Land under water: What causes extreme flooding?

If rivers overflow their banks, the consequences can be devastating -- just like the catastrophic floods in North Rhine-Westphalia and Rhineland-Palatinate of 2021 showed. In order to limit flood damage and optimise flood risk assessment, we need to better understand what factors can lead to extreme forms of flooding and to what extent. Using methods of explainable machine learning, researchers at the Helmholtz Centre for Environmental Research (UFZ) have shown that floods are more extreme when several factors are involved in their development. The research was published in Science Advances.

There are several factors that play an important role in the development of floods: air temperature, soil moisture, snow depth, and the daily precipitation in the days before a flood. In order to better understand how individual factors contribute to flooding, UFZ researchers examined more than 3,500 river basins worldwide and analysed flood events between 1981 and 2020 for each of them. The result: precipitation was the sole determining factor in only around 25% of the almost 125,000 flood events. Soil moisture was the decisive factor in just over 10% of cases, and snow melt and air temperature were the sole factors in only around 3% of cases. In contrast, 51.6% of cases were caused by at least two factors. At around 23%, the combination of precipitation and soil moisture occurs most frequently.

However, when analysing the data, the UFZ researchers discovered that three -- or even all four -- factors can be jointly responsible for a flood event. For example, temperature, soil moisture, and snow depth were decisive factors in around 5,000 floods whilst all four factors were decisive in around 1,000 flood events. And not only that: "We also showed that flood events become more extreme when more factors are involved," says Dr Jakob Zscheischler, Head of the UFZ Department "Compound Environmental Risks" and senior author of the article. In the case of one-year floods, 51.6% can be attributed to several factors; in the case of five- and ten-year floods, 70.1% and 71.3% respectively can be attributed to several factors. The more extreme a flood is, the more driving factors there are and the more likely they are to interact in the event generation. This correlation often also applies to individual river basins and is referred to as flood complexity.

According to the researchers, river basins in the northern regions of Europe and America as well as in the Alpine region have a low flood complexity. This is because snow melt is the dominant factor for most floods regardless of the flood magnitude. The same applies to the Amazon basin, where the high soil moisture resulting from the rainy season is often a major cause of floods of varying severity. In Germany, the Havel and the Zusam, a tributary of the Danube in Bavaria, are river basins that have a low flood complexity. Regions with river basins that have a high flood complexity primarily include eastern Brazil, the Andes, eastern Australia, the Rocky Mountains up to the US west coast, and the western and central European plains. In Germany, this includes the Moselle and the upper reaches of the Elbe. "River basins in these regions generally have several flooding mechanisms," says Jakob Zscheischler. For example, river basins in the European plains can be affected by flooding caused by the combination of heavy precipitation, active snow melt, and high soil moisture.

However, the complexity of flood processes in a river basin also depends on the climate and land surface conditions in the respective river basin. This is because every river basin has its own special features. Among other things, the researchers looked at the climate moisture index, the soil texture, the forest cover, the size of the river basin, and the river gradient. "In drier regions, the mechanisms that lead to flooding tend to be more heterogeneous. For moderate floods, just a few days of heavy rainfall is usually enough. For extreme floods, it needs to rain longer on already moist soils," says lead author Dr Shijie Jiang, who now works at the Max Planck Institute for Biogeochemistry in Jena.

The scientists used explainable machine learning for the analysis. "First, we use the potential flood drivers air temperature, soil moisture, and snow depth as well as the weekly precipitation -- each day is considered as an individual driving factor -- to predict the run-off magnitude and thus the size of the flood," explains Zscheischler. The researchers then quantified which variables and combinations of variables contributed to the run-off of a particular flood and to which extent. This approach is referred to as explainable machine learning because it uncovers the predictive relationship between flood drivers and run-off during a flood in the trained model. "With this new methodology, we can quantify how many driving factors and combinations thereof are relevant for the occurrence and intensity of floods," adds Jiang.

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Polar bears unlikely to adapt to longer summers

More time stranded on land means greater risk of starvation for polar bears, a new study indicates.

