Climate & Climate Change

Quantifying the benefits of cutting carbon

September 18, 2017 | Perhaps tens of thousands of studies have detailed the possible impacts of a changing climate: the amount of sea level rise we can expect by the end of the century, for example, or the regions of the world that will likely experience more severe droughts.But rarely do these studies outline the actual benefits that society would gain from emitting fewer greenhouse gases instead of more. Instead, they often compare what is likely to happen in a particular scenario — such as emissions continuing unabated — to what the climate was like in the past."Most studies don't give us a good sense of what we have to gain by mitigating. What are the benefits of capping warming at 2 degrees Celsius instead of 3, for example?" said NCAR scientist Brian O'Neill. "That's important information for policy makers because mitigation has a price, and you need to understand the benefits to weigh them against the cost."O'Neill and dozens of colleagues from NCAR and universities across the country have recently wrapped up a major effort to wrestle with this problem. The results from the project — the Benefits of Reduced Anthropogenic Climate changE (BRACE) — are published in a special issue of the journal Climatic Change.For BRACE, scientists explored what the world would look like through 2100 in two different scenarios: a business-as-usual emissions scenario with a temperature increase of about 3.7 degrees Celsius over preindustrial levels and a moderate mitigation scenario where warming is reduced to about 2.5 degrees Celsius. By comparing the two worlds, they were able to calculate the benefits of moderate mitigation for a range of areas, from agriculture to health to extreme events.Additionally, the researchers looked at how different pathways of societal development — including population growth, economic growth, and technological development — might play a role or even outweigh the climate effects."It's important to analyze the effects of societal change alongside the effects of climate change," O'Neill said. "Societal change can matter as much as climate change. And in some cases, it matters more."The results of the studies vary significantly across sectors. When it comes to extreme heat, for example, the benefits of reducing emissions are clear and significant. In other areas, the findings are more nuanced. In agriculture, for example, the potential fertilizing effects of higher levels of carbon dioxide in the atmosphere and the ability of some farmers to adapt — by adding nitrogen fertilizer in regions where it isn't currently used, for example — could in some cases mute, or even cancel out, the benefits of mitigation.The BRACE project has studied the benefits of reducing greenhouse gas emissions in a number of areas, including extreme heat, mosquito-borne diseases, droughts, tropical cyclones, and agriculture. (Photo collage, UCAR.)Natural variability or climate change?For the studies, the scientists relied on two sets of simulations from the NCAR-based Community Earth System Model. For each set — one representing a future with no mitigation and the other a future with moderate mitigation — the scientists drew from more than a dozen simulations. Having an "ensemble" of model runs allowed them to better estimate extreme events, which occur rarely by definition and are therefore difficult to capture with just one or two simulations. It also allowed them to distinguish the effects of human-caused climate change from natural climate variability.In many cases, the ensembles allowed the scientists to demonstrate a substantial benefit of mitigating greenhouse gas emissions. One study found that extreme heat waves that today have just a one in 20 chance of occurring in any given year could become six to 20 times more common if greenhouse gas emissions continue unabated. In the moderate mitigation scenario, such heat waves would likely become just 2 to 4 times more common.Other studies of extreme heat showed similar benefits. For example, scientists found the likelihood that a single summer between 2060 and 2080 would be hotter than any in the historical record to be 80 percent if greenhouse gas emissions continue unabated. But that risk is cut in half, to 41 percent, with moderate mitigation.In some areas of study, however, natural variability still overwhelms the impact of climate change, making it difficult to measure the benefits of mitigation. This was true for several BRACE studies that looked at how tropical cyclones — and the damage they can cause — would differ between the two scenarios. The studies found that tropical cyclone damage (and number of storms) might actually increase if greenhouse gas emissions are reduced, though the intensity of storms might also be lessened.However, the storm findings were not statistically robust, meaning the data were too noisy to detect a clear signal."Tropical cyclones are highly variable, so to show a real difference