Clouds from a HiFLOR global model simulation, showing three hurricanes in the Atlantic and one in West Pacific (Figure Remik Ziemlinski)
New NOAA research that looks at the devastating 2017 Atlantic hurricane season projects that if similar weather conditions occur in the future, it’s likely that the number of major hurricanes (category 3 and higher) would increase by two in a similar active year at the end of century.
This increase would be driven by predicted climate warming, according to the research appearing today in Science.
Last year’s six major Atlantic hurricanes included landfalling hurricanes Harvey, Irma and Maria that unleashed destruction on and caused loss of life in communities across Texas, Florida and Puerto Rico. The three hurricanes caused an estimated $265 billion in damages during a year that shattered all records for U.S. economic losses due to severe weather.
Global model identifies climate influence
Hiroyuki Murakami. Photo by Maria Setzer
Using a high-resolution global climate model called HiFLOR, developed at GFDL, scientists were able to accurately predict the active hurricane season in June 2017. Scientists then conducted additional experiments with HiFLOR that found it was a remarkably warm tropical Atlantic Ocean, relative to the global tropical ocean, which was the main driver of 2017 hurricane activity.
“This new method allows us to predict hurricane activity as the season is happening as well as take into consideration the likely contribution of climate warming,” said Murakami, a climate researcher at GFDL. NOAA scientists use ocean temperature data showing the relative warmth of the tropical Atlantic to help create vital hurricane season outlooks.
In addition to Murakami, the research team included Emma Levin of Paul D. Schreiber High School in Port Washington, NY, Thomas Delworth and Richard Gudgel of GFDL, and Pang-Chi Hsu of Nanjing University of Information Science and Technology, China.
The study, “Dominant Effect of Relative Tropical Atlantic Warming on Major Hurricane Occurrence,” by H. Murakami, E. Levin, T. L. Delworth, R. Gudgel and P-C Hsu, was published in the journal Science on Sept. 27, 2018.
Funding for this study was provided by the National Oceanic and Atmospheric Administration (NOAA).
The 2015 Paris climate agreement sought to stabilize global temperatures by limiting warming to well below 2.0 degrees Celsius above pre-industrial levels and to pursue limiting warming even further, to 1.5 C.
To quantify what that would mean for people living in coastal areas, a group of researchers employed a global network of tide gauges and a local sea level projection framework to explore differences in the frequency of storm surges and other extreme sea-level events across three scenarios: global temperature increases of 1.5, 2.0 and 2.5 C.
DJ Rasmussen. Photo courtesy of DJ Rasmussen
They concluded that by 2150, the seemingly small difference between an increase of 1.5 and 2.0 C would mean the permanent inundation of lands currently home to about 5 million people, including 60,000 who live on small island nations.
The study, conducted by researchers at Princeton University and colleagues at Rutgers and Tufts Universities, the independent scientific organization Climate Central, and ICF International, was published in the journal Environmental Research Letters on March 15, 2018.
“People think the Paris Agreement is going to save us from harm from climate change, but we show that even under the best-case climate policy being considered today, many places will still have to deal with rising seas and more frequent coastal floods,” said D.J Rasmussen, a graduate student in Princeton’s Program in Science, Technology and Environmental Policy in the Woodrow Wilson School of Public and International Affairs, and first author of the study.
The researchers found that higher temperatures will make extreme sea level events much more common. They used long-term hourly tide gauge records and extreme value theory to estimate present and future return periods of extreme sea-level events through the 22nd century. Under the 1.5 C scenario, the frequency of extreme sea level events is still expected to increase. For example, by the end of the 21st century, New York City is expected to experience one Hurricane Sandy-like flood event every five years.
The study found that New York City can expect two Hurricane Sandy-like floods per decade by the year 2100 if global temperatures rise 1.5 degrees Celsius above pre-industrial levels, and three such events per decade if temperatures rise to 2.0 and 2.5 C.
Extreme sea levels can arise from high tides or storm surge or a combination of surge and tide (sometimes called the storm tide). When driven by hurricanes or other large storms, extreme sea levels flood coastal areas, threatening life and property. Rising mean sea levels are already magnifying the frequency and severity of extreme sea levels, and experts predict that by the end of the century, coastal flooding may be among the costliest impacts of climate change in some regions.
Future extreme events will be exacerbated by the rising global sea level, which in turn depends on the trajectory of global mean surface temperature. Even if global temperatures are stabilized, sea levels are expected to continue to rise for centuries, due to the fact that carbon dioxide stays in the atmosphere for a long time and the ice sheets are slow to respond to warming.
Overall, the researchers predicted that by the end of the century, a 1.5 C temperature increase could drive the global mean sea level up by roughly 1.6 feet (48 cm) while a 2.0 C increase will raise oceans by about 1.8 feet (56 cm) and a 2.5 C increase will raise sea level by an estimated 1.9 feet (58 cm).