During three summer weeks, 20 polar bears closely observed by scientists tried different strategies to maintain energy reserves, including resting, scavenging and foraging.

Yet nearly all of them lost weight rapidly: on average around 1 kilogram, or 2.2 pounds, per day.

Some have speculated that polar bears might adapt to the longer ice-free seasons due to climate warming by acting like their grizzly bear relatives and either rest or eat terrestrial food.

The polar bears in this study tried versions of both strategies -- with little success.

"Neither strategy will allow polar bears to exist on land beyond a certain amount of time. Even those bears that were foraging lost body weight at the same rate as those that laid down," said Charles Robbins, director of the Washington State University Bear Center and co-author of the study in the journal Nature Communications.

"Polar bears are not grizzly bears wearing white coats. They're very, very different."

Usually larger than grizzly bears, adult male polar bears can reach 10 feet in length and weigh 1,500 pounds compared to grizzly bears' 8 feet and 800 pounds.

To maintain that great mass, polar bears rely on the energy-rich fat of seals, which they best catch on the ice.

Little has been known about polar bear energy expenditure and behavior when confined to land, so researchers used collars with video cameras and GPS to track polar bears summering in the western Hudson Bay region of Manitoba, Canada.

They wanted to see what the specialized ice-hunters ate and did during the extended time on land when their preferred seal prey was out of reach.

The researchers also weighed the bears before and after the observation period and measured their energy expenditures.

"We found a real diversity of bear behaviors, and as a result, we saw a diverse range of energy expenditures," said lead author Anthony Pagano, research wildlife biologist with the U.S. Geological Survey Polar Bear Research Program and former WSU post-doctoral researcher.

Many of the adult male polar bears simply laid down to conserve energy, burning calories at rates similar to hibernation.

Others, actively searched for food, consuming bird and caribou carcasses as well as berries, kelp and grasses.

In all, the researchers found a five-fold range in energy expenditure from an adult male that rested 98% of the time to the most active who clocked 330 kilometers (205 miles). Some adult females spent as much as 40% of their time foraging.

Yet all that activity didn't pay off.

"The terrestrial foods did give them some energetic benefit, but ultimately, the bears had to spend more energy to access those resources," said Pagano.

Three polar bears went for long swims -- one swimming 175 kilometers (about 110 miles) across the bay.

Two found carcasses in the water, a beluga and a seal, but neither bear could feed on their finds while swimming nor bring them back to land.

Only one bear out of the 20 gained weight after stumbling across a dead marine mammal on land.

The study focused on the southern-most extent of polar bear range in the western Hudson Bay, where climate warming is likely impacting the bears at a faster rate than other Arctic regions.

The polar bear population in the area has already declined by an estimated 30% since 1987.

This study indicates that polar bears across the Arctic are at risk of starvation as the ice-free period continues to grow.

"As polar bears are forced on land earlier, it cuts into the period that they normally acquire the majority of the energy they need to survive," said Pagano.

"With increased land use, the expectation is that we'll likely see increases in starvation, particularly with adolescents and females with cubs."

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The entire planet's ecosystems classified

A global cross-disciplinary team of scientists led by UNSW Sydney researchers has developed the first comprehensive classification of the world's ecosystems across land, rivers and wetlands, and seas. The ecosystem typology will enable more coordinated and effective biodiversity conservation, critical for human wellbeing.

The extensive collaboration includes the International Union for Conservation of Nature (IUCN), which comprises about 1400 member organisations, including countries; the IUCN Commission on Ecosystem Management; the PLuS Alliance -- Arizona State University, King's College London and UNSW Sydney; and more than 100 specialist ecosystem scientists around the world.

The study, published today in Nature, explores the science that underpins the typology, as well as how it can help achieve objectives in global policy that flow to individual countries. With UNSW's support, IUCN launched the first public version of the typology in 2020 and, since then, the researchers have refined and updated it.

The research team was led by Professor David Keith with Professor Richard Kingsford from UNSW's Centre for Ecosystem Science, and Professor Emily Nicholson from Deakin University.