between the present and the future, we would need to run the model many more times to accurately describe how present and future storms are different," said NCAR scientist Andrew Gettelman, who led one of the studies.Still, there are several plausible explanations for why hurricane activity might decrease if greenhouse gas emissions continue on their current trajectory. For example, uneven warming of the ocean basins could generate high-level winds, which make it more difficult for storms to form.However, storms might still be more intense due to increases in ocean temperature, and indeed Gettelman and colleagues found increases in frequency for the most intense landfalling storms, despite decreases in overall storm numbers.The BRACE researchers' work on this topic is made more difficult by the relatively short historical record that exists for hurricanes. Though observations of landfalling hurricanes stretch back as long as people have lived near coasts, a comprehensive record of how many cyclones form across the globe's oceans only began at the dawn of the satellite era in 1979. With only a few decades of data to create a baseline, it's difficult to detect future trends, Gettelman said.Additionally, global climate models are typically run at a resolution that is too coarse for individual cyclones to form. While hurricanes do form in climate models when run at a high resolution, it is not yet clear that those storms are behaving in a way that matches what would happen in the real world.The societal effect Even if climate models could accurately simulate tropical cyclones, it might still be difficult to determine the benefit, at least in terms of decreased damage, that might be realized if greenhouse gas emissions were cut. That's because societal choices also play an important role. For example, in the study led by Gettelman, the researchers found that any potential changes in storms and the damage they cause were swamped by the projected increase in societal vulnerability: more people and more buildings are projected to be near coastlines, in harm's way.This societal effect also dominated the climate effect in other areas studied for the BRACE project, including heat-related mortality in Houston, and total population exposure to the Aedes aegypti mosquito, which carries diseases including Zika, dengue, and chikungunya."When it comes to human health outcomes, climate change usually becomes an exacerbating factor," said NCAR scientist Andrew Monaghan, who led the study on Aedes aegypti. "Other important factors are governance decisions, socioeconomic development, and population growth trajectories."Monaghan and colleagues found that the amount of land that is climatically suitable for the mosquito to live would increase by 13 percent without any mitigation of greenhouse gases, compared to 8 percent in a moderate mitigation scenario. But the number of people who would be exposed to the mosquito would increase by as much as 130 percent as the population grows in regions where the mosquito can thrive.The societal effect may also be dominant in certain aspects of agriculture, according to the BRACE studies. Fertilizing or irrigating additional land could increase crop yields, for example. And crops that are particularly vulnerable to climate change could be replaced with hardier varieties.Agricultural uncertaintyThe BRACE studies that tried to quantify the agricultural benefits of mitigation also had to contend with another area of uncertainty: How much of a fertilizing effect will increased carbon dioxide in the atmosphere have on crop yield? Plants need carbon dioxide to grow, and greater availability of carbon dioxide can bolster crop yields. But there are other limiting factors at play, including available water, soil nutrients, and extreme heat.One BRACE study that included the fertilizing effects of increased carbon dioxide found that crop yields would grow by a global average of 11 percent if greenhouse gas emissions continue on their current trajectory. By comparison, the global average would increase just 6 percent with moderate mitigation."We've already observed a massive ramp-up in crop yields from the 1960s through the current day, due to changes in fertilizer, crop varieties, management, and carbon dioxide levels," said NCAR scientist Peter Lawrence, who worked on the agricultural aspects of the BRACE project. "The big question is what happens as we move forward. We know that our models can accurately represent the relationship between increased carbon dioxide and increased plant growth. But there are a lot of processes our models don't capture, including flooding and the effects of heat waves while plants are flowering."Other agricultural studies in the BRACE project that looked at specific crops found net positive benefits from reducing greenhouse gases. For example, one study found that climate change would decrease potential maize yields in the future (relative to what they would be in the absence of climate change). Maize