Land & Vegetation

Soil moisture, snowpack data could help predict 'flash droughts'

BOULDER, Colo. — New research suggests that "flash droughts" — like the one that unexpectedly gripped the Southern Rockies and Midwest in the summer of 2012 — could be predicted months in advance using soil moisture and snowpack data. Researchers at the National Center for Atmospheric Research (NCAR) analyzed the conditions leading up to the 2012 drought, which ultimately caused $30 billion in economic losses, looking for any warning signs that a drought was on the way. In a study funded by the National Science Foundation and published in the Journal of Geophysical Research-Atmospheres, the scientists find that observations of snowmelt and soil moisture could have predicted the ensuing drought up to four months in advance."The 2012 drought over the Midwest was one of the most severe and extensive U.S. droughts since the 1930s Dust Bowl, but it was also extremely challenging to predict," said Debasish PaiMazumder, lead author of the study. "This study demonstrated the potential to improve seasonal drought outlooks in the future, giving farmers, water planners, and others more time to prepare."The official U.S. Drought Monitor issued on Aug. 21, 2012. The map shows the exceptionally severe drought across the middle of the country. Just three months before, drought forecasts failed to predict that a drought was on the way. Click to enlarge. (Image courtesy National Drought Mitigation Center.)Seasonal drought forecasts issued in May 2012 for the upcoming summer did not foresee a drought forming in the country's midsection. But by the end of August, a drought that had started in the Southern Rockies had spread across the Midwest, parching Oklahoma, Kansas, Nebraska, and Missouri.These flash droughts — which form and intensify rapidly — can catch forecasters off guard because they are not preceded by any large-scale climate patterns that could act as a warning signal. For example, one contributor to the recent California drought was a persistent high-pressure system parked off the west coast of Canada that deflected storms away from the state. Because forecasters could identify the high-pressure system, they could also accurately predict fewer storms and a worsening of the drought.Previous research has shown that looking at soil moisture alone could improve the lead-time of drought predictions by one to two months. PaiMazumder and NCAR colleague James Done were interested in whether they could extend this further by adding snowpack into the equation.“Advance knowledge of a drought even a month or two ahead of time can greatly minimize the effects on society,” said Anjuli Bamzai, program director in NSF’s Division of Atmospheric and Geospace Sciences, which funded the research.  “This study highlights the role of snowpack and soil moisture conditions in predicting the sudden onset of drought.”To explore the physical connections among snowpack, soil moisture, and drought, the researchers analyzed data collected between 1980-2012. To supplement those observations, they also explored the physical connections in a new NCAR-based community Weather Research and Forecasting (WRF) model dataset comprising 24 simulations of the period 1990-2000 and 2012. Because each simulation was run with small tweaks to the way the model represents atmospheric physics, the result was a broad look at different climate scenarios that could have plausibly unfolded during the study period."The model helped us get a handle on how robust the relationships between snowpack, soil moisture, and drought are," Done said. "The stronger the relationship, the better a predictor is."While observations of snowpack and soil moisture could have helped predict the 2012 drought, the method does not replace other drought prediction measures that identify large-scale phenomena that frequently lead to drought conditions."This is another ingredient that could be used when making seasonal drought forecasts," Done said. "But it's not the only ingredient, and for many droughts that are tied to large-scale precursors, it may not be the most important one." About the articleTitle: Potential Predictability Sources of the 2012 US Drought in Observations and a Regional Model EnsembleAuthors: Debasish PaiMazumder and James DoneJournal: Journal of Geophysical Research – Atmospheres, DOI: 10.1002/2016JD025322Writer:Laura Snider, Senior Science Writer and Public Information Officer   