"For the first time, we have a common platform that identifies, defines and describes the full suite of the whole planet's ecosystems," said Professor Keith.

"It may seem rather odd that we haven't had this before, but historically scientists have forged advances by working somewhat separately in marine, freshwater and terrestrial ecosystems. This is the first time that all of this detailed knowledge has been brought together into a single framework taking advantage of common theory across the disciplines."

The typology allows us to understand broad global patterns, including the transformation of ecosystems by people. Ten per cent of ecosystems are artificially created and maintained by humans but occupy more than 30 per cent of the Earth's land surface -- what is left is home to 94 per cent of threatened species on the IUCN Red List.

At a policy level, this is the first time we've had this kind of overview, Professor Kingsford said.

"It's very hard to see the big picture on a jigsaw puzzle until you have all the pieces in place -- and that's what we now have. We have a much more substantial foundation to move ahead with a new era of ecosystem conservation and management policy."

At a more general level, the overview allows policymakers and industry to plan their initiatives in full context. For governments and non-government organisations (NGOs) working in a range of countries, the overview can inform decisions about how ecosystem protection and restoration efforts can achieve maximum conservation benefit, and where development infrastructure is best placed to minimise impact.

"Efforts on biodiversity conservation have largely centered at the species level, because it's seen to be more tangible," said Professor Keith. "But a broader focus on both ecosystems and species is more likely to succeed in conserving all plants and animals, as well as the essential services that nature provides people."

Globally, countries coordinate their efforts under the umbrella of the United Nations Convention on Biological Diversity (CBD), which is coming up for renewal at the end of 2022. Delegates from 193 countries will meet in December at the 15th Conference of Parties in Montreal, Canada, to agree on the post-2020 agenda for CBD. Preparations for that meeting indicate a stronger emphasis on ecosystem conservation and management in the coming decades.

"The global ecosystem typology will make it possible to account for ongoing ecosystem change, identify threatened ecosystem types, and plan better preventative action and restoration under a renewed agenda for the CBD," said Professor Nicholson.

This typology marks a breakthrough for sustainable management of the world's ecosystems, said Dr Angela Andrade, Chair of IUCN's Commission on Ecosystem Management and one of the authors.

"It will enable real progress on United Nations Sustainable Development Goals and Environmental Accounting, and should help place ecosystems at the forefront of the United Nations' post-2020 agenda for conserving biological diversity."

To make that a reality, we need a full set of high quality maps for all major ecosystem types, Professor Keith said.

"We are already well down that path, but we need help to surmount the considerable challenges by exploiting recent advances in computer and satellite technology, along with global networks of citizen scientists."

The ecosystem typology

Ecosystems provide homes and vital life support for all plants and animals, and supply essential ecosystem services that sustain business, culture and human wellbeing. Those services -- such as provision of clean air and water, carbon sequestration, reduced risks of disasters and outdoor recreational opportunities that sustain mental health -- are sometimes regarded as free, but ecosystem degradation incurs costs for tapping alternative resources, disaster relief and reconstruction, and to health budgets.

All of the world's ecosystems show hallmarks of human influence, and many are under acute risks of collapse, with consequences for habitats of species, genetic diversity, ecosystem services, sustainable development and human wellbeing.

The global ecosystem typology describes the diversity of tropical forests, big rivers, coral reefs and other ecosystems that have typically been the focus of public attention. But it also includes little-known ecosystems of deep ocean trenches, seamounts, lakes beneath the ice sheets and microscopic ecosystems within rocks.

"We don't think often about what's in the deep oceans, for example," said Professor Keith. "There's a tremendous variety of life down there and it's organised into a number of different ecosystems. And those ecosystems are beginning to feel the impact of human expansion.

"The deep trenches in the ocean are filling up with microplastics, and we're starting to look at mining volcanic vents for minerals. We need to make decisions about those kinds of environments, just as we do about coral reefs and rainforests."