yields would decrease by a global mean of 27 percent if emissions continue unabated vs. 15 percent with moderate mitigation. Another study found that the exposure of crops to damaging heat above a critical threshold for growth would be reduced by about a third in the moderate mitigation scenario compared to the scenario with no mitigation.The agricultural studies highlight the stubborn uncertainties that persist when trying to determine how crops will be affected in a warming world. And while this makes quantifying the benefits of mitigation more difficult, it also helps set the direction for future research.O'Neill says he hopes all of the more than 20 studies published so far in the BRACE framework will inform the direction of future work, including what questions need to be asked and what kinds of model simulations would prove most useful for finding answers."Even though the BRACE project involved dozens of scientists to research a broad range of sectors, it is really just a start," said O'Neill.About the articleTitle: The Benefits of Reduced Anthropogenic Cliamte changE (BRACE): a synthesisAuthors: Brian C. O’Neill1, James M. Done, Andrew Gettelman, Peter Lawrence, Flavio Lehner, Jean-Francois Lamarque, Lei Lin, Andrew J. Monaghan, Keith Oleson, Xiaolin Ren, Benjamin M. Sanderson, Claudia Tebaldi, Matthias Weitzel, Yangyang Xu, Brooke Anderson, Miranda J. Fix, and Samuel LevisJournal: Climatic Change DOI: 10.1007/s10584-017-2009-xCollaboratorsArgonne National LaboratoryBrigham Young UniversityCity University of New YorkColorado School of MinesColorado State UniversityJohns HopkinsIndian Space Research Organization, IndiaLanzhou University, ChinaLawrence Berkeley National LaboratoryNational Oceanic and Atmospheric AdministrationStanford UniversityStony Brook UniversitySun Yat-sen University, ChinaSwiss Federal Institute of TechnologyTexas A&MThe Climate CorporationUniversity of ArizonaUniversity of Colorado BoulderUniversity of Illinois UrbanaUniversity of WashingtonWriter/contact:Laura Snider, Senior Science Writer and Public Information Officer 

Dino-killing asteroid could have thrust Earth into two years of darkness

BOULDER, Colo. — Tremendous amounts of soot, lofted into the air from global wildfires following a massive asteroid strike 66 million years ago, would have plunged Earth into darkness for nearly two years, new research finds. This would have shut down photosynthesis, drastically cooled the planet, and contributed to the mass extinction that marked the end of the age of dinosaurs.These new details about how the climate could have dramatically changed following the impact of a 10-kilometer-wide asteroid will be published Aug. 21 in the Proceedings of the National Academy of Sciences. The study, led by the National Center for Atmospheric Research (NCAR) with support from NASA and the University of Colorado Boulder, used a world-class computer model to paint a rich picture of how Earth’s conditions might have looked at the end of the Cretaceous Period, information that paleobiologists may be able to use to better understand why some species died, especially in the oceans, while others survived.Scientists estimate that more than three-quarters of all species on Earth, including all non-avian dinosaurs, disappeared at the boundary of the Cretaceous-Paleogene periods, an event known as the K-Pg extinction. Evidence shows that the extinction occurred at the same time that a large asteroid hit Earth in what is now the Yucatán Peninsula. The collision would have triggered earthquakes, tsunamis, and even volcanic eruptions.Scientists also calculate that the force of the impact would have launched vaporized rock high above Earth's surface, where it would have condensed into small particles known as spherules. As the spherules fell back to Earth, they would have been heated by friction to temperatures high enough to spark global fires and broil Earth's surface. A thin layer of spherules can be found worldwide in the geologic record."The extinction of many of the large animals on land could have been caused by the immediate aftermath of the impact, but animals that lived in the oceans or those that could burrow underground or slip underwater temporarily could have survived," said NCAR scientist Charles Bardeen, who led the study. "Our study picks up the story after the initial effects — after the earthquakes and the tsunamis and the broiling. We wanted to look at the long-term consequences of the amount of soot we think was created and what those consequences might have meant for the animals that were left."Other study co-authors are Rolando Garcia and Andrew Conley, both NCAR scientists, and Owen “Brian” Toon, a researcher at the University of Colorado Boulder.An illustration of an asteroid impacting Earth. (Image courtesy NASA.)A world without photosynthesisIn past studies, researchers have estimated the amount of soot that might have been produced by global wildfires by measuring soot deposits still preserved in