The research team included Klaus Bittermann, a postdoctoral researcher at Tufts University who is associated with the Potsdam Institute for Climate Impact Research in Germany; Maya Buchanan, who earned a doctorate in 2017 from the Woodrow Wilson School of Public and International Affairs and is now at ICF International; Scott Kulp, a senior computational scientist and senior developer for the Program on Sea Level Rise at Climate Central, an independent organization of scientists and journalists located in Princeton, NJ; Benjamin Strauss, who received a Ph.D. in 2007 from Princeton’s Department of Ecology and Evolutionary Biology and is chief scientist at Climate Central; and Robert Kopp, a professor of Earth and Planetary Sciences at Rutgers University.
The senior author on the study was Michael Oppenheimer, the Albert G. Milbank Professor of Geosciences and International Affairs and the Princeton Environmental Institute. Kopp developed the sea level projection framework, and Buchanan, Kopp, and Oppenheimer developed the flood projection framework. Researchers at Climate Central conducted the population inundation analysis.
Drifting dust burying farm abandoned farm equipment in 1935. Image courtesy of NOAA.
By Pooja Makhijani for the Office of Communications
Could the storms that once engulfed the Great Plains in clouds of black dust in the 1930’s once again wreak havoc in the U.S.? A new statistical model developed by researchers at Princeton University and the National Oceanic and Atmospheric Administration (NOAA)’s Geophysical Fluid Dynamics Laboratory (GFDL) predicts that climate change will amplify dust activity in parts of the U.S. in the latter half of the 21st century, which may lead to the increased frequency of spectacular dust storms that have far-reaching impacts on public health and infrastructure.
The model, detailed in a study published July 17 in the journal Scientific Reports, eliminates some of the uncertainty found in previous dust activity models by using present-day satellite data such as dust optical depth, which measures to what extent dust particles block sunlight, as well as leafy green coverage over land and other factors.
“Few existing climate models have captured the magnitude and variability of dust across North America,” said Bing Pu, the study’s lead author and an associate research scholar in the Program in Atmospheric and Oceanic Sciences (AOS), a collaboration between Princeton and GFDL.
Dust storms happen when wind blows soil particles into the atmosphere. Dust storms are most frequent and destructive in arid climates with loose soil — especially on lands affected by drought and deforestation. Certain regions of the U.S., such as the southwestern deserts and the central plains, are dust-prone. Most importantly, existing climate models predict “unprecedented” dry conditions in the late-21st century due to an increase in greenhouse gases in these very areas.
It is this “perfect storm” of geography and predicted drought and drought-like conditions that led Pu and her colleague Paul Ginoux, a physical scientist at GFDL, to examine the influence of climate change on dust. They analyzed satellite data about the frequency of dust events and the land’s leafy green coverage over the contiguous U.S., as well as precipitation and surface wind speed, and reported that climate change will increase dust activity in the southern Great Plains from spring to fall in the late half of the 21st century due to reduced rainfall, increased land surface bareness and increased surface wind speed. Conversely, they predicted reduced dust activity in the northern Great Plains in spring during the same time period due to increased precipitation and increased surface vegetation.
Although it is still unclear if rising temperatures themselves trigger the release of yet more dust into the atmosphere, this research offers a glimpse of what the future might hold. “This is an early attempt to project future changes in dust activity in parts of the United States caused by increasing greenhouse gases,” Pu said. Nonetheless, these findings are important given the huge economic and health consequences of severe dust storms, as they can disrupt public transportation systems and trigger respiratory disease epidemics. “Our specific projections may provide an early warning on erosion control, and help improve risk management and resource planning,” she said.
The paper, “Projection of American dustiness in the late 21st century due to climate change,” was published July 17, 2017 in the journal Scientific Reports (doi 10.1038/s41598-017-05431-9 ) and is available online.
This research was supported by NOAA, Princeton University’s Cooperative Institute for Climate Science, and NASA grantNNH14ZDA001N-ACMAP.
Deep sea corals reveal that efficient nutrient consumption by plankton drove carbon sequestration in the deep ocean during the ice ages. Photo courtesy of Caltech.
By Robert Perkins, Caltech
We know a lot about how carbon dioxide (CO2) levels can drive climate change, but how about the way that climate change can cause fluctuations in CO2 levels? New research from an international team of scientists reveals one of the mechanisms by which a colder climate was accompanied by depleted atmospheric CO2 during past ice ages.
The overall goal of the work is to better understand how and why the earth goes through periodic climate change, which could shed light on how man-made factors could affect the global climate.