A hierarchical structure

The new typology has a hierarchical structure with six levels. The top level divides the planet into major realms, including terrestrial, freshwater, marine and subterranean ecosystems. The second and third levels include 25 biomes and 110 ecosystem functional groups, based on the ecological processes that shape different ecosystems and the functions that their key components perform. These functional groups will frame blueprints for sustainable ecosystem management.

The lower levels of the hierarchy are based on finer ecosystem features and enable the integration of existing national classifications. These national ecosystem classifications and maps benefit from detailed scientific observations and considerable investment over many years. They are critical to conservation because many countries have built their environmental governance and regulations around them, as well as their protected area networks. For the first time, a globally agreed typology enables these many different systems to be reconciled across national borders, while supporting their ongoing use in each country.

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Land plants changed Earth's composition

Scientists at the University of Southampton have discovered that the evolution of land plants caused a sudden shift in the composition of Earth's continents.

The Southampton researchers, led by Dr Tom Gernon, working with Queen's University Canada, led by Dr Christopher Spencer, and colleagues at the University of Cambridge, the University of Aberdeen, and the China University of Geosciences, Wuhan, studied the effects of land plant evolution on Earth's chemical composition over the past 700 million years.

The researchers' findings are published in the journal Nature Geoscience.

The evolution of land plants took place about 430 million years ago during the Silurian Period, when North America and Europe were conjoined in a landmass called Pangaea.

The proliferation of plants completely transformed Earth's biosphere -- those parts of the planet's surface where life thrives -- paving the way for the advent of dinosaurs about 200 million years later.

"Plants caused fundamental changes to river systems, bringing about more meandering rivers and muddy floodplains, as well as thicker soils," says Dr Christopher Spencer, Assistant Professor at Queen's University in Kingston, Ontario, lead author of the study. "This shift was tied to the development of plant rooting systems that helped produce colossal amounts of mud (by breaking down rocks) and stabilised river channels, which locked up this mud for long periods."

The team recognised that Earth's surface and deep interior are linked by plate tectonics -- rivers flush mud into the oceans, and this mud then gets dragged into the Earth's molten interior (or mantle) at subduction zones where it gets melted to form new rocks.

"When these rocks crystallise, they trap in vestiges of their past history," says Dr Tom Gernon, Associate Professor of Earth Science at the University of Southampton and co-author of the study. "So, we hypothesised that the evolution of plants should dramatically slow down the delivery of mud to the oceans, and that this feature should be preserved in the rock record -- it's that simple."

To test this idea, the team studied a database of over five thousand zircon crystals formed in magmas at subduction zones -- essentially 'time capsules' that preserve vital information on the chemical conditions that prevailed on Earth when they crystallised.

The team uncovered compelling evidence for a dramatic shift in the composition of rocks making up Earth's continents, which coincides almost precisely with the onset of land plants.

Notably, the scientists also found that the chemical characteristics of zircon crystals generated at this time indicate a significant slowing down of sediment transfer to the oceans, just as they had hypothesised.

The researchers show that vegetation changed not only the surface of the Earth, but also the dynamics of melting in Earth's mantle.

"It is amazing to think that the greening of the continents was felt in the deep Earth," concludes Dr Spencer.

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New fossil shows four-legged fishapod that returned to the water while Tiktaalik ventured onto land

A meme has been circulating online during the pandemic featuring Tiktaalik roseae, the iconic, four-legged "fishapod" that first made the transition from water to land 375 million years ago. Most variations show Tiktaalik poking its head out of the water and ready to crawl ashore, while an out of frame hand threatens it with a rolled-up newspaper or a stick. The joke is that those of us exhausted by the modern world wish we could go back in time, shoo it back into the water, and stop evolution in its tracks, sparing ourselves the present day of war, pestilence, and internet memes.