Population trumps climate and carbon in shaping the future of wildfires

May 23, 2016 | All other things being equal, a warming climate would likely expand the amount of land scorched each year by wildfires across the globe.  But all other things aren't equal. A new study by a team of scientists, including Leiwen Jiang at the National Center for Atmospheric Research (NCAR), found that the future pattern of population growth, not climate change, is likely to be the dominant factor in determining whether the amount of land burned by fires increases or decreases this century. Jiang and his colleagues — Wolfgang Knorr of Lund University in Sweden and Almut Arneth of the Karlsruhe Institute of Technology in Germany — also found that the anticipated increase in total burned area due to a warmer climate will likely be offset by the carbon dioxide itself, which can act as a fertilizer, affecting plant growth and driving down fire risk globally. When climate change, carbon dioxide concentration, and population are all considered, the total area burned across the globe could very well decrease over the rest of this century, according to the study, published in the journal Nature Climate Change. "You cannot look at the impact of climate change alone to predict future wildfire risk," Jiang said. "You have to put population changes in the model, and not just population size, but also spatial distribution. There is a big difference between population growth in rural areas compared to urban areas." Humans and fires: A complex relationship A wildfire burns across a Kansas grassland. On average, grassland fires account for 70 percent of the global land area burned each year. (Photo courtesy U.S. Fish and Wildlife Service.) Climate change generally increases global fire risk by drying the fuel — trees, grasses, and other vegetation — that feeds the flames. At the same time, the additional carbon dioxide in the atmosphere tends to decrease global fire risk, largely by encouraging the growth of shrubs in areas that are now grasslands. On average, more than 70 percent of the total area burned each year across the world is on grassland savannah, where fire can spread very quickly. As shrubs encroach, they fragment the grassland and create natural firebreaks. People, on the other hand, have a complex effect on fire risk. "People not only act as fire igniters, but they also act as fire suppressors," Jiang said. In general, humans suppress fires more than they ignite them, leading to an overall downward trend in acres burned when population increase is considered on its own, the study found. This helps explain why global burned area has actually decreased over the last century, despite a warming climate, Jiang said. Impacts of urbanization vary regionally In the future, however, the way population growth is distributed across rural and urban areas will have more of an impact on future fires than total population growth alone. The pace of urbanization also has a different effect, depending on the region of the world. For example, in densely populated parts of sub-Saharan Africa, a future with generally low population growth but fast urbanization could lead to a decrease in average area burned annually, from about 39,000 square miles (101,000 square kilometers) between 1971 and 2000 to about 30,000 square miles (78,000 square kilometers) between 2071 and 2100. Conversely, relatively high population growth with a slow rate of urbanization would lead to an increase in annual burned area from about 63,000 square miles (101,000 square kilometers) to about 75,000 square miles (194,000 square kilometers) over the same time periods. Both scenarios assume that greenhouse gas emissions continue unabated. In other regions of relatively high population density, the study finds that total burned area increases regardless of urbanization patterns, but the scale of the increase varies. For example, in heavily populated parts of Asia, North Africa, Latin America, and the Caribbean, a scenario of low population growth and fast urbanization would lead to an increase in average burned area annually from 21,000 square miles (55,000 square kilometers) to 23,500 square miles (61,000 square kilometers). However, high population growth coupled with slow urbanization would drive an even greater increase, to 33,600 square miles (87,000 square kilometers). Regardless of the population scenario, the new study does not project a large future increase in burned area. Instead, it shows that the total area burned by wildfires globally could further decrease this century or perhaps increase to levels seen in the late twentieth century, depending on future population growth patterns. "This study underscores how important it is to consider demographics as we measure the future impacts of climate change," said Jiang. "The way our population grows will have impacts far beyond wildfires, and may determine how resilient society is in the future." About the article W. Knorr, A. Arneth and L. Jiang, Demographic controls of future global fire risk, Nature Climate Change, DOI: 10.1038/nclimate2999 Writer/contactLaura Snider, Senior Science Writer and Public Information Officer Collaborating organizationsLund University, SwedenKarlsruhe Institute of Technology, Germany  FunderEuropean Commission 

Searing heat waves detailed in study of future climate

Sweltering heat waves that typically strike once every 20 years could become yearly events across 60 percent of Earth's land surface by 2075, if human-produced greenhouse gas emissions continue unchecked.  If stringent emissions-reduction measures are put in place, however, these extreme heat events could be reduced significantly. Even so, 18 percent of global land areas would still be subjected yearly to these intense heat waves, defined as three exceptionally hot days in a row. These are among the findings of a new study by Claudia Tebaldi of the National Center for Atmospheric Research and Michael Wehner of the Lawrence Berkeley National Laboratory. The study, funded by the U.S. Department of Energy and published in the journal Climatic Change, quantifies the benefits society would reap, in terms of avoiding extreme heat events, if action is taken now to mitigate climate change. "The study shows that aggressive cuts in greenhouse gas emissions will translate into sizable benefits, starting in the middle of the century, for both the number and intensity of extreme heat events," Tebaldi said. "Even though heat waves are on the rise, we still have time to avoid a large portion of the impacts." More frequent, more severe Tebaldi and Wehner used data generated by the NCAR-based Community Earth System Model to study 20-year extreme heat events—those intense enough to have just a 1-in-20 chance of occurring in any given year. The model was developed with support from the Department of Energy and the National Science Foundation, NCAR’s sponsor. The researchers looked at two things: how frequently today's typical 20-year heat wave may occur in the future, as well as how much more intense future 20-year heat waves will be. For large portions of the world's land surface, future heat waves with a 1-in-20 chance of occurring in any given year are projected to become more extreme than heat waves with the same chance of occurring today. Stringent efforts to mitigate human-produced carbon emissions would reduce the amount of land area at risk for these intense heat waves—defined as three days of exceptionally hot temperature. Click to enlarge. (This table is freely available for media & nonprofit use.) Besides finding that today's 20-year heat waves could become annual occurrences across more than half of the world's land areas by 2075, the study also concluded that heat waves with a 1-in-20 chance of occurring during a future year will be much more extreme than heat waves with the same probability of occurring today. For example, if emissions remain unabated, a heat wave with a 1-in-20 chance of occurring in 2050 would be at least 3 degrees Celsius (5.4 degrees Fahrenheit) hotter for 60 percent of the world's land areas. For 10 percent of land areas, a 20-year heat wave in 2050 would be at least 5 degrees C (9 degrees F) hotter. A few degrees may not seem like much on a mild day, but during extreme heat events, they can mean the difference between life and death for vulnerable populations, Wehner said. "It’s the extreme weather that impacts human health; this week could be 2 degrees Celsius hotter than last week, and that doesn’t matter," he said. "Now, imagine the hottest day that you can remember and instead of 42 degrees C (107.6 degrees F) it’s now 45 degrees C (113 degrees F). That’s going to have a dangerous impact on the poor, the old and the very young, who are typically the ones dying in heat waves." By 2075, the situation is likely to become much more dire if greenhouse gas emissions—produced largely by the burning of fossil fuels—are not reduced. The percent of land areas subject to 20-year events that are at least 5 degrees C hotter swells from 10 to 54 percent. However, if emissions are aggressively cut, the severity of these 20-year events could be significantly reduced over the majority of the world's land areas, though portions of the Earth would still face dangerous heat extremes. For example, in 2075, almost a quarter—instead of more than a half—of land areas could experience 20-year heat waves that are at least 5 degrees C hotter than today's. “But even with such dramatic reductions in carbon dioxide emissions, future heat waves will be far more dangerous than they are now,” Wehner said. The researchers also looked at single-day extreme heat events, as well as single-day and three-day blocks when the overnight low temperature remained exceptionally warm. Past research has shown that human health is especially endangered when temperatures do not cool off significantly at night. All of these events had similar increases in frequency and intensity. A tool for cost-benefit analysis The fact that extreme heat events are expected to increase in the future as the climate changes—and the fact that emission reductions could ameliorate that increase—is not a surprise, Tebaldi said. But this study is important because it puts hard numbers to the problem. "There is a cost attached to reducing emissions," Tebaldi said. "Decision makers are interested in being able to quantify the expected benefits of reductions so they can do a cost-benefit analysis." Tebaldi and Wehner's paper is part of a larger project based at NCAR called the Benefits of Reduced Anthropogenic Climate Change, or BRACE. For the project, researchers from across NCAR and partner organizations are working to quantify how emission reductions may affect health, agriculture, hurricanes, sea level rise, and drought. About the article Title: Benefits of mitigation for future heat extremes under RCP4.5 compared to RCP8.5Authors: Claudia Tebaldi and Michael F. WehnerPublication: Climatic Change Writer:Laura Snider, Senior Science Writer and Public Information Officer