Now, an international team of scientists has shown that periods of colder climates are associated with higher phytoplankton efficiency and a reduction in nutrients in the surface of the Southern Ocean (the ocean surrounding the Antarctic), which is related to an increase in carbon sequestration in the deep ocean. A paper about their research appears this week in the online edition of the Proceedings of the National Academy of Sciences.
“It is critical to understand why atmospheric CO2 concentration was lower during the ice ages. This will help us understand how the ocean will respond to ongoing anthropogenic CO2 emissions,” says Xingchen (Tony) Wang, lead author of the study. Wang was a graduate student at Princeton University while conducting the research in the lab of Daniel Sigman, the Dusenbury Professor of Geological and Geophysical Sciences. Wang is now a Simons Foundation Postdoctoral Fellow on the Origins of Life at Caltech. The study used a library of 10,000 deep-sea corals collected by Caltech’s Jess Adkins.
Xingchen (Tony) Wang and Jess Adkins. Photo courtesy of Caltech
Earth’s average temperature has naturally fluctuated by about 4 to 5 degrees Celsius over the course of the past million years as the planet has cycled in and out of glacial periods. During that time, the earth’s atmospheric CO2 levels have fluctuated between roughly 180 and 280 parts per million (ppm) every 100,000 years or so. (In recent years, man-made carbon emissions have boosted that concentration up to over 400 ppm.)
About 10 years ago, researchers noticed a close correspondence between the fluctuations in CO2 levels and in temperature over the last million years. When the earth is at its coldest, the amount of CO2 in the atmosphere is also at its lowest. During the most recent ice age, which ended about 11,000 years ago, global temperatures were 5 degrees Celsius lower than they are today, and atmospheric CO2 concentrations were at 180 ppm.
There is 60 times more carbon in the ocean than in the atmosphere—partly because the ocean is so big. The mass of the world’s oceans is roughly 270 times greater than that of the atmosphere. As such, the ocean is the greatest regulator of carbon in the atmosphere, acting as both a sink and a source for atmospheric CO2.
Biological processes are the main driver of CO2 absorption from the atmosphere to the ocean. Just like photosynthesizing trees and plants on land, plankton at the surface of the sea turn CO2 into sugars that are eventually consumed by other creatures. As the sea creatures who consume those sugars—and the carbon they contain—die, they sink to the deep ocean, where the carbon is locked away from the atmosphere for a long time. This process is called the “biological pump.”
A healthy population of phytoplankton helps lock away carbon from the atmosphere. In order to thrive, phytoplankton need nutrients—notably, nitrogen, phosphorus, and iron. In most parts of the modern ocean, phytoplankton deplete all of the available nutrients in the surface ocean, and the biological pump operates at maximum efficiency.
However, in the modern Southern Ocean, there is a limited amount of iron—which means that there are not enough phytoplankton to fully consume the nitrogen and phosphorus in the surface waters. When there is less living biomass, there is also less that can die and sink to the bottom—which results in a decrease in carbon sequestration. The biological pump is not currently operating as efficiently as it theoretically could.
To track the efficiency of the biological pump over the span of the past 40,000 years, Adkins and his colleagues collected more than 10,000 fossils of the coral Desmophyllum dianthus.
Why coral? Two reasons: first, as it grows, coral accretes a skeleton around itself, precipitating calcium carbonate (CaCO3) and other trace elements (including nitrogen) out of the water around it. That process creates a rocky record of the chemistry of the ocean. Second, coral can be precisely dated using a combination of radiocarbon and uranium dating.
“Finding a few centimeter-tall fossil corals 2,000 meters deep in the ocean is no trivial task,” says Adkins, the Smits Family Professor of Geochemistry and Global Environmental Science at Caltech.
Adkins and his colleagues collected coral from the relatively narrow (500-mile) gap known as the Drake Passage between South America and Antarctica (among other places). Because the Southern Ocean flows around Antarctica, all of its waters funnel through that gap—making the samples Adkins collected a robust record of the water throughout the Southern Ocean.
Coauthors include scientists from Caltech, Princeton University, Pomona College, the Max Planck Institute for Chemistry in Germany, University of Bristol, and ETH Zurich in Switzerland.
Wang analyzed the ratios of two isotopes of nitrogen atoms in these corals – nitrogen-14 (14N, the most common variety of the atom, with seven protons and seven neutrons in its nucleus) and nitrogen-15 (15N, which has an extra neutron). When phytoplankton consume nitrogen, they prefer 14N to 15N. As a result, there is a correlation between the ratio of nitrogen isotopes in sinking organic matter (which the corals then eat as it falls to the seafloor) and how much nitrogen is being consumed in the surface ocean—and, by extension, the efficiency of the biological pump.
A higher amount of 15N in the fossils indicates that the biological pump was operating more efficiently at that time. An analogy would be monitoring what a person eats in their home. If they are eating more of their less-liked foods, then one could assume that the amount of food in their pantry is running low.