As it turns out, one of Tiktaalik's close relatives did just that, opting to return to living in open water instead of venturing onto land. A new study from the laboratory of Neil Shubin, PhD, who co-discovered Tiktaalik in 2004, describes a fossil species that closely resembles Tiktaalik but has features that made it more suited to life in the water than its adventurous cousin. Qikiqtania wakei was small -- just 30 inches long -- compared to Tiktaalik, which could grow up to nine feet. The new fossil includes partial upper and lower jaws, portions of the neck, and scales. Mostly importantly, it also features a complete pectoral fin with a distinct humerus bone that lacks the ridges that would indicate where muscles and joints would be on a limb geared toward walking on land. Instead, Qikiqtania's upper arm was smooth and curved, more suited for a life paddling underwater. The uniqueness of the arm bones of Qikiqtania suggest that it returned to paddling the water after its ancestors began to use their appendages for walking.

"At first we thought it could be a juvenile Tiktaalik, because it was smaller and maybe some of those processes hadn't developed yet," Shubin said. "But the humerus is smooth and boomerang shaped, and it doesn't have the elements that would support it pushing up on land. It's remarkably different and suggests something new."

The paper, "A New Elpistostegalian from the Late Devonian of the Canadian Arctic and the diversity of stem tetrapods," was published July 20, 2022, in Nature.

A prehistoric pandemic project

Shubin, who is the Robert R. Bensley Distinguished Service Professor of Organismal Biology and Anatomy at the University of Chicago, found the fossildays before Tiktaalik was discovered, at a site about one mile east on southern Ellesmere Island in the territory of Nunavut in northern Arctic Canada. The name Qikiatania comes from the Inuktitut word Qikiqtaaluk or Qikiqtani, the traditional name for the region where the fossil site is located. The species designation wakei is in memory of the late David Wake, an eminent evolutionary biologist from the University of California at Berkeley.

Shubin and his field partner, Ted Daeschler, PhD, from the Academy of Natural Sciences of Drexel University, collected the specimens from a quarry after spotting a few promising looking rocks with distinctive, white scales on the surface. But they sat in storage, mostly unexamined, while the team focused on preparing Tiktaalik.

Fifteen years later, the discovery of Qikiqtania became another pandemic story. Postdoctoral researchers Justin Lemberg, PhD, and Tom Stewart, PhD, CT-scanned one of the larger rock specimens in March 2020 and realized that it contained a pectoral fin. Unfortunately, it was too deep inside the rock to get a high-resolution image, and they couldn't do much more with it once the pandemic forced labs to close.

"We were trying to collect as much CT-data of the material as we could before the lockdown, and the very last piece we scanned was a large, unassuming block with only a few flecks of scales visible from the surface," said Lemberg, who is now doing cultural resource management fieldwork in Southern California. "We could hardly believe it when the first, grainy images of a pectoral fin came into view. We knew we could collect a better scan of the block if we had the time, but that was March 13th, 2020, and the University shut down all non-essential operations the following week."

In the summer of 2020 when campus facilities reopened, they contacted Mark Webster, PhD, Associate Professor of Geophysical Sciences, who had access to a saw that could trim pieces off the specimen so that a CT scanner could get closer and produce a better image. Stewart and Lemberg carefully marked the boundaries on the block and arranged an exchange outside their lab in Culver Hall. The resulting images revealed a nearly complete pectoral fin and upper limb, including the distinctive humerus bone.

"That's what blew our minds," Shubin said. "This was by no means a fascinating block at first, but we realized during the COVID lockdown when we couldn't get in the lab that the original scan wasn't good enough and we needed to trim the block. And when we did, look at what happened. It gave us something exciting to work on during the pandemic. It's a fabulous story."

Glimpses into vertebrate history

Qikiqtania is slightly older than Tiktaalik but not by much. The team's analysis of where it sits on the tree of life places it, like Tiktaalik, adjacent to the earliest creatures known to have finger-like digits. But even though Qikiqtania's distinct pectoral fin was more suited for swimming, it wasn't entirely fish-like either. Its curved paddle shape was a distinct adaptation, different from the jointed, muscled legs or fan-shaped fins we see in tetrapods and fish today.

We tend to think animals evolved in a straight line that connects their prehistoric forms to some living creature today, but Qikiqtania shows that some animals stayed on a different path that ultimately didn't work out. Maybe that's a lesson for those wishing Tiktaalik had stayed in the water with it.