Global food system faces multiple threats from climate change

BOULDER — Climate change is likely to have far-reaching impacts on food security throughout the world, especially for the poor and those living in tropical regions, according to a new international report that includes three co-authors from the National Center for Atmospheric Research (NCAR).The report, issued today at the Paris 2015 United Nations Climate Conference (also known as COP21) warns that warmer temperatures and altered precipitation patterns can threaten food production, disrupt transportation systems, and degrade food safety, among other impacts. As a result, international progress in the past few decades toward improving food security will be difficult to maintain.The report, Climate Change, Global Food Security, and the U.S. Food System, provides an overview of recent research in climate change and agriculture. Led by the U.S. Department of Agriculture and published under the auspices of the U.S. Global Change Research Program, it includes contributors from 19 federal, academic, nongovernmental, intergovernmental, and private organizations in the United States, Argentina, Britain, and Thailand.“If society continues on a path of high emissions of greenhouse gases, there is no way around the fact that climate change is going to be a primary challenge for producing and distributing food,” said NCAR scientist Claudia Tebaldi, a co-author of the report. “If society lowers emissions, climate change will still be a stressor on food security, but other factors such as socioeconomic conditions could be more critical.”Two other NCAR scientists—Caspar Ammann and Brian O’Neill—also served as co-authors. The report was produced as part of a collaboration between NCAR, the Department of Agriculture, and the University Corporation for Atmospheric Research, which manages NCAR on behalf of the National Science Foundation.A farmer tills his field. (Photo by Keith Weller, U.S. Department of Agriculture.)The report focuses on identifying climate change impacts on global food security through 2100. The authors emphasize that food security – the ability of people to obtain and use sufficient amounts of safe and nutritious food – will be affected by several factors in addition to climate change, such as technological advances, increases in population, the distribution of wealth, and changes in eating habits.“Changes in society and changes in climate will both be critically important to food security in the coming decades,” O’Neill said. “This means we have to do a better job of anticipating possible changes in income, governance, inequality, and other factors, and a better job understanding how they interact with food security and climate change.”Among the report’s key findings:The impact of climate change on crop and livestock productivity is projected to be larger for tropical and subtropical regions such as Africa and South Asia, although there will be regional variations. Wealthy populations and temperate regions are less at risk, and some high-latitude regions may temporarily experience productivity increases, in part because of warmer temperatures and more precipitation. However, if society continues to emit more carbon dioxide and other greenhouse gases that cause climate change, even those regions will face damaging outcomes during the second half of this century.Climate change has important implications for food producers and consumers in the United States. The nation is likely to experience changes in the types and cost of food available for import. It can also expect to face increased demand for agricultural exports from regions coping with production difficulties.Climate change risks extend beyond agricultural production to critical elements of global food systems, including processing, storage, transportation, and consumption. For example, warmer temperatures can have a negative impact on food storage and increase food safety risks; higher sea levels and changes to lake and river levels can impede transportation.Risks to food security will increase with a higher magnitude and faster rate of climate change. In a worst-case scenario based on high greenhouse gas concentrations, high population growth, and low economic growth, the number of people at risk of undernourishment would increase by as much as 175 million by 2080 over today’s level of about 805 million. This would reverse recent gains, as the number of people at risk of undernourishment has dropped from about 1 billion since the early 1990s.Society can take steps to reduce the food system’s vulnerability to climate change, ranging from more advanced growing methods to cold storage, improvements in transportation infrastructure, and other strategies. Such adaptations, however, may be difficult to implement in some regions due to availability of water, soil nutrients, infrastructure, funding, or other factors.More information:Climate Change, Global Food Security, and the U.S. Food SystemUSDA and UCAR videos about the new report (UCARConnect)

