts occurred in the Yucatan Peninsula of southeastern Mexico and northern Central America at the end of the ninth century, when Maya cities mysteriously began to be depopulated. Believing the Maya were mostly dependent on drought-sensitive corn, beans, and squash, some scholars assume the droughts resulted in starvation.

However, a new analysis by UC Riverside archaeologist Scott Fedick and plant physiologist Louis Santiago shows the Maya had nearly 500 edible plants available to them, many of which are highly drought resistant. The results of this analysis have now been published in the Proceedings of the National Academy of Sciences.

"Even in the most extreme drought situation -- and we have no clear evidence the most extreme situation ever occurred -- 59 species of edible plants would still have persisted," Santiago said.

Some of the toughest plants the Maya would have turned to include cassava with its edible tubers, and hearts of palm. Another is chaya, a shrub domesticated by the Maya and eaten today by their descendants. Its leaves are high in protein, iron, potassium, and calcium.

"Chaya and cassava together would have provided a huge amount of carbohydrates and protein," Santiago said.

Unable to find a master list of indigenous Maya food plants, Fedick recently compiled and published one that draws on decades of Maya plant knowledge. Faced with much speculation about drought as the cause of Maya social disruptions, he and Santiago decided to examine all 497 plants on the list for drought tolerance.

"When botanists study drought resistance, they're usually talking about a specific plant, or a particular ecosystem," Fedick said. "One of the reasons this project was so challenging is because we examined the dietary flora of an entire civilization -- annuals, perennials, herbs, trees, domesticates, and wild species. It was a unique endeavor."

Though the researchers do not have a clear answer about why ancient Maya society unraveled, they suspect social and economic upheaval played a role.

"One thing we do know is the overly simplistic explanation of drought leading to agricultural collapse is probably not true," Fedick said.

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Global warming begets more warming, new paleoclimate study finds

It is increasingly clear that the prolonged drought conditions, record-breaking heat, sustained wildfires, and frequent, more extreme storms experienced in recent years are a direct result of rising global temperatures brought on by humans' addition of carbon dioxide to the atmosphere. And a new MIT study on extreme climate events in Earth's ancient history suggests that today's planet may become more volatile as it continues to warm.

The study, appearing today in Science Advances, examines the paleoclimate record of the last 66 million years, during the Cenozoic era, which began shortly after the extinction of the dinosaurs. The scientists found that during this period, fluctuations in the Earth's climate experienced a surprising "warming bias." In other words, there were far more warming events -- periods of prolonged global warming, lasting thousands to tens of thousands of years -- than cooling events. What's more, warming events tended to be more extreme, with greater shifts in temperature, than cooling events.

The researchers say a possible explanation for this warming bias may lie in a "multiplier effect," whereby a modest degree of warming -- for instance from volcanoes releasing carbon dioxide into the atmosphere -- naturally speeds up certain biological and chemical processes that enhance these fluctuations, leading, on average, to still more warming.

Interestingly, the team observed that this warming bias disappeared about 5 million years ago, around the time when ice sheets started forming in the Northern Hemisphere. It's unclear what effect the ice has had on the Earth's response to climate shifts. But as today's Arctic ice recedes, the new study suggests that a multiplier effect may kick back in, and the result may be a further amplification of human-induced global warming.

"The Northern Hemisphere's ice sheets are shrinking, and could potentially disappear as a long-term consequence of human actions" says the study's lead author Constantin Arnscheidt, a graduate student in MIT's Department of Earth, Atmospheric and Planetary Sciences. "Our research suggests that this may make the Earth's climate fundamentally more susceptible to extreme, long-term global warming events such as those seen in the geologic past."

Arnscheidt's study co-author is Daniel Rothman, professor of geophysics at MIT, and co-founder and co-director of MIT's Lorenz Center.

A volatile push

For their analysis, the team consulted large databases of sediments containing deep-sea benthic foraminifera -- single-celled organisms that have been around for hundreds of millions of years and whose hard shells are preserved in sediments. The composition of these shells is affected by the ocean temperatures as organisms are growing; the shells are therefore considered a reliable proxy for the Earth's ancient temperatures.

For decades, scientists have analyzed the composition of these shells, collected from all over the world and dated to various time periods, to track how the Earth's temperature has fluctuated over millions of years.

"When using these data to study extreme climate events, most studies have focused on individual large spikes in temperature, typically of a few degrees Celsius warming," Arnscheidt says. "Instead, we tried to look at the overall statistics and consider all the fluctuations involved, rather than picking out the big ones."

The team first carried out a statistical analysis of the data and observed that, over the last 66 million years, the distribution of global temperature fluctuations didn't resemble a standard bell curve, with symmetric tails representing an equal probability of extreme warm and extreme cool fluctuations. Instead, the curve was noticeably lopsided, skewed toward more warm than cool events. The curve also exhibited a noticeably longer tail, representing warm events that were more extreme, or of higher temperature, than the most extreme cold events.

"This indicates there's some sort of amplification relative to what you would otherwise have expected," Arnscheidt says. "Everything's pointing to something fundamental that's causing this push, or bias toward warming events."

"It's fair to say that the Earth system becomes more volatile, in a warming sense," Rothman adds.

A warming multiplier

The team wondered whether this warming bias might have been a result of "multiplicative noise" in the climate-carbon cycle. Scientists have long understood that higher temperatures, up to a point, tend to speed up biological and chemical processes. Because the carbon cycle, which is a key driver of long-term climate fluctuations, is itself composed of such processes, increases in temperature may lead to larger fluctuations, biasing the system towards extreme warming events.

In mathematics, there exists a set of equations that describes such general amplifying, or multiplicative effects. The researchers applied this multiplicative theory to their analysis to see whether the equations could predict the asymmetrical distribution, including the degree of its skew and the length of its tails.

In the end, they found that the data, and the observed bias toward warming, could be explained by the multiplicative theory. In other words, it's very likely that, over the last 66 million years, periods of modest warming were on average further enhanced by multiplier effects, such as the response of biological and chemical processes that further warmed the planet.

As part of the study, the researchers also looked at the correlation between past warming events and changes in Earth's orbit. Over hundreds of thousands of years, Earth's orbit around the sun regularly becomes more or less elliptical. But scientists have wondered why many past warming events appeared to coincide with these changes, and why these events feature outsized warming compared with what the change in Earth's orbit could have wrought on its own.

So, Arnscheidt and Rothman incorporated the Earth's orbital changes into the multiplicative model and their analysis of Earth's temperature changes, and found that multiplier effects could predictably amplify, on average, the modest temperature rises due to changes in Earth's orbit.

"Climate warms and cools in synchrony with orbital changes, but the orbital cycles themselves would predict only modest changes in climate," Rothman says. "But if we consider a multiplicative model, then modest warming, paired with this multiplier effect, can result in extreme events that tend to occur at the same time as these orbital changes."

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