Future floods: Global warming intensifies heavy rain -- even more than expected

The intensity and frequency of extreme rainfall increases exponentially with global warming, a new study finds. The analysis by researchers from the Potsdam Institute of Climate Impact Research (PIK) shows that state-of-the-art climate models significantly underestimate how much extreme rainfall increases under global warming -- meaning that extreme rainfall could increase quicker than climate models suggest.

"Our study confirms that the intensity and frequency of heavy rainfall extremes are increasing exponentially with every increment of global warming," explains Max Kotz, lead-author of the study published in the Journal of Climate. These changes follow the physical theory of the classic Clausius-Clapeyron relation of 1834, which established that warmer air can hold more water vapour.

"State-of-the-art climate models vary on how strongly extreme rainfall scales with global warming and that they underestimate it compared to historical observations."

"Climate impacts on society have been calculated using climate models. Now our findings suggest that these impacts could be much worse than we thought. Extreme rainfall will be heavier and more frequent. Society needs to be prepared for this," says PIK department head and author of the study Anders Levermann.

Changes in the frequency and intensity of daily rainfall extremes over land can impact social welfare, the economy and social stability, given their link to flooding but also ground-water availability, which can cause considerable loss of life and financial losses.

Stronger increases of extremes across tropical regions

The researchers at PIK analysed the intensity and frequency of daily precipitation extremes over land in 21 state-of-the-art climate simulations (CMIP-6) and compared the changes projected by CMIP-6 models to those observed historically.

The method they applied draws on pattern-filtering techniques, allowing them to separate which changes in the climate system are forced by human emissions, and which are not.

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Fewer but more intense tropical storms predicted over the Ganges and Mekong

Climate experts project a decline in the frequency of future tropical storms but an increase in their strength across the Ganges and Mekong basins allowing for better future planning.

The Newcastle University-led team focused on the Ganges and Mekong basins and evaluated the simulation of tropical storms. Their analyses show an increase in tropical storms frequency up until the early 2010s but that climate models project a frequency decline of over 50% on average across both basins by 2050.

In contrast, the results from high resolution climate models show an increase in the future intensity of tropical storms for both basins, with the largest increases for the most intense tropical storms.

These findings can be used to assess the future resilience of existing infrastructure systems to tropical storms across these densely populated basins.

The team, involving scientists from the Met Office and the University of Reading, published their findings in the journal Geophysical Research Letters. The scientists used European Union Horizon 2020 project PRIMAVERA models, which are available at up to 25 km resolution. They also employed two storm tracking algorithms, TRACK and TempExt.

Study lead author, Dr Haider Ali, of Newcastle University's School of Engineering, said: "Tropical storms are one of the world's most damaging natural hazards which result in colossal socioeconomic losses to life, infrastructure and property, especially in low-lying delta rivers basins like the Ganges and Mekong.

"Knowledge of changes to tropical storms activity under climate change can therefore be helpful in developing better disaster risk mitigation and for climate adaptation. Previous modelling studies have used coarse-resolution global climate models unable to capture key tropical storm characteristics.

"In this study, we used finer resolution models and two different tracking algorithms to resolve a part of this uncertainty."

Study author, Hayley Fowler, Professor of Climate Change Impacts, Newcastle University School of Engineering, added: "Our results are consistent with those found for tropical storms and Tropical Cyclones in the Atlantic Basin, where they also project an overall decline in frequency but an increase in the frequency of the most intense TCs. These systems cause massive impacts on society from high winds, rainfall and storm surges causing flooding. Quantifying these changes will allow us to better plan for future events."

Implications for Climate Adaptation Policies

The Ganges and Mekong basins are two significant river systems in Asia that play vital roles in the lives of millions of people living in the region. The basins are essential for agriculture, water supply, and transportation.

However, both the Ganges and Mekong basins are highly vulnerable to the impacts of climate change, including changes in precipitation patterns, extreme weather events, and sea-level rise.

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We've pumped so much groundwater that we've nudged Earth's spin

By pumping water out of the ground and moving it elsewhere, humans have shifted such a large mass of water that the Earth tilted nearly 80 centimeters (31.5 inches) east between 1993 and 2010 alone, according to a new study published in Geophysical Research Letters, AGU's journal for short-format, high-impact research with implications spanning the Earth and space sciences.

Based on climate models, scientists previously estimated humans pumped 2,150 gigatons of groundwater, equivalent to more than 6 millimeters (0.24 inches) of sea level rise, from 1993 to 2010. But validating that estimate is difficult.