"Tiktaalik is often treated as a transitional animal because it's easy to see the stepwise pattern of changes from life in the water to life on land. But we know that in evolution things aren't always so simple," said Stewart, who will be joining the faculty at Penn State University this summer. "We don't often get glimpses into this part of vertebrate history. Now we're starting to uncover that diversity and to get a sense of the ecology and unique adaptations of these animals. It's more than simple transformation with just a limited number of species."

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How inland and coastal waterways influence climate

"Streams to the river, river to the sea." If only it were that simple.

Most global carbon-budgeting efforts assume a linear flow of water from the land to the sea, which ignores the complex interplay between streams, rivers, lakes, groundwater, estuaries, mangroves and more. A study co-led by climate scientist Laure Resplandy, an assistant professor of geosciences and the High Meadows Environmental Institute (HMEI) at Princeton University, details how carbon is stored and transported through the intricacy of inland and coastal waterways. Published in the current issue of the journal Nature, the work has significant implications for enforcing the carbon calculations that are part of international climate accords.

Terrestrial and marine ecosystems have a powerful influence on climate by regulating the level of atmospheric carbon dioxide (CO2). These ecosystems, however, are often viewed as disconnected from each other, which ignores the transfer of carbon from land to the open ocean through a complex network of water bodies -- the continuum of streams, rivers, estuaries and other bodies carrying water from land to the sea.

In a detailed analysis, the team of researchers from Belgium, the United States and France discovered that this land-to-ocean aquatic continuum (LOAC) carries a substantial amount of carbon of anthropogenic (e.g., fossil-fuel) origin. Thus, the carbon removed from the atmosphere by terrestrial ecosystems is not all stored locally, as is commonly assumed, which has implications for global agreements that require countries to report their carbon inventories. The researchers also found that the land-to-ocean carbon transfer of natural origin was larger than previously thought, with far-reaching implications for the assessment of the anthropogenic CO2 uptake by the ocean and the land.

"The complexity of the LOAC, which includes rivers, groundwater, lakes, reservoirs, estuaries, tidal marshes, mangroves, seagrasses, and waters above continental shelves, has made it challenging to assess its influence on the global carbon cycle," said Pierre Regnier, a professor at the University of Brussels who co-led the study with Resplandy.

Because of that complexity, important global carbon-budgeting efforts, such as those of the U.N. Intergovernmental Panel on Climate Change and the Global Carbon Project, typically assume a direct "pipeline" transfer of carbon from river mouths to the open ocean. Another common assumption is that all the transported carbon is natural, neglecting the impacts of human perturbations on this aquatic continuum, such as damming and the decimation of coastal vegetation.

In this study, the researchers synthesized more than 100 individual studies of the various components of the continuum. From this synthesis, LOAC carbon budgets were developed for two time periods: the pre-industrial period and the present day. Their results confirm the well-known pre-industrial carbon "loop" in which carbon is taken up from the atmosphere by terrestrial ecosystems, transferred by rivers to the ocean, and then outgassed back to the atmosphere.

"We find the amount of carbon carried by this natural land-to-ocean loop, 0.65 billion tons per year, is roughly 50% greater than previously thought," Resplandy said.

Furthermore, this loop is comprised of two smaller loops, one that transfers carbon from terrestrial ecosystems to inland waters and another from coastal vegetation (so-called "blue carbon ecosystems") to the open ocean.

"A larger pre-industrial land-to-ocean carbon transport implies that the ocean uptake of anthropogenic CO2 previously inferred from observations was underestimated," Resplandy said.

"The flip side is that the land uptake of anthropogenic CO2 was overestimated," added Regnier.

The study demonstrates that anthropogenic carbon carried by rivers is either outgassed back to the atmosphere or eventually stored in aquatic sediments and the open ocean.

Philippe Ciais, a research director at the Laboratoire des Sciences du Climat et de l'Environnement and a co-author of the study explained: "This new view of the anthropogenic CO2 budget may have a silver lining because sediments and the ocean offer arguably more stable repositories than terrestrial biomass and soil carbon, which are vulnerable to droughts, fires and land-use change."

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