Have it your way: New hydrologic model lets the user decide

June 17, 2015 | Water falls from the sky, runs across the earth, funnels into drainages, and fills rivers.  It seems simple. But total precipitation does not equal total streamflow. Along the way, some of the water gets sucked up by plants, absorbed by the soil, or evaporated into the air. A complex array of factors—from wind speed to geology—influences how much precipitation converts into streamflow and at what speed. Hydrologic models for predicting streamflow each capture these variables in different ways. This can lead to large disparities in their results and, therefore, large headaches for water managers who depend on the forecasts to plan for the future. Scientists at the National Center for Atmospheric Research and their collaborators have now developed a way to bring these diverse modeling approaches together, dig into their differences, and ultimately paint a fuller picture of what streamflow may look like in the future, especially in the face of a changing climate. Led by NCAR scientist Martyn Clark, the research team has developed the Structure for Unifying Multiple Modeling Alternatives. SUMMA allows users to customize the model by making individual decisions about how to treat a vast range of variables, a capability that allows SUMMA to both mimic existing models and create something entirely new. Hydrologic models vary in the way they represent physical processes. For example, they can differ in their assumptions about the temperature at which snow is most prone to stick to the tree canopy. SUMMA allows users to customize how the model treats a range of variables to uncover the impact of any single assumption. (©UCAR. Photo by Carlye Calvin. This image is freely available for media & nonprofit use.) Flipping the switch on model comparison Because hydrologic models can vary so drastically in their approaches, water managers sometimes use several different models and then compare the results. When the future scenarios differ significantly, however, it can be difficult to pinpoint the reason. Is it due, for example, to the way the model divides the landscape into smaller units? Or maybe how it connects those units? Something else? "There can be hundreds of differences between the models," Clark said. "You can make broad conclusions, but you need a more controlled and systematic approach to model evaluation to understand why the differences occurred." SUMMA, which Clark and his collaborators describe in detail in a pair of papers recently published in Water Resources Research, tackles this problem by allowing researchers to drill down into the construction of existing models. By essentially flipping switches within SUMMA, a user can recreate the parameters hard-wired into existing hydrologic models. Then, by changing one setting at a time, the user can begin to untangle what actually accounts for differences in the modeling results. For instance, some existing hydrologic models assume that snow tends to stick better to the tree canopy when the temperature is hovering near freezing. Other models assume the opposite—that the trees intercept more snow as the temperature drops. Snow that sticks to trees is more likely to evaporate directly into the air. SUMMA will let a user uncover the impact of a single assumption—such as when snow sticks to trees or how water flows through soil—on predictions produced by multiple models, even when those models have dozens of other differences. "The idea is to be more explicit about the decisions we're making when building models," Clark said. "The result can be more accurate and detailed representations of the watershed." Characterizing uncertainty SUMMA is also a tool that can help scientists grapple with some of the challenges of assessing the impact of climate change on water supplies. Water resource managers have begun to incorporate climate change into their long-term planning. In order to make good decisions, however, they need to know how confident to be in any one model's results. Uncertainty can be introduced into projections of future impacts in multiple ways, including when the results from global climate models are downscaled to a single river basin. Uncertainty also trickles in when a hydrologic model doesn't do a good job of representing one particular process that's important in one particular watershed. Because SUMMA can be easily modified, users can produce an ensemble of future streamflow predictions that characterize how great the uncertainty is—or isn't—by describing an entire range of possible scenarios. Water resource managers can be more confident in the results where the scenarios tend to look the same. Where the scenarios differ, they may need to plan for a wider spectrum of potential future conditions. "We want to allow for a fuller characterization of model uncertainty so that we can plan for water security," Clark said. "And SUMMA is a way to bring this together." In graphic terms Users of the SUMMA hydrologic model can make myriad choices about how the model functions, such as deciding how to represent the variability across a watershed. For example, users can decide first whether to break a river basin up into a grid (top left), into sub-catchments (top right), or into any other shape. The runoff computed for each of these individual divisions, known as Grouped Response Units is added to the total river flow.But SUMMA also allows users to further customize each GRU by dividing them into Hydrologic Response Units, which characterize how vegetation, geology, and other variables affect the way water moves through the soil column. Users have the option of treating each GRU as a single HRU (a) or of dividing each GRU into a grid, (b) with each cell representing a different HRU.Finally, users can exercise even more control over the way the model represents water movement within a GRU by customizing the shape of each HRU so that it reflects as accurately as possible the actual conditions on the ground (c). (©UCAR. Illustration courtesy Martyn Clark, NCAR. This image is freely available for media & nonprofit use.)    For a more detailed graphic, click here. About the articles Martyn P. Clark, Bart Nijssen, Jessica D. Lundquist, Dmitri Kavetski, David E. Rupp, Ross A. Woods, Jim E. Freer, Ethan D. Gutmann, Andrew W. Wood, Levi D. Brekke, Jeffrey R. Arnold, David J. Gochis, and Roy M. Rasmussen, A unified approach for process-based hydrologic modeling: 1. Modeling concept, Water Resources Research, doi: 10.1002/2015WR017198 Martyn P. Clark, Bart Nijssen, Jessica D. Lundquist, Dmitri Kavetski, David E. Rupp, Ross A. Woods, Jim E. Freer, Ethan D. Gutmann, Andrew W. Wood, David J. Gochis, Roy M. Rasmussen, David G. Tarboton, Vinod Mahat, Gerald N. Flerchinger, and Danny G. Marks, A unified approach for process-based hydrologic modeling: 2. Model implementation and case studies, Water Resources Research, doi: 10.1002/2015WR017200 Writer/contact Laura Snider Collaborating institutionsBureau of ReclamationColorado State UniversityNational Center for Atmospheric ResearchOregon State UniversityUniversity of AdelaideUniversity of BristolUniversity of WashingtonU.S. Army Corps of EngineersUSDA Agricultural Research ServiceUtah State University Funders Bureau of ReclamationNational Science FoundationNASA Advanced Information Systems Technology (AIST) ProgramNOAA Modeling Analysis Predictions and Projections (MAPP) ProgramU.S. Army Corps of Engineers