One approach lies with the Earth's rotational pole, which is the point around which the planet rotates. It moves during a process called polar motion, which is when the position of the Earth's rotational pole varies relative to the crust. The distribution of water on the planet affects how mass is distributed. Like adding a tiny bit of weight to a spinning top, the Earth spins a little differently as water is moved around.

"Earth's rotational pole actually changes a lot," said Ki-Weon Seo, a geophysicist at Seoul National University who led the study. "Our study shows that among climate-related causes, the redistribution of groundwater actually has the largest impact on the drift of the rotational pole."

Water's ability to change the Earth's rotation was discovered in 2016, and until now, the specific contribution of groundwater to these rotational changes was unexplored. In the new study, researchers modeled the observed changes in the drift of Earth's rotational pole and the movement of water -- first, with only ice sheets and glaciers considered, and then adding in different scenarios of groundwater redistribution.

The model only matched the observed polar drift once the researchers included 2150 gigatons of groundwater redistribution. Without it, the model was off by 78.5 centimeters (31 inches), or 4.3 centimeters (1.7 inches) of drift per year.

"I'm very glad to find the unexplained cause of the rotation pole drift," Seo said. "On the other hand, as a resident of Earth and a father, I'm concerned and surprised to see that pumping groundwater is another source of sea-level rise."

"This is a nice contribution and an important documentation for sure," said Surendra Adhikari, a research scientist at the Jet Propulsion Laboratory who was not involved in this study. Adhikari published the 2016 paper on water redistribution impacting rotational drift. "They've quantified the role of groundwater pumping on polar motion, and it's pretty significant."

The location of the groundwater matters for how much it could change polar drift; redistributing water from the midlatitudes has a larger impact on the rotational pole. During the study period, the most water was redistributed in western North America and northwestern India, both at midlatitudes.

Countries' attempts to slow groundwater depletion rates, especially in those sensitive regions, could theoretically alter the change in drift, but only if such conservation approaches are sustained for decades, Seo said.

The rotational pole normally changes by several meters within about a year, so changes due to groundwater pumping don't run the risk of shifting seasons. But on geologic time scales, polar drift can have an impact on climate, Adhikari said.

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Humidity may increase heat risk in urban climates

As temperatures across the globe reach record-level highs, urban areas are facing increased heat stress. Cities are generally warmer and dryer than adjacent rural land. But in the Global South, there is an additional complicating factor -- urban humid heat.

A new study, led by Yale School of the Environment scientists and published in Nature, investigated the combined effect of temperature and humidity on urban heat stress using observational data and an urban climate model calculation. Researchers found that the heat stress burden is dependent on local climate and a humidifying effect can erase the cooling benefits that would come from trees and vegetation.

"A widely held view is that urban residents suffer more heat burden than the general population owing to the urban heat island phenomenon. This view is incomplete because it omits another ubiquitous urban microclimate phenomenon called the urban dry island -- that urban land tends to be less humid than the surrounding rural land," says Xuhui Lee, Sara Shallenberger Brown Professor of Meteorology, who directed the study. "In dry, temperate, and boreal climates, urban residents are actually less heat-stressed than rural residents. But in the humid Global South, the urban heat island is dominant over the urban dry island, resulting in two to six extra dangerous heat stress days per summer."

Lee and YSE doctoral student Keer Zhang, lead author of the study, say they were motivated to investigate the issue for several reasons: a large percentage of the global population lives in urban areas; many people in informal urban settlements do not have access to air conditioning; and the problem is going to get worse as temperatures rise and more people move to cities. About 4.3 billion people, or 55% of the world's population, live in urban settings, and the number is expected to rise to 80% by 2050, according to the World Economic Forum.

The researchers developed a theoretical framework on how urban land modifies both air temperature and air humidity and showed that these two effects have equal weight in heat stress as measured by the wet-bulb temperature, in contrary to other heat indexes, which weigh temperature more heavily than humidity. Wet-bulb temperature combines dry air temperature with humidity to measure humid heat. The results of the study, the authors note, raise important questions.

"Green vegetation can lower air temperature via water evaporation, but it can also increase heat burden because of air humidity. The question then is to what extent this humidifying effect erases the cooling benefit arising from temperature reduction. We hope to answer this question in a follow-up study, where we are comparing observations of the wet-bulb temperature in urban greenspaces (with dense tree cover) and those in built-up neighborhoods," Lee says.

Zhang says she hopes the study can lead to further research on how cities can mitigate heat stress.