Indeed, Wang found that higher amounts of 15N were present in fossils corresponding to the last ice age, indicating that the biological pump was operating more efficiently during that time. As such, the evidence suggests that colder climates allow more biomass to grow in the surface Southern Ocean—likely because colder climates experience stronger winds, which can blow more iron into the Southern Ocean from the continents. That biomass consumes carbon, then dies and sinks, locking it away from the atmosphere.
Adkins and his colleagues plan to continue probing the coral library for further details about the cycles of ocean chemistry changes over the past several hundred thousand years.
The research was funded by the National Science Foundation, Princeton University, the European Research Council, and the Natural Environment Research Council.
The study, “Deep-sea coral evidence for lower Southern Ocean surface nitrate concentrations during the last ice age,” Xingchen Tony Wang, Daniel M. Sigman, Maria G. Prokopenko, Jess F. Adkins, Laura F. Robinson, Sophia K. Hines, Junyi Chai, Anja S. Studer, Alfredo Martínez-García, Tianyu Chen, and Gerald H. Haug, was published in the journal Proceedings of the National Academy of Sciences early edition the week of March 13, 2017. doi: 10.1073/pnas.1615718114
Researchers found a clear difference in the rate of global surface warming (left panel) and the rate of subtropical rainfall decline (indicated by the brown shading in the right panel) when forced with an instantaneous increase of CO2. (Credit: Jie He, Ph.D., Princeton University and Brian J. Soden, Ph.D., University of Miami Rosenstiel School of Marine and Atmospheric Science)
By Diana Udel, University of Miami
Terrestrial rainfall in the subtropics — including the southeastern United States — may not decline in response to increased greenhouse gases as much as it could over oceans, according to a study from Princeton University and the University of Miami (UM). The study challenges previous projections of how dry subtropical regions could become in the future, and it suggests that the impact of decreased rainfall on people living in these regions could be less severe than initially thought.
“The lack of rainfall decline over subtropical land is caused by the fact that land will warm much faster than the ocean in the future — a mechanism that has been overlooked in previous studies about subtropical precipitation change,” said first author Jie He, a postdoctoral research associate in Princeton’s Program in Atmospheric and Oceanic Sciences who works at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory located on Princeton’s Forrestal Campus.
In the new study, published in the journal Nature Climate Change, He and co-author Brian Soden, a UM professor of atmospheric sciences, used an ensemble of climate models to show that rainfall decreases occur faster than global warming, and therefore another mechanism must be at play. They found that direct heating from increasing greenhouse gases is causing the land to warm faster than the ocean. The associated changes in atmospheric circulation are thus driving rainfall decline over the oceans rather than land.
Subtropical rainfall changes have been previously attributed to two mechanisms related to global warming: greater moisture content in air that is transported away from the subtropics, and a pole-ward shift in air circulation. While both mechanisms are present, this study shows that neither one is responsible for a decline in rainfall.
“It has been long accepted that climate models project a large-scale rainfall decline in the future over the subtropics. Since most of the subtropical regions are already suffering from rainfall scarcity, the possibility of future rainfall decline is of great concern,” Soden said. “However, most of this decline occurs over subtropical oceans, not land, due to changes in the atmospheric circulation induced by the more rapid warming of land than ocean.”
Most of the reduction in subtropical rainfall occurs instantaneously with an increase of greenhouse gases, independent of the warming of the Earth’s surface, which occurs much more slowly. According to the authors, this indicates that emission reductions would immediately mitigate subtropical rainfall decline, even though the surface will continue to warm for a long time.
He is supported by the Visiting Scientist Program at the department of Atmospheric and Oceanic Science, Princeton University.
Princeton University researchers used ice cores collected in Greenland (pictured here) and Antarctica to study 800,000 years of atmospheric oxygen. Image source: Stolper, et al.
By Morgan Kelly, Office of Communications
Princeton University researchers have compiled 30 years of data to construct the first ice core-based record of atmospheric oxygen concentrations spanning the past 800,000 years, according to a paper published today in the journal Science.
The record shows that atmospheric oxygen has declined 0.7 percent relative to current atmospheric-oxygen concentrations, a reasonable pace by geological standards, the researchers said. During the past 100 years, however, atmospheric oxygen has declined by a comparatively speedy 0.1 percent because of the burning of fossil fuels, which consumes oxygen and produces carbon dioxide.
Curiously, the decline in atmospheric oxygen over the past 800,000 years was not accompanied by any significant increase in the average amount of carbon dioxide in the atmosphere, though carbon dioxide concentrations do vary over individual ice age cycles. To explain this apparent paradox, the researchers called upon a theory for how the global carbon cycle, atmospheric carbon dioxide and Earth’s temperature are linked on geologic timescales.