Curbing carbon: Not enough plant food to go around?

April 20, 2015 | One of the great climate mysteries that scientists are working to solve is how trees and other plants respond to a more carbon-rich atmosphere. Most climate scenarios, including those of the Intergovernmental Panel on Climate Change, assume that more carbon dioxide (CO2) in the atmosphere will accelerate plant growth, thereby drawing down more of this greenhouse gas from the atmosphere. But a number of studies have indicated that plants can’t keep absorbing more CO2 because there aren’t enough nutrients in the soil to sustain their growth. A new study in Nature Geoscience, led by NCAR scientist Will Wieder, underscores what’s at stake. If society stays on its current trajectory of CO2 emissions, and the growth rates of plants aren't as robust as many models project, the result by the end of the century could mean in an additional 10 percent of the greenhouse gas in the atmosphere, the study finds. While there is uncertainty around this estimate, that amount of CO2—an estimated 140 petagrams—would be equivalent to about 14 years of CO2 emissions from all human-related sources worldwide at current rates, or about as much CO2 as has been released so far this century. New estimates indicate that plants, such as these trees in the Osa Peninsula in Costa Rica, may absorb less carbon dioxide then previously thought. This could result in more global warming. (©UCAR. Photo by Will Wieder, NCAR.) “Humanity so far has greatly benefited from plants removing carbon dioxide from the atmosphere,” said Wieder, who also works at the Institute for Arctic and Alpine Research at the University of Colorado Boulder. “But if a lack of nutrients limits their ability to keep soaking up CO2, then climate change becomes an even bigger problem then we thought—unless society can cut back on emissions.” The role of nutrients Most of the world’s leading climate models assume that plants will respond to increased atmospheric levels of CO2 by growing more and more, which is known as the CO2 fertilization effect. The more the plants grow, the more CO2 they absorb from the atmosphere, thereby slowing climate change. But CO2 is far from the only determinant of plant growth. Nutrients in the soil, especially nitrogen and phosphorus, are also critical. Because the supply of such nutrients is limited, scientists have warned that plant growth will be less than indicated in climate models. Most climate models so far have not included nutrients because such biogeochemical processes are difficult to simulate and vary greatly from one type of terrestrial ecosystem to another. The NCAR-based Community Earth System Model, jointly funded by the National Science Foundation and U.S. Department of Energy, is one of the first to begin considering the role of soil nutrients in the models that are used for climate change projections. In the new study, Wieder and his co-authors turned to the world’s leading climate models that were used in an international study known as CMIP5 (the Coupled Model Intercomparison Project, Phase 5). They focused on how the 11 models represented plant growth in specific geographic regions, comparing that to changes in nitrogen and phosphorus availability caused by deposition of airborne particles and other factors. Their results showed that nitrogen limitation could reduce plant uptake of CO2 by 19 percent, and nitrogen and phosphorus limitation combined could reduce plant uptake by 25 percent, compared to the average results of the climate models. Instead of acting as a carbon sink and drawing down CO2, the terrestrial biosphere would become a net source of the greenhouse gas to the atmosphere by the end of the century, with soil microbes releasing more carbon than growing plants could absorb. The role of uncertainty Wieder stressed that significant uncertainties remain. One of the questions, for example, is how soil microbes—which free up nitrogen in the soil, but also release carbon dioxide into the atmosphere—will respond to warming temperatures. Similarly, scientists don’t know if plants will become more efficient at drawing up additional nutrients in the soil. But the overall picture is that Earth’s limited amounts of nitrogen and phosphorus mean that "plants will not be able to keep up with society’s CO2 emissions," Wieder said. “To store that much carbon on land, plants will need more nitrogen and phosphorus,” he said. “If they can’t get it, we’re going to go from terrestrial ecosystems sponging up CO2 to actually having them contributing to the problem.” Writer/contactDavid Hosansky CollaboratorsUniversity of Colorado Boulder, Institute of Arctic and Alpine ResearchUniversity of Montana, MissoulaUniversity of Oklahoma, NormanUniversity of Minnesota Institute on the Environment, St. PaulPacific Northwest National Laboratories FundersNational Science FoundationAndrew W. Mellon Foundation 
  

How will climate change affect tropical forests?