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Researchers correlate Arctic warming to extreme winter weather in midlatitude and its future

A warmer Arctic has been linked to extreme winter weather in the midlatitude regions. But, it is not clear how global warming affects this link. In a new study, researchers from Korea and USA show, using weather data and climate models, that while the "Warm Arctic-Cold Continent" pattern will continue as the climate continues to warm, Arctic warming will become a less reliable predictor of extreme winter weather in the future.

Pictures of melting glaciers and stranded polar bears on shrinking sea ice in the Arctic are perhaps the most striking images that have been used to highlights the effects of global warming. However, they do not convey the full extent of the consequences of warmer Arctic. In recent years, there has been growing recognition of the Arctic's role in driving extreme weather events in other parts of the world. While the Arctic has been warming at a rate twice as fast as the global average, winters in the midlatitude regions have experienced colder and more severe weather events. For instance, the winter of 2022-2023 saw record-breaking cold temperatures and snowfall in Japan, China, and Korea. Similarly, many parts of Eurasia and North America have experienced severe cold snaps, with heavy snowfall and prolonged periods of sub-zero temperatures.

While there are multiple theories for this climate phenomenon, an international team of researchers led by Professor Jin-Ho Yoon from Gwangju Institute of Science and Technology (GIST), Korea set out to examine the relationship between the severe winters in the Northern Hemisphere and the melting sea ice in the Arctic region, a phenomenon referred to as the "Warm Arctic-Cold Continent" (WACC), and how this relationship changed with the warming climate.

In their study published online on 27 March 2023 in the journal npj Climate and Atmospheric Science, the researchers looked at historic climate data and turned to climate projection models to explore the potential connection and assess how this phenomenon might be influenced by different global warming scenarios.

Based on the climate data from the European Center for Medium-Range Weather Forecasting (ECMWF) going back almost 40 years, the researchers correlated winter temperatures in East Asia and North America to the temperatures of the Barents-Kara Sea and the East Siberian-Chukchi Sea in the Arctic region. They observed that lower winter temperatures in East Asia and North America are usually accompanied by warmer Arctic Sea temperatures. However, they also found that in some winters, such as the 2017/18 winter in East Asia, this pattern did not hold, suggesting that this linkage include uncertainty likely due to factors other than Arctic Sea temperatures were at play.

Nonetheless, using climate projections from the Half degree Additional warming, Prognosis and Projected Impacts (HAPPI) experiments which were targeted to project future climate under 1.5°C to 2°C warming scenarios, the researchers found the WACC pattern to persist even when global temperatures rose. However, they found that the correlation between the Arctic Sea temperature and the East Asia temperatures became more uncertain with the intensification of global warming. "We found that the relationship between Arctic warming and cold weather events in midlatitude would become more uncertain under warmer climates, challenging the forecast of winter temperature in the future," says Mr. Yungi Hong, a Ph.D. student at GIST and a member of the research team.

"Our study shows that while one can expect the Arctic warming-triggered cold waves in the midlatitudes to persist in a warmer future, they will become more difficult to predict," adds Prof. Jin-Ho Yoon.

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Feedback loops make climate action even more urgent, scientists say

An international collaboration led by Oregon State University scientists has identified 27 global warming accelerators known as amplifying feedback loops, including some that the researchers say may not be fully accounted for in climate models.

They note that the findings, published today in the journal One Earth, add urgency to the need to respond to the climate crisis and provide a roadmap for policymakers aiming to avert the most severe consequences of a warming planet.

In climate science, amplifying feedback loops are situations where a climate-caused alteration can trigger a process that causes even more warming, which in turn intensifies the alteration. An example would be warming in the Arctic, leading to melting sea ice, which results in further warming because sea water absorbs rather than reflects solar radiation.

OSU College of Forestry postdoctoral scholar Christopher Wolf and distinguished professor William Ripple led the study, which in all looked at 41 climate change feedbacks.

"Many of the feedback loops we examined significantly increase warming because of their connection to greenhouse gas emissions," Wolf said. "To the best of our knowledge, this is the most extensive list available of climate feedback loops, and not all of them are fully considered in climate models. What's urgently needed is more research and modeling and an accelerated cutback of emissions."

The paper makes two calls to action for "immediate and massive" emissions reductions:
 

• Minimize short-term warming given that "climate disasters" in the form of wildfires, coastal flooding, permafrost thaw, intense storms and other extreme weather are already occurring.
• Mitigate the possible major threats looming from climate tipping points that are drawing ever-closer due to the prevalence of the many amplifying feedback loops. A tipping point is a threshold after which a change in a component of the climate system becomes self-perpetuating.