“The planet has various processes that can keep carbon dioxide levels in check,” said first author Daniel Stolper, a postdoctoral research associate in Princeton’s Department of Geosciences. The researchers discuss a process known as silicate weathering in particular, wherein carbon dioxide reacts with exposed rock to produce, eventually, calcium carbonate minerals, which trap carbon dioxide in a solid form. As temperatures rise due to higher carbon dioxide in the atmosphere, silicate-weathering rates are hypothesized to increase and remove carbon dioxide from the atmosphere faster.
Researchers at Princeton University analyzed ice cores collected in Greenland (pictured here) and Antarctica to determine levels of atmospheric oxygen over the last 800,000 years. (Image: Stolper, et al.)
Stolper and his co-authors suggest that the extra carbon dioxide emitted due to declining oxygen concentrations in the atmosphere stimulated silicate weathering, which stabilized carbon dioxide but allowed oxygen to continue to decline.
“The oxygen record is telling us there’s also a change in the amount of carbon dioxide [that was created when oxygen was removed] entering the atmosphere and ocean,” said co-author John Higgins, Princeton assistant professor of geosciences. “However, atmospheric carbon dioxide levels aren’t changing because the Earth has had time to respond via increased silicate-weathering rates.
“The Earth can take care of extra carbon dioxide when it has hundreds of thousands or millions of years to get its act together. In contrast, humankind is releasing carbon dioxide today so quickly that silicate weathering can’t possibly respond fast enough,” Higgins continued. “The Earth has these long processes that humankind has short-circuited.”
The researchers built their history of atmospheric oxygen using measured ratios of oxygen-to-nitrogen found in air trapped in Antarctic ice. This method was established by co-author Michael Bender, professor of geosciences, emeritus, at Princeton.
Because oxygen is critical to many forms of life and geochemical processes, numerous models and indirect proxies for the oxygen content in the atmosphere have been developed over the years, but there was no consensus on whether oxygen concentrations were rising, falling or flat during the past million years (and before fossil fuel burning). The Princeton team analyzed the ice-core data to create a single account of how atmospheric oxygen has changed during the past 800,000 years.
“This record represents an important benchmark for the study of the history of atmospheric oxygen,” Higgins said. “Understanding the history of oxygen in Earth’s atmosphere is intimately connected to understanding the evolution of complex life. It’s one of these big, fundamental ongoing questions in Earth science.”
Daniel A. Stolper, Michael L. Bender, Gabrielle B. Dreyfus, Yuzhen Yan, and John A. Higgins. 2016. A Pleistocene ice core record of atmospheric oxygen concentrations. Science. Article published Sept. 22, 2016. DOI: 10.1126/science.aaf5445
The work was supported by a National Oceanic and Atmospheric Administration Climate and Global Change postdoctoral fellowship, and the National Science Foundation (grant no. ANT-1443263).
Amazonian tropical rainforest near Manaus, Brazil. Photo courtesy of William Anderegg, Princeton University.
By Morgan Kelly, Office of Communications
The warming effects of climate change usually conjure up ideas of parched and barren landscapes broiling under a blazing sun, its heat amplified by greenhouse gases. But a study led by Princeton University researchers suggests that hotter nights may actually wield much greater influence over the planet’s atmosphere as global temperatures rise — and could eventually lead to more carbon flooding the atmosphere.
Since measurements began in 1959, nighttime temperatures in the tropics have had a strong influence over year-to-year shifts in the land’s carbon-storage capacity, or “sink,” the researchers report in the journal Proceedings of the National Academy of Sciences. Earth’s ecosystems absorb about 25% of the excess carbon from the atmosphere, and tropical forests account for about one-third of land-based plant productivity.
During the past 50 years, the land-based carbon sink’s “interannual variability” has grown by 50 to 100 percent, the researchers found. The researchers used climate- and satellite-imaging data to determine which of various climate factors — including rainfall, drought and daytime temperatures — had the most effect on the carbon sink’s swings. They found the strongest association with variations in tropical nighttime temperatures, which have risen by about 0.6 degrees Celsius since 1959.
First author William Anderegg, an associate research scholar in the Princeton Environmental Institute, explained that he and his colleagues determined that warm nighttime temperatures lead plants to put more carbon into the atmosphere through a process known as respiration.
Just as people are more active on warm nights, so too are plants. Although plants take up carbon dioxide from the atmosphere, they also internally consume sugars to stay alive. That process, known as respiration, produces carbon dioxide. Plants step up respiration in warm weather, Anderegg said. The researchers found that yearly variations in the carbon sink strongly correlated with variations in plant respiration.
“When you heat up a system, biological processes tend to increase,” Anderegg said. “At hotter temperatures, plant respiration rates go up and this is what’s happening during hot nights. Plants lose a lot more carbon than they would during cooler nights.”