April 1, 2015 | Tropical forests play a major role in the planet’s carbon cycle, but there are a lot of uncertainties about how they will respond to climate change. A new international project aims to bring the future of tropical forests into much clearer focus. The project, called the Next Generation Ecosystem Experiment–Tropics, or NGEE-Tropics, is led by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab). It includes scientists from the National Center for Atmospheric Research (NCAR). The project will explore how tropical forests respond to increasing atmospheric carbon dioxide (CO2) levels, rising temperatures, shifting precipitation patterns, and other natural and human-induced changes. A major new international project focuses on the potential impacts of climate change on tropical forests such as this one in Caxiuanã, Brazil. (Photo by Hugo Glendinning.) Over the next decade, NGEE-Tropics scientists will collaborate with other researchers to carry out experiments in tropical forests around the globe. This research will fuel the development of a first-of-its-kind tropical forest ecosystem model that extends from the bedrock to the top of the forest canopy. The model will capture myriad soil and vegetation processes at a resolution better than 10 kilometers. This is the resolution that next-generation Earth system models will achieve during the project’s lifetime. Exploring uncharted territory At NCAR, scientists will study the strategies that rainforest trees use to conserve water in order to better predict the response of different forest types to drought. This is because one particular concern for tropical biomes is their resilience to changing rainfall patterns. Separately, the NCAR team also will use data from large experimental burning projects in Brazil to refine a computer model that simulates the complex interactions between vegetation, fires and climate. In both projects, the research team will work closely with community ecologists to better understand how different plant types are organized into ecosystems. “NGEE is an unprecedented opportunity to bring the sciences of ecology and Earth system modeling together around a powerful set of experiments and observation campaigns to predict how ecosystems respond to changes in climate,” said NCAR scientist Rosie Fisher. “This is quite uncharted territory, and we're excited about what we might find."  In addition to NCAR, NGEE-Tropics brings together scientists from Brookhaven, Los Alamos, Oak Ridge, and Pacific Northwest national laboratories. It also includes researchers from NASA, the Smithsonian Tropical Research Institute, the U.S. Forest Service, and several institutions from other nations, including Brazil’s National Institute of Amazonian Research. The 10-year, $100 million project is supported by the Department of Energy’s Office of Science. For more about the project, see the Berkeley Lab news release.   Writer/contactDavid Hosansky FunderU.S. Department of Energy CollaboratorsLawrence Berkeley National LaboratoryBrookhaven National Laboratory Los Alamos National LaboratoryOak Ridge National LaboratoryPacific Northwest National LaboratoryNCARNASA, the Smithsonian Tropical Research Institute, the U.S. Forest Service,Brazil’s National Institute of Amazonian Research and several institutions from other nationsFor the executive committee and scientific advisory board, see the NGEE-Tropics webpage.        