the geologic record. For the new study, Bardeen and his colleagues used the NCAR-based Community Earth System Model (CESM) to simulate the effect of the soot on global climate going forward. They used the most recent estimates of the amount of fine soot found in the layer of rock left after the impact (15,000 million tons), as well as larger and smaller amounts, to quantify the climate's sensitivity to more or less extensive fires.In the simulations, soot heated by the Sun was lofted higher and higher into the atmosphere, eventually forming a global barrier that blocked the vast majority of sunlight from reaching Earth's surface. “At first it would have been about as dark as a moonlit night," Toon said.While the skies would have gradually brightened, photosynthesis would have been impossible for more than a year and a half, according to the simulations. Because many of the plants on land would have already been incinerated in the fires, the darkness would likely have had its greatest impact on phytoplankton, which underpin the ocean food chain. The loss of these tiny organisms would have had a ripple effect through the ocean, eventually devastating many species of marine life.The research team also found that photosynthesis would have been temporarily blocked even at much lower levels of soot. For example, in a simulation using only 5,000 million tons of soot — about a third of the best estimate from measurements — photosynthesis would still have been impossible for an entire year.In the simulations, the loss of sunlight caused a steep decline in average temperatures at Earth's surface, with a drop of 50 degrees Fahrenheit (28 degrees Celsius) over land and 20 degrees Fahrenheit (11 degrees Celsius) over the oceans.While Earth's surface cooled in the study scenarios, the atmosphere higher up in the stratosphere actually became much warmer as the soot absorbed light from the Sun. The warmer temperatures caused ozone destruction and allowed for large quantities of water vapor to be stored in the upper atmosphere. The water vapor then chemically reacted in the stratosphere to produce hydrogen compounds that led to further ozone destruction. The resulting ozone loss would have allowed damaging doses of ultraviolet light to reach Earth's surface after the soot cleared.The large reservoir of water in the upper atmosphere formed in the simulations also caused the layer of sunlight-blocking soot to be removed abruptly after lingering for years, a finding that surprised the research team. As the soot began to settle out of the stratosphere, the air began to cool. This cooling, in turn, caused water vapor to condense into ice particles, which washed even more soot out of the atmosphere. As a result of this feedback loop — cooling causing precipitation that caused more cooling — the thinning soot layer disappeared in just a few months.Challenging the modelWhile the scientists think the new study gives a robust picture of how large injections of soot into the atmosphere can affect the climate, they also caution that the study has limitations.For example, the simulations were run in a model of modern-day Earth, not a model representing what Earth looked like during the Cretaceous Period, when the continents were in slightly different locations. The atmosphere 66 million years ago also contained somewhat different concentrations of gases, including higher levels of carbon dioxide.Additionally, the simulations did not try to account for volcanic eruptions or sulfur released from the Earth's crust at the site of the asteroid impact, which would have resulted in an increase in light-reflecting sulfate aerosols in the atmosphere.The study also challenged the limits of the computer model's atmospheric component, known as the Whole Atmosphere Community Climate Model (WACCM)."An asteroid collision is a very large perturbation — not something you would normally see when modeling future climate scenarios," Bardeen said. "So the model was not designed to handle this and, as we went along, we had to adjust the model so it could handle some of the event's impacts, such as warming of the stratosphere by over 200 degrees Celsius."These improvements to WACCM could be useful for other types of studies, including modeling a "nuclear winter" scenario. Like global wildfires millions of years ago, the explosion of nuclear weapons could also inject large amounts of soot into the atmosphere, which could lead to a temporary global cooling."The amount of soot created by nuclear warfare would be much less than we saw during the K-Pg extinction," Bardeen said. "But the soot would still alter the climate in similar ways, cooling the surface and heating the upper atmosphere, with potentially devastating effects."Writer:Laura Snider, Senior Science Writer

UCAR statement on U.S. withdrawal from Paris climate agreement