Previous research has shown that nighttime temperatures have risen significantly faster as a result of climate change than daytime temperatures, Anderegg said. This means that in future climate scenarios respiration rates could increase to the point that the land is putting more carbon into the atmosphere than it’s taking out, “which would be disastrous,” he said.
Of course, plants consume carbon dioxide as a part of photosynthesis, during which they convert sunlight into energy. Photosynthesis also is sensitive to rises in temperature, but it occurs only during the day, whereas respiration occurs at all hours and thus is more sensitive to nighttime warming, Anderegg said.
“Nighttime temperatures have been increasing faster than daytime temperatures and will continue to rise faster,” Anderegg said. “This suggests that tropical ecosystems might be more vulnerable to climate change than previously thought, risking crossing the threshold from a carbon sink to a carbon source. But there’s certainly potential for plants to acclimate their respiration rates and that’s an area that needs future study.”
This research was supported by the National Science Foundation MacroSystems Biology Grant (EF-1340270), RAPID Grant (DEB-1249256) and EAGER Grant (1550932); and a National Oceanic and Atmospheric Administration (NOAA) Climate and Global Change postdoctoral fellowship administered by the University Corporation of Atmospheric Research.
William R. L. Anderegg, Ashley P. Ballantyne, W. Kolby Smith, Joseph Majkut, Sam Rabin, Claudie Beaulieu, Richard Birdsey, John P. Dunne, Richard A. Houghton, Ranga B. Myneni, Yude Pan, Jorge L. Sarmiento, Nathan Serota, Elena Shevliakova, Pieter Tan and Stephen W. Pacala. “Tropical nighttime warming as a dominant driver of variability in the terrestrial carbon sink.” Proceedings of the National Academy of Sciences, published online in-advance of print Dec. 7, 2015. DOI: 10.1073/pnas.1521479112.
More rain on African savanna leads to fewer trees, a Princeton study found. (Credit PEI)
by Angela Page for the Princeton Environmental Institute
In 2011, an influx of remote sensing data from satellites scanning the African savannas revealed a mystery: these rolling grasslands, with their heavy rainfalls and spells of drought, were home to significantly fewer trees than researchers had previously expected given the biome’s high annual precipitation. In fact, the 2011 study found that the more instances of heavy rainfall a savanna received, the fewer trees it had.
This paradox may finally have a solution due to new work from Princeton University recently published in the Proceeding of the National Academy of Sciences. In the study, researchers use mathematical equations to show that physiological differences between trees and grasses are enough to explain the curious phenomenon.
“A simple way to view this is to think of rainfall as annual income,” said Xiangtao Xu, a doctoral candidate in David Medvigy’s lab and first author on the paper. “Trees and grasses are competing over the amount of money the savanna gets every year and it matters how they use their funds.” Xu explained that when the bank is full and there is a lot of rain, the grasses, which build relatively cheap structures, thrive. When there is a deficit, the trees suffer less than grasses and therefore win out.
To establish these findings, Xu and his Princeton collaborators Medvigy, assistant professor in geosciences, and Ignacio Rodriguez-Iturbe, professor of civil and environmental engineering, created a numerical model that mimics the actual mechanistic functions of the trees and grasses. “We put in equations for how they photosynthesize, how they absorb water, how they steal water from each other—and then we coupled it all with a stochastic rainfall generator,” said Xu.
Whereas former analyses only considered total annual or monthly rainfall, understanding how rainfall is distributed across the days is critical here, Xu said, because it determines who will win in a competition between grasses and trees for the finite resource of water availability.
The stochastic rainfall generator draws on rainfall parameters derived from station observations across the savanna. By coupling it with the mechanistic equations describing how the trees and grasses function, the team was able to observe how the plants would respond under different local climate conditions.
The research team found that under very wet conditions, grasses have an advantage because they can quickly absorb water and support high photosynthesis rates. Trees, with their tougher leaves and roots, are able to survive better in dry periods because of their ability to withstand water stress. But this amounts to a disadvantage for trees in periods of intense rainfall, as they are comparatively less effective at utilizing the newly abundant water.
“We put realistic rainfall schemes into the model, then generated corresponding grass or tree abundance, and compared the numerical results with real-world observations,” Xu said. If the model looked like the real-world data, then they could say it offered a viable explanation for the unexpected phenomenon, which is not supported by traditional models—and that is exactly what they found. They tested the model using both field measurements from a well-studied savanna in Nylsvley, South Africa and nine other sites along the Kalahari Transect, as well as remote sensing data across the whole continent. With each site, the model accurately predicted observed tree abundances in those locations.