Tropical forests have large appetite for carbon dioxide

BOULDER — A new study led by NASA and the National Center for Atmospheric Research (NCAR) shows that tropical forests may be absorbing far more human-emitted carbon dioxide than many scientists thought. The study estimates that tropical forests absorb 1.4 billion metric tons of carbon dioxide out of a total global absorption of 2.5 billion, in response to rising atmospheric levels of the greenhouse gas. This means, if left undisturbed, the tropical trees should be able to continue reducing the rate of global warming. “This is good news, because uptake in northern forests may already be slowing, while tropical forests may continue to take up carbon for many years,” said David Schimel of NASA’s Jet Propulsion Laboratory in Pasadena, California. Schimel is lead author of a paper on the new research, appearing this week in the Proceedings of National Academy of Sciences. The question of which type of forest is the bigger carbon absorber “is not just an accounting curiosity,” said NCAR scientist Britton Stephens, a co-author on the paper. “It has big implications for our understanding of whether global terrestrial ecosystems might continue to offset our carbon dioxide emissions or might begin to exacerbate climate change.” Forests and other land vegetation currently remove up to 30 percent of human carbon dioxide emissions from the atmosphere by absorbing carbon dioxide during photosynthesis. If the rate of absorption were to slow down, the rate of global warming would speed up in return. Tropical forests like this one in the Serra do Mar Paranaense in Brazil may be absorbing far more human-emitted carbon dioxide than many scientists thought. (Photo by Deyvid Setti e Eloy Olindo Setti via Wikimedia Commons.) The new study is the first to devise a way to make apples-to-apples comparisons of carbon dioxide uptake estimates from many sources at different scales: computer models of ecosystem processes, atmospheric models used to deduce the sources of today’s concentrations (called atmospheric inverse models), satellite images, data from routine and experimental forest plots, and more. The researchers reconciled these analyses and assessed the accuracy of the inverse models based on how well they reproduced independent, airborne and ground-based measurements. They obtained their new estimate of the tropical carbon absorption from the weighted average of atmospheric, ecosystem model, and ground-based data. “Until our analysis, no one had successfully completed a global reconciliation of information about carbon dioxide effects from the atmospheric, forestry, and modeling communities,” said coauthor Joshua Fisher of the Jet Propulsion Laboratory. “It is incredible that all these different types of independent data sources start to converge on an answer.” The research was funded by NASA and by the National Science Foundation, which sponsors NCAR. Growing forests, more fires As human-caused emissions add more carbon dioxide to the atmosphere, forests worldwide are using it to grow faster, reducing the amount that stays airborne. This effect is called carbon dioxide fertilization. But climate change also decreases water availability in some regions and makes Earth warmer, leading to more frequent droughts and larger wildfires. In the tropics, humans compound the problem by burning wood during deforestation. Fires don’t just stop carbon absorption by killing trees, they also spew huge amounts of previously-stored carbon into the atmosphere as the wood burns. For about 25 years, most atmospheric inverse models have been showing that mid-latitude forests in the Northern Hemisphere absorb more carbon than tropical forests. That result was initially based on the then-current understanding of global air flows and limited data suggesting that deforestation was causing tropical forests to release more carbon dioxide than they were absorbing. In the mid-2000s, Stephens used measurements of carbon dioxide made from aircraft to show that many atmospheric inverse models were not correctly representing flows of carbon dioxide in the air above ground level. Models that matched the aircraft measurements better showed more carbon absorption in the tropical forests. However, there were still not enough global data sets to validate the idea of large tropical-forest absorption. Schimel said that their new study took advantage of a great deal of work other scientists have done since Stephens’ paper to pull together national and regional data of various kinds into robust, global data sets. He noted that the new paper reconciles results at every scale from the pores of a single leaf, where photosynthesis takes place, to the whole Earth, as air moves carbon dioxide around the globe. “What we’ve had up till this paper was a theory of carbon dioxide fertilization based on phenomena at the microscopic scale and observations at the global scale that appeared to contradict those phenomena,” he said. “Here, at least, is a hypothesis that provides of a consistent explanation that includes both how we know photosynthesis works and what’s happening at the planetary scale.” Atmospheric models have improved over the past decade, but there is still considerable disagreement among atmospheric inverse estimates of the distribution of carbon uptake, owing to remaining differences in modeling global air flows. “It is critical that we rigorously test these models against observations so that we can further reduce uncertainty on the terrestrial feedback to climate change,” Stephens said. About the article Title: The effect of increasing CO2 on the terrestrial carbon cycle Authors: David Schimel, Britton Stephens, and Joshua B. Fisher Journal: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.1407302112

Ground-level ozone's toll in India

September 9, 2014 | Ozone pollution in India is damaging millions of tons of the country’s major crops, according to a new study by an international team of researchers. The pollution caused losses of more than $1 billion in a single year, destroying enough food to feed tens of millions of people living in poverty in the country. The study used crop production data and a powerful, NCAR-based computer model to estimate the impacts of ozone. It was led by scientists at the Indian Institute of Tropical Meteorology (IITM) in Pune, India, with co-authors from the Scripps Institution of Oceanography and NCAR. The research was published last month in Geophysical Research Letters, a journal of the American Geophysical Union. The researchers looked at the agricultural effects in 2005 of high levels of ground-level ozone, which damages plants and affects human health as well. They estimated that the pollutant ruined 6.7 million U.S. tons (6 million metric tons) of India’s wheat, rice, soybean, and cotton crops that year. India could feed 94 million people with the lost wheat and rice crops, about one-third of the country’s population living in poverty, according to Sachin Ghude, an atmospheric scientist at the Indian Institute of Tropical Meteorology and lead author of the new study. A traffic jam in Delhi, India. Automobile emissions in the presence of sunlight contribute to the formation of low-level ozone, a significant threat to human health and crop productivity. (Wikimedia Commons image courtesy NOMAD.) Despite air quality standards passed in the 1980s that were designed to curb industrial and vehicle emissions, pollution remains a major challenge for India. Long-term measurements of surface ozone across India—measured on the ground or by aircraft—are not available, making it difficult to get a clear picture of how levels of the pollutant have changed. But satellite-based studies show ozone has increased in the last two decades, Ghude said. To estimate ozone levels over specific regions of the country, the researchers turned to the chemistry version of the NCAR-based Weather Research and Forecasting (WRF) model. This powerful software tool enabled the researchers to simulate hourly ozone levels, based on six estimates of emissions. They then calculated the impact on yields of wheat, soybeans, cotton, and rice. Although previous studies had looked at ozone damage at local levels, the study’s authors set out to conduct the first comprehensive look at how ozone pollution has affected agriculture in India on a national level. Ghude said the new paper could help policymakers craft new ozone pollution standards and plan mitigation strategies to reduce damage to plants. NCAR postdoctoral researcher Rajesh Kumar, a co-author of the study, said India's ozone problems will likely get worse with climate change. Research has shown that warmer temperatures tend to produce higher levels of ozone. “This is a huge impact, but what we have now is not the worst-case scenario,” he said. “It will most likely get worse.” Ghude, S. D., C. Jena, D. M. Chate, G. Beig, G. G. Pfister, R. Kumar, and V. Ramanathan (2014), Reductions in India's crop yield due to ozone, Geophysical Research Letters, 41, 5685–5691, doi:10.1002/2014GL060930. Writer/ContactDavid Hosansky, NCAR/UCAR Communications Collaborating institutionsIndian Institute of Tropical Meteorology National Center for Atmospheric Research Scripps Institution of Oceanography FunderNational Science Foundation  