BOULDER, Colo. — President Donald Trump today announced he is withdrawing the United States from the Paris Agreement on climate change, a global pact signed by more than 190 countries to cut carbon dioxide emissions. He also said he would seek to renegotiate it or forge a new agreement. Antonio J. Busalacchi, the president of the University Corporation for Atmospheric Research (UCAR), issued the following statement:Today's decision to begin withdrawing from the Paris Agreement under its current terms creates new uncertainties about the future of our climate. At a time when our economic well-being and national security depend increasingly on accurate predictions of the impacts of greenhouse gas emissions, investments in climate research are even more necessary so scientists can project climate change in the new policy environment.Climate change poses major risks to food and water supplies, transportation systems, and other resources in the United States and worldwide. Rising temperatures and their impacts on weather patterns are creating additional stress at a time of international conflicts, endangering our economic and military security. If average global temperatures rise more than 2 degrees Celsius — the target of the Paris Agreement — research indicates that damaging impacts, such as sea level rise, intense heat waves and droughts, and shifts in weather patterns and storms will become more severe. With today’s decision, scientists will need to focus more attention on the potential ramifications of failing to curb emissions sufficiently to meet the 2-degree target.Nations are amassing information about future climate conditions as a necessary precondition for competing in the global marketplace. Multinational corporations are seeking to mitigate their exposure to climate risks, and if they cannot get the needed information from U.S.-funded research they will go elsewhere to get the most authoritative information. U.S. rivals, including China, are conducting vigorous climate research projects that support their economic and military investments and expand their influence worldwide. Even if the United States no longer participates in climate agreements, it cannot afford to cede climate knowledge to overseas competitors.Climate research is fundamentally nonpartisan. The work under way at the National Center for Atmospheric Research, in collaboration with our partners at government agencies, the university community, and the private sector, builds an evidence-based picture of the possible future impacts of climate change. As always, we stand ready to provide the results of our scientific inquiry to Congress and the administration in order to keep our nation secure and prosperous.Today's decision does not mean that climate change will go away. To the contrary, the heightened potential for increased greenhouse gas emissions poses a substantial threat to our communities, businesses, and military. The work by U.S. researchers — to understand and anticipate changes in our climate system and determine ways to mitigate or adapt to the potential impacts — is now more vital than ever.

Decades of data on world's oceans reveal a troubling oxygen decline

NCAR scientist Matthew Long is a co-author of a new study appearing in Geophysical Research Letters. This is an excerpt from a news release by Georgia Tech, a UCAR member institution. May 11, 2017 | A new analysis of decades of data on oceans across the globe has revealed that the amount of dissolved oxygen contained in the water – an important measure of ocean health – has been declining for more than 20 years.Researchers at Georgia Institute of Technology looked at a historic dataset of ocean information stretching back more than 50 years and searched for long term trends and patterns. They found that oxygen levels started dropping in the 1980s as ocean temperatures began to climb.“The oxygen in oceans has dynamic properties, and its concentration can change with natural climate variability,” said Taka Ito, an associate professor in Georgia Tech’s School of Earth and Atmospheric Sciences who led the research. “The important aspect of our result is that the rate of global oxygen loss appears to be exceeding the level of nature's random variability.”The study, which was published April in Geophysical Research Letters, was sponsored by the National Science Foundation and the National Oceanic and Atmospheric Administration. The team included researchers from the National Center for Atmospheric Research, the University of Washington-Seattle, and Hokkaido University in Japan.Read the full news release from Georgia Tech.Global map of the linear trend of dissolved oxygen at the depth of 100 meters. (Image courtesy Georgia Tech.) About the articleTitle: Upper ocean O2 trends: 1958–2015Authors: Takamitsu Ito, Shoshiro Minobe, Matthew C. Long, and Curtis DeutschJournal: Geophysical Research Letters, DOI: 10.1002/2017GL073613

Warmer temperatures cause decline in key runoff measure

BOULDER, Colo. — Since the mid-1980s, the percentage of precipitation that becomes streamflow in the Upper Rio Grande watershed has fallen more steeply than at any point in at least 445 years, according to a new study led by the National Center for Atmospheric Research (NCAR).While this decline was driven in part by the transition from an unusually wet period to an unusually dry period, rising temperatures deepened the trend, the researchers said.The study paints a detailed picture of how temperature has affected the runoff ratio — the amount of snow and rain that actually makes it into the river — over time, and the findings could help improve water supply forecasts for the Rio Grande, which is a source of water for an estimated 5 million people.The study results also suggest that runoff ratios in the Upper Rio Grande and other neighboring snow-fed watersheds, such as the Colorado River Basin, could decline further as the climate continues to warm.Sandhill cranes in the San Luis Valley of Colorado. The mountains ringing the valley form the headwaters of the Rio Grande River, which flows south into New Mexico and along the border between Texas and Mexico. (Photo courtesy of the National Park Service.)"The most important variable for predicting streamflow is how much it has rained or snowed," said NCAR scientist Flavio Lehner, lead author of the study. "But when we looked back hundreds of years, we found that temperature has also had an important influence  — which is not currently factored into water supply forecasts. We believe that incorporating temperature in future forecasts will increase their accuracy, not only in general but also in the face of climate change."The study, published in the journal Geophysical Research Letters, was funded by the Bureau of Reclamation, Army Corps of Engineers, National Oceanic and Atmospheric Administration (NOAA), and National Science Foundation, which is NCAR's sponsor.Co-authors of the paper are Eugene Wahl, of NOAA; Andrew Wood, of NCAR; and Douglas Blatchford and Dagmar Llewellyn, both of the Bureau of Reclamation.Over-predicting water supplyBorn in the Rocky Mountains of southern Colorado, the Rio Grande cuts south across New Mexico before hooking east and forming the border between Texas and Mexico. Snow piles up on the peaks surrounding the headwaters throughout the winter, and in spring the snowpack begins to melt and feed the river.The resulting streamflow is used both by farmers and cities, including Albuquerque, New Mexico, and El Paso, Texas, and water users depend on the annual water supply forecasts to determine who gets how much of the river. The forecast is also used to determine whether additional water needs to be imported from the San Juan River, on the other side of the Continental Divide, or pumped from groundwater.Current operational streamflow forecasts depend on estimates of the amount of snow and rain that have fallen in the basin, and they assume that a particular amount of precipitation and snowpack will always yield a particular amount of streamflow.In recent years, those forecasts have tended to over-predict how much water will be available, leading to over-allocation of the river. In an effort to understand this changing dynamic, Lehner and his colleagues investigated how the relationship between precipitation and streamflow, known as the runoff ratio, has evolved over time.Precipitation vs. streamflow: Tree rings tell a new storyThe scientists used tree ring-derived streamflow data from outside of the Upper Rio Grande basin to reconstruct estimates of precipitation within the watershed stretching back to 1571. Then they combined this information with a separate streamflow reconstruction within the basin for the same period. Because these two reconstructions were independent, it allowed the research team to also estimate runoff ratio for each year: the higher the ratio, the greater the share of precipitation that was actually converted into streamflow."For the first time, we were able to take these two quantities and use them to reconstruct runoff ratios over the past 445 years," Wahl said.They found that the runoff ratio varies significantly from year to year and even decade to decade. The biggest factor associated with this variation was precipitation. When it snows less over the mountains in the headwaters of the Rio Grande, not only is less water available to become streamflow, but the runoff ratio also decreases. In other words, a smaller percentage of the snowpack becomes streamflow during drier years.But the scientists also found that another factor affected the runoff ratio: temperature. Over the last few centuries, the runoff ratio was reduced when temperatures were warmer. And the influence of temperature strengthened during drier years: When the snowpack was shallow, warm temperatures reduced the runoff ratio more than when the snowpack was deep, further exacerbating drought conditions. The low runoff ratios seen in dry years were two and a half to three times more likely when temperatures were also warmer."The effect of temperature on runoff ratio is relatively small compared to precipitation," Lehner said. "But because its greatest impact is when conditions are dry, a warmer year can make an already bad situation much worse."A number of factors may explain the influence of temperature on runoff ratio. When it's warmer, plants take up more water from the soil and more water can evaporate directly into the air. Additionally, warmer temperatures can lead snow to melt earlier in the season, when the days are shorter and the angle of the sun is lower. This causes the snow to melt more slowly, allowing the meltwater to linger in the soil and giving plants added opportunity to use it.The extensive reconstruction of historical runoff ratio in the Upper Rio Grande also revealed that the decline in runoff ratio over the last three decades is unprecedented in the historical record. The 1980s were an unusually wet period for the Upper Rio Grande, while the 2000s and 2010s have been unusually dry. Pair that with an increase in temperatures over the same period, and the decline in runoff ratio between 1986 and 2015 was unlike any other stretch of that length in the last 445 years.The graph shows changes to runoff ratio in the Upper Rio Grande over time. (Image courtesy Flavio Lehner, NCAR.) Upgrading the old approachesThis new understanding of how temperature influences runoff ratio could help improve water supply forecasts, which do not currently consider whether the upcoming months are expected to be hotter or cooler than average. The authors are now assessing the value of incorporating seasonal temperature forecasts into water supply forecasts to account for these temperature influences. The study complements a multi-year NCAR project funded by the Bureau of Reclamation and the Army Corps of Engineers that is evaluating prospects for enhancing seasonal streamflow forecasts for reservoir management.“Forecast users and stakeholders are increasingly raising questions about the reliability of forecasting techniques if climate is changing our hydrology," said Wood, who led the effort. "This study helps us think about ways to upgrade one of our oldest approaches — statistical water supply forecasting — to respond to recent trends in temperature. Our current challenge is to find ways to make sure the lessons of this work can benefit operational streamflow forecasts.” Because the existing forecasting models were calibrated on conditions in the late 1980s and 1990s, it's not surprising that they over-predicted streamflow in the drier period since 2000, Lehner said."These statistical models often assume that the climate is stable," Lehner said. "It's an assumption that sometimes works, but statistical forecasting techniques will struggle with any strong changes in hydroclimatology from decade to decade, such as the one we have just experienced."Lehner is a Postdoc Applying Climate Expertise (PACE) fellow, which is part of the Cooperative Programs for the Advancement of Earth System Science (CPAESS). CPAESS is a community program of the University Corporation for Atmospheric Research (UCAR).About the articleTitle: Assessing recent declines in Upper Rio Grande River runoff efficiency from a paleoclimate perspectiveAuthors: Flavio Lehner, Eugene R. Wahl, Andrew W. Wood, Douglas B. Blatchford, and Dagmar LlewellynJournal: Geophysical Research Letters, DOI: 10.1002/2017GL073253Writer:Laura Snider, Senior Science Writer and Public Information Officer

Building roads to match tomorrow's weather

April 20, 2017 | When engineers design roads, bridges, and other types of transportation infrastructure, they need to account for local weather patterns. Extreme heat or freeze-thaw cycles can lead to ruts and cracks in roads, and heavy rains can overwhelm inadequate drainage systems, washing out bridges and flooding key transportation corridors.But how should engineers design new transportation projects, which may last for a half-century, if climate change will greatly alter weather patterns? The extent to which temperatures and precipitation may change in the future has become a major concern for the transportation industry.To address this issue, climate scientists at the National Center for Atmospheric Research (NCAR) are launching an innovative collaboration with civil and environmental engineers at Carnegie Mellon University and the RAND Corporation. They're using global and regional computer models, along with statistical techniques, to generate projections of future climate in ways that will be most helpful to infrastructure designers and planners, especially when it comes to drainage.A girl looks at a washed-out road in Louisville, Colorado, after damaging floods in 2013. Engineers are teaming up with climate scientists to design transportation infrastructure that can withstand shifting weather patterns. (Photo by David Hosansky.)The three-year project, funded by the National Science Foundation, will focus on Pittsburgh and several other cities across the country that will likely be affected in different ways by future climate."Our overriding goal is to enable transportation agencies to maximize the lifetime performance of new infrastructure while minimizing the costs to ensure its resilience to extreme weather events," said NCAR senior scientist Linda Mearns, the principal investigator on the project.Several recent studies led by NCAR scientists have underscored the extent to which climate change may affect future temperature and precipitation extremes in the United States. One concluded that, if emissions of greenhouse gases continue along a business-as-usual course, record daily high temperatures will outpace record daily lows by about 15 to 1 later in the century. A second study, also looking at emissions continuing on a business-as-usual path, concluded that incidents of extreme rainfall may increase by as much as five times in parts of the country.More detail means more uncertaintyTo conduct the new project, Mearns and her colleagues are working closely with local transportation officials and other stakeholders. Rather than analyzing the overall ways that climate is likely to change in the target cities, they're focusing on information that will be most useful to the design and construction of drainage infrastructure and other transportation systems."This requires very active engagement with stakeholders," Mearns said. "It's working together to determine what they want versus what we can actually provide and coming up with measures of uncertainty that are meaningful for them. This is in the realm of true coproduction of knowledge."For example, an engineer designing a drainage system along a highway might want an estimate of how much precipitation will fall in 15-minute increments. Although climate models do not provide such detailed information, Mearns and her colleagues can provide a partial answer by using a combination of techniques to produce projections of future precipitation every hour to several hours, as well as characterizing the uncertainty around those projections.A major challenge is that more detailed projections have greater uncertainty. While climate models consistently show that emissions of greenhouse gases lead to higher average global temperatures, the outlook is less clear for temperature and precipitation patterns by region. The type of information most needed by infrastructure planners and designers—projections of extreme temperatures and precipitation for specific locations and time periods—is even more uncertain. As a result, the study team will have to make compromises between the need for high-resolution data and the need for reliable data.Mearns said it's critical to give engineers a clear understanding of the uncertainty of a particular projection in order to prevent transportation projects from being based on a false sense of precision in climate projections. "The challenge," she said, "is developing sound engineering strategies for extremes under uncertainty."In addition to Mearns, the NCAR scientists working on the project include Seth McGinnis, Melissa Bukovsky, Rachel McCrary, and Doug Nychka. The Carnegie Mellon team is being led by Costa Samaras, who directs the school's Center for Engineering and Resilience for Climate Adaptation.“This project is a unique interdisciplinary collaboration that will advance the ways engineers and climate scientists will work together in the future,” said Samaras. “Infrastructure can last for many decades, and engineers need to design infrastructure to be resilient at the end of the infrastructure life span as well as in the beginning. Working with NCAR is critical to advancing the research needed to transform the way we design infrastructure in the United States."The benefit of different techniquesTo generate climate projections, Mearns and her colleagues will use two types of techniques to translate the coarse resolution of a global computer model, which typically simulates climate processes that are larger than about 100 miles, into the localized weather events that are of interest to transportation experts.One of these techniques, known as dynamical downscaling, will use a combination of three coarser-resolution global climate models and two higher-resolution regional models (including the NCAR-based Weather Research and Forecasting model, or WRF). This will enable the researchers to simulate the entire globe in coarse resolution while zooming in on selected regions with much higher resolution. This approach doesn't need as much supercomputing power as trying to simulate the entire globe in high resolution, although it still can be computationally intensive.The other technique, known as statistical downscaling, involves developing statistical relationships between large-scale atmospheric patterns and local temperatures and precipitation. This technique, which requires even less computing, can help scientists link conditions in a global model (such as a large area of low pressure) to a localized weather event (such as intermittent downpours).The combined approaches will enable the scientists to generate projections for