The work rejects the long held theory of root niche separation, which predicts that trees will outcompete grasses under intense rainfall when the soil becomes saturated, because their heavy roots penetrate deeper into the ground. “But this ignores the fact that grasses and trees have different abilities for absorbing and utilizing water,” Xu said. “And that’s one of the most important parts of what we found. Grasses are more efficient at absorbing water, so in a big rainfall event, grasses win.”
“Models are developed to understand and predict the past and present state — they offer a perspective on future states given the shift in climatic conditions,” said Gaby Katul, a Professor of Hydrology and Micrometeorology in the Nicholas School of the Environment at Duke University, who was not involved in the research. “This work offers evidence of how shifts in rainfall affect the tree-grass interaction because rainfall variations are large. The approach can be used not only to ‘diagnose’ the present state where rainfall pattern variations dominate but also offers a ‘prognosis’ as to what may happen in the future.”
Several high profile papers over the last decade predict that periods of intense rainfall like those described in the paper will become more frequent around the globe, especially in tropical areas, Xu said. His work suggests that these global climate changes will eventually lead to diminished tree abundance on the savannas.
“Because the savanna takes up a large area, which is home to an abundance of both wild animals and livestock, this will influence many people who live in those areas,” Xu said. “It’s important to understand how the biome would change under global climate change.”
Furthermore, the study highlights the importance of understanding the structure and pattern of rainfall, not just the total annual precipitation—which is where most research in this area has traditionally focused. Fifty years from now, a region may still experience the same overall depth of precipitation, but if the intensity has changed, that will induce changes to the abundance of grasses and trees. This, in turn, will influence the herbivores that subsist on them, and other animals in the biome — essentially, affecting the entire complex ecosystem.
Xu said it would be difficult to predict whether such changes would have positive or negative impacts. But he did say that more grasses mean more support for cows and horses and other herbivores. On the other hand, fewer trees mean less CO2 is captured out of the atmosphere, as well as diminished habitat for birds and other animals that rely on the trees for survival.
What the model does offer is an entry point for better policies and decisions to help communities adapt to future changes. “It’s just like with the weather,” Xu said. “If you don’t read the weather report, you have to take what nature gives you. But if you know in advance that it will rain tomorrow, you know to bring an umbrella.”
Xiangtao Xua, David Medvigy, and Ignacio Rodriguez-Iturbe. Relation between rainfall intensity and savanna tree abundance explained by water use strategies. Published online September 29, 2015, doi: 10.1073/pnas.1517382112. PNAS October 5, 2015.
By John Sullivan, School of Engineering and Applied Science
Toward the end of this century (project here for the years 2068 to 2098) the possibility of storm surges of eight to 11 meters (26 to 36 feet) increases significantly in cities not usually expected to be vulnerable to tropical storms, such as Tampa, Florida, according to recent research in the journal Nature Climate Change.
Researchers at Princeton and MIT have used computer models to show that severe tropical cyclones could hit a number of coastal cities worldwide that are widely seen as unthreatened by such powerful storms.
The researchers call these potentially devastating storms Gray Swans in comparison with the term Black Swan, which has come to mean truly unpredicted events that have a major impact. Gray Swans are highly unlikely, the researchers said, but they can be predicted with a degree of confidence.
“We are considering extreme cases,” said Ning Lin, an assistant professor of civil and environmental engineering at Princeton. “These are relevant for policy making and planning, especially for critical infrastructure and nuclear power plants.”
In an article published Aug. 31 in Nature Climate Change, Lin and her coauthor Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology, examined potential storm hazards for three cities: Tampa, Fla.; Cairns, Australia; and Dubai, United Arab Emirates.
The researchers concluded that powerful storms could generate dangerous storm surge waters in all three cities. They estimated the levels of devastating storm surges occurring in these cities with odds of 1 in 10,000 in an average year, under current climate conditions.
Tampa Bay, for example, has experienced very few extremely damaging hurricanes in its history, the researchers said. The city, which lies on the central-west coast of Florida, was hit by major hurricanes in 1848 and in 1921.
The researchers entered Tampa Bay area climate data recorded between 1980 and 2005 into their model and ran 7,000 simulated hurricanes in the area. They concluded that, although unlikely, a Gray Swan storm could bring surges of up to roughly six meters (18 feet) to the Tampa Bay area. That level of storm surge could dwarf those of the storms of 1848 and 1921, which reached about 4.6 meters and 3.5 meters respectively.
The researchers said their model also indicates that the probability of such storms will increase as the climate changes.
“With climate change, these probabilities can increase significantly over the 21st century,” the researchers said. In Tampa, the current storm surge likelihood of 1 in 10,000 is projected to increase to between 1 in 3,000 and 1 in 1,100 by mid-century and between 1 in 2,500 and 1 in 700 by the end of the century.
The work was supported in part by Princeton’s Project X Fund, the Andlinger Center for Energy and the Environment’s Innovation Fund, and the National Science Foundation.