Climate experts estimate risk of rapid crop slowdown

BOULDER – The world faces a small but substantially increased risk over the next two decades of a major slowdown in the growth of global crop yields because of climate change, new research finds. The authors, from Stanford University and the National Center for Atmospheric Research, say the odds of a major production slowdown of wheat and corn, even with a warming climate, are not very high. But the risk is about 20 times more significant than it would be without global warming, and it may require planning by organizations that are affected by international food availability and price.  “Climate change has substantially increased the prospect that crop production will fail to keep up with rising demand in the next 20 years,” said NCAR scientist Claudia Tebaldi, a co-author of the study. Stanford professor David Lobell said he wanted to study the potential impact of climate change on agriculture in the next two decades because of questions he has received from stakeholders and decision makers in governments and the private sector. “I’m often asked whether climate change will threaten food supply, as if it’s a simple yes or no answer,” Lobell said. “The truth is that over a 10- or 20-year period, it depends largely on how fast the Earth warms, and we can’t predict the pace of warming very precisely. So the best we can do is try to determine the odds.” A storm looms behind wheat fields in eastern Colorado, where recurrent drought has had major impacts on agriculture over the last 15 years. (©UCAR, photo by Carlye Calvin. This image is freely available for media & nonprofit use.) Lobell and Tebaldi used computer models of global climate, as well as data about weather and crops, to calculate the chances that climatic trends would have a negative effect of 10 percent on yields in the next 20 years. This would have a major impact on food supply. Yields would continue to increase but the slowdown would effectively cut the projected rate of increase by about half at the same time that demand is projected to grow sharply. They found that the likelihood of natural climate shifts causing such a slowdown over the next 20 years is only 1 in 200. But when the authors accounted for human-induced global warming, they found that the odds jumped to 1 in 10 for corn and 1 in 20 for wheat. The study appears in this month’s issue of Environmental Research Letters. It was funded by the National Science Foundation (NSF), which is NCAR’s sponsor, and by the U.S. Department of Energy (DOE). More crops needed worldwide Global yields of crops such as corn and wheat have typically increased by about 1-2 percent per year in recent decades, and the U.N. Food and Agriculture Organization projects that global production of major crops will increase by 13 percent per decade through 2030—likely the fastest rate of increase during the coming century. However, global demand for crops is also expected to rise rapidly during the next two decades because of population growth, greater per-capita food consumption, and increasing use of biofuels. Lobell and Tebaldi set out to estimate the odds that climate change could interfere with the ability of crop producers to keep up with demand. Whereas other climate research had looked at the crop impacts that were most likely, Lobell and Tebaldi decided to focus on the less likely but potentially more dangerous scenario that climate change would reduce yield growth by 10 percent or more. The researchers used simulations available from an NCAR-based climate model (developed by teams of scientists with support from NSF and DOE), as well as several other models, to provide trends in temperature and precipitation over the next two decades for crop-intensive regions under a scenario of increasing carbon dioxide. They also used the same model simulations without human-caused increases in carbon dioxide to assess the same trends in a natural climate. In addition, they ran statistical analyses to estimate the impacts of changes in temperature and precipitation on wheat and corn yields in various regions of the globe and during specific times of the year that coincide with the most important times of the growing seasons for those two crops. The authors quantified the extent to which warming temperatures would correlate with reduced yields. For example, an increase of 1 degree Celsius (1.8 degrees Fahrenheit) would slow corn yields by 7 percent and wheat yields by 6 percent. Depending on the crop-growing region, the odds of such a temperature increase in the next 20 years were about 30 to 40 percent in simulations that included increases in carbon dioxide. In contrast, such temperature increases had a much lower chance of occurring in stimulations that included only natural variability, not human-induced climate change. Although society could offset the climate impacts by planting wheat and corn in cooler regions, such planting shifts to date have not occurred quickly enough to offset warmer temperatures, the study warned. The authors also found little evidence that other adaptation strategies, such as changes in crop varieties or growing practices would totally offset the impact of warming temperatures. “Although further study may prove otherwise, we do not anticipate adaptation being fast enough to significantly alter the near-term risks estimated in this paper,” they wrote. “We can’t predict whether a major slowdown in crop growth will actually happen, and the odds are still fairly low,” said Tebaldi. “But climate change has increased the odds to the point that organizations concerned with food security or global stability need to be aware of this risk.” About the article Title: Getting caught with our plants down: the risks of a global crop yield slowdown from climate trends in the next two decades Authors: David B. Lobell and Claudia Tebaldi Publication: Environmental Research Letters – doi:10.1088/1748-9326/9/7/074003

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