"Transformative, socially just changes in global energy and transportation, short-lived air pollution, food production, nature preservation and the international economy, together with population policies based on education and equality, are needed to meet these challenges in both the short and long term," Ripple said. "It's too late to fully prevent the pain of climate change, but if we take meaningful steps soon while prioritizing human basic needs and social justice, it could still be possible to limit the harm."

Ripple, Wolf and co-authors from the University of Exeter, the Potsdam Institute for Climate Impact Research, the Woodwell Climate Research Center and Terrestrial Ecosystems Research Associates considered both biological and physical feedbacks. Biological feedbacks include forest dieback, soil carbon loss and wildfire; physical feedbacks involve changes such as reduced snow cover, increased Antarctic rainfall and shrinking arctic sea ice.

Even comparatively modest warming is expected to heighten the likelihood that the Earth will cross various tipping points, the researchers say, causing big changes in the planet's climate system and potentially strengthening the amplifying feedbacks.

"Climate models may be underestimating the acceleration in global temperature change because they aren't fully considering this large and related set of amplifying feedback loops," Wolf said. "The accuracy of climate models is crucial as they help guide mitigation efforts by telling policymakers about the expected effects of human-caused greenhouse gas emissions. While recent climate models do a much better job of incorporating diverse feedback loops, more progress is needed."

Emissions have risen substantially over the last century, the researchers note, despite several decades of warnings that they should be significantly curbed. The scientists say interactions among feedback loops could cause a permanent shift away from the Earth's current climate state to one that threatens the survival of many humans and other life forms.

"In the worst case, if amplifying feedbacks are strong enough, the result is likely tragic climate change that's moved beyond anything humans can control," Ripple said. "We need a rapid transition toward integrated Earth system science because the climate can only be fully understood by considering the functioning and state of all Earth systems together. This will require large-scale collaboration, and the result would provide better information for policymakers."

In addition to the 27 amplifying climate feedbacks the scientists studied were seven that are characterized as dampening -- they act to stabilize the climate system. An example is carbon dioxide fertilization, where rising concentrations of atmospheric CO2 lead to increasing carbon uptake by vegetation.

The effects of the remaining seven feedbacks, including increased atmospheric dust and reduced ocean stability, are not yet known.

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The last 12,000 years show a more complex climate history than previously thought

We rely on climate models to predict the future, but models cannot be fully tested as climate observations rarely extend back more than 150 years. Understanding the Earth's past climate history across a longer period gives us an invaluable opportunity to test climate models on longer timescales and reduce uncertainties in climate predictions. In this context, changes in the average surface temperature of the Earth during the current interglacial Epoch, the Holocene (approximately the past 12,000 years), have been thoroughly debated over the past decades. Reconstructions of past temperature seem to indicate that global mean temperature showed a maximum around 6,000 years ago and has cooled until the onset of the current climate crisis during the industrial revolution.

Climate model simulations, on the other hand, suggest continuous warming since the start of the Holocene. In 2014, researchers named this major mismatch between models and past climate observations the "Holocene Temperature Conundrum."

In this new study, scientists used the largest available database of past temperature reconstructions extending back 12,000 years to carefully investigate the geographic pattern of temperature change during the Holocene. Olivier Cartapanis and colleagues find that, contrary to previously thought, there is no globally synchronous warm period during the Holocene. Instead, the warmest temperatures are found at different times not only in different regions but also between the ocean and on land. This questions how meaningful comparisons of the global mean temperature between reconstructions and models actually are.

According to the lead author Olivier Cartapanis, "the results challenge the paradigm of a Holocene Thermal Maximum occurring at the same time worldwide." And, while the warmest temperature was reached between 4,000 and 8,000 years ago in western Europe and northern America, the surface ocean temperature cooled since about 10,000 years ago at mid-high latitudes and remained stable in the tropics. The regional variability in the timing of maximum temperature suggests that high latitude insolation and ice extent played major roles in driving climate changes throughout the Holocene.

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Scientists identify the most extreme heatwaves ever recorded globally

A new study has revealed the most intense heatwaves ever across the world -- and remarkably some of these went almost unnoticed decades ago.

The research, led by the University of Bristol, also shows heatwaves are projected to get hotter in future as climate change worsens.

The western North America heatwave last summer was record-breaking with an all-time Canadian high of 49.6 °C in Lytton, British Columbia, on June 29, an increase of 4.6 °C from the previous peak.

The new findings, published today in Science Advances, uncovered five other heatwaves around the world which were even more severe, but went largely underreported.

Lead author, climate scientist Dr Vikki Thompson at the University of Bristol, said: "The recent heatwave in Canada and the United States shocked the world. Yet we show there have been some even greater extremes in the last few decades. Using climate models, we also find extreme heat events are likely to increase in magnitude over the coming century -- at the same rate as the local average temperature."

Heatwaves are one of the most devastating extreme weather events. The western North America heatwave was the most deadly weather event ever in Canada, resulting in hundreds of fatalities. The associated raging wildfires also led to extensive infrastructure damage and loss of crops.

But the study, which calculated how extreme heatwaves were relative to the local temperature, showed the top three hottest-ever in the respective regions were in Southeast Asia in April 1998, which hit 32.8 °C, Brazil in November 1985, peaking at 36.5 °C, and Southern USA in July 1980, when temperatures rose to 38.4 °C.

Dr Vikki Thompson, from the university's Cabot Institute for the Environment, said: "The western North America heatwave will be remembered because of its widespread devastation. However, the study exposes several greater meteorological extremes in recent decades, some of which went largely under the radar likely due to their occurrence in more deprived countries. It is important to assess the severity of heatwaves in terms of local temperature variability because both humans and the natural eco-system will adapt to this, so in regions where there is less variation, a smaller absolute extreme may have more harmful effects."

The team of scientists also used sophisticated climate model projections to anticipate heatwave trends in the rest of this century. The modelling indicated levels of heatwave intensity are set to rise in line with increasing global temperatures.

Although the highest local temperatures do not necessarily cause the biggest impacts, they are often related. Improving understanding of climate extremes and where they have occurred can help prioritise measures to help tackle this in the most vulnerable regions.

Co-author Professor Dann Mitchell, Professor in Climate Sciences at the University of Bristol, said: "Climate change is one of the greatest global health problems of our time, and we have showed that many heatwaves outside of the developed world have gone largely unnoticed. The country-level burden of heat on mortality can be in the thousands of deaths, and countries which experience temperatures outside their normal range are the most susceptible to these shocks."

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Lakes are changing worldwide: Human activities to blame

International research led by Luke Grant, Inne Vanderkelen and Prof Wim Thiery of the VUB research group BCLIMATE shows that global changes in lake temperature and ice cover are not due to natural climate variability and can only be explained by massive greenhouse gas emissions since the Industrial Revolution. The influence of human-induced climate change is evident in rising lake temperatures and in the fact that the ice cover forms later and melts sooner.

"These physical properties are fundamental to lake ecosystems," says Grant, a researcher at VUB and lead author of the study. "As impacts continue to increase in the future, we risk severely damaging lake ecosystems, including water quality and populations of native fish species. This would be disastrous for the many ways in which local communities depend on lakes, ranging from drinking water supply to fishing."

The team also predicted future development under different warming scenarios. In a low-emission scenario, the average warming of lakes in the future is estimated to stabilise at +1.5°C above pre-industrial levels and the duration of ice cover to be 14 days shorter. In a high-emission world, these changes could lead to an increase of +4.0 °C and 46 fewer days of ice.

At the beginning of the project, the authors observed changes in lakes around the world: temperatures are rising and seasonal ice cover is shorter. However, the role of climate change in these trends had not yet been demonstrated.

"In other words, we had to rule out the possibility that these changes were caused by the natural variability of the climate system," says fellow VUB researcher and study co-author Vanderkelen.

The team therefore developed multiple computer simulations with models of lakes on a global scale, on which they then ran a series of climate models. Once the team had built up this database, they applied a methodology described by the Intergovernmental Panel on Climate Change (IPCC). After determining the historical impact of climate change on lakes, they also analysed various future climate scenarios.

The results show that it is highly unlikely that the trends in lake temperatures and ice cover in recent decades can be explained solely by natural climate variability. Moreover, the researchers found clear similarities between the observed changes in lakes and model simulations of lakes in a climate influenced by greenhouse gas emissions.

"This is very convincing evidence that climate change caused by humans has already impacted lakes," says Grant. Projections of lake temperatures and ice cover loss unanimously indicate increasing trends for the future. For every 1°C increase in global air temperature, lakes are estimated to warm by 0.9°C and lose 9.7 days of ice cover. In addition, the analysis revealed significant differences in the impact on lakes at the end of the century, depending on the measures taken by humans to combat climate change.

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