McGill Arctic Research Station during late-spring at Expedition Fjord, Axel Heiberg Island, Nunavut, Canada. (Photo by Nadia Mykytczuk, Laurentian University)
By Morgan Kelly, Office of Communications
In addition to melting icecaps and imperiled wildlife, a significant concern among scientists is that higher Arctic temperatures brought about by climate change could result in the release of massive amounts of carbon locked in the region’s frozen soil in the form of carbon dioxide and methane. Arctic permafrost is estimated to contain about a trillion tons of carbon, which would potentially accelerate global warming. Carbon emissions in the form of methane have been of particular concern because on a 100-year scale methane is about 25-times more potent than carbon dioxide at trapping heat.
However, new research led by Princeton University researchers and published in The ISME Journal in August suggests that, thanks to methane-hungry bacteria, the majority of Arctic soil might actually be able to absorb methane from the atmosphere rather than release it. Furthermore, that ability seems to become greater as temperatures rise.
The researchers found that Arctic soils containing low carbon content — which make up 87 percent of the soil in permafrost regions globally — not only remove methane from the atmosphere, but also become more efficient as temperatures increase. During a three-year period, a carbon-poor site on Axel Heiberg Island in Canada’s Arctic region consistently took up more methane as the ground temperature rose from 0 to 18 degrees Celsius (32 to 64.4 degrees Fahrenheit). The researchers project that should Arctic temperatures rise by 5 to 15 degrees Celsius over the next 100 years, the methane-absorbing capacity of “carbon-poor” soil could increase by five to 30 times.
The researchers found that this ability stems from an as-yet unknown species of bacteria in carbon-poor Arctic soil that consume methane in the atmosphere. The bacteria are related to a bacterial group known as Upland Soil Cluster Alpha, the dominant methane-consuming bacteria in carbon-poor Arctic soil. The bacteria the researchers studied remove the carbon from methane to produce methanol, a simple alcohol the bacteria process immediately. The carbon is used for growth or respiration, meaning that it either remains in bacterial cells or is released as carbon dioxide.
First author Chui Yim “Maggie” Lau, an associate research scholar in Princeton’s Department of Geosciences, said that although it’s too early to claim that the entire Arctic will be a massive methane “sink” in a warmer world, the study’s results do suggest that the Arctic could help mitigate the warming effect that would be caused by a rising amount of methane in the atmosphere. In immediate terms, climate models that project conditions on a warmer Earth could use this study to more accurately calculate the future methane content of the atmosphere, Lau said.
“At our study sites, we are more confident that these soils will continue to be a sink under future warming. In the future, the Arctic may not have atmospheric methane increase as much as the rest of the world,” Lau said. “We don’t have a direct answer as to whether these Arctic soils will offset global atmospheric methane or not, but they will certainly help the situation.”
The researchers want to study the bacteria’s physiology as well as test the upper temperature threshold and methane concentrations at which they can still efficiently process methane, Lau said. Field observations showed that the bacteria are still effective up to 18 degrees Celsius (64.4 degrees Fahrenheit) and can remove methane down to one-quarter of the methane level in the atmosphere, which is around 0.5 parts-per-million.
“If these bacteria can still work in a future warmer climate and are widespread in other Arctic permafrost areas, maybe they could regulate methane for the whole globe,” Lau said. “These regions may seem isolated from the world, but they may have been doing things to help the world.”
From Princeton, Lau worked with geoscience graduate student and second author Brandon Stackhouse; Nicholas Burton, who received his bachelor’s degree in geosciences in 2013; David Medvigy, an assistant professor of geosciences; and senior author Tullis Onstott, a professor of geosciences. Co-authors on the paper were from the University of Tennessee-Knoxville; the Oak Ridge National Laboratory; McGill University; Laurentian University in Canada; and the University of Texas at Austin.
The research was supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research (DE-SC0004902); the National Science Foundation (grant no. ARC-0909482); the Canada Foundation for Innovation (grant no. 206704); the Natural Sciences and Engineering Research Council of Canada Discovery Grant Program (grant no. 298520-05); and the Northern Research Supplements Program (grant no. 305490-05)
M.C.Y. Lau, B.T. Stackhouse, A.C. Layton, A. Chauhan, T. A. Vishnivetskaya, K. Chourey, J. Ronholm, N.C.S. Mykytczuk, P.C. Bennett, G. Lamarche-Gagnon, N. Burton, W.H. Pollard, C.R. Omelon, D.M. Medvigy, R.L. Hettich, S.M. Pfiffner, L.G. Whyte, and T.C. Onstott. 2015. An active atmospheric methane sink in high Arctic mineral cryosols. The ISME Journal. Article published in print August 2015. DOI:10.1038/ismej.2015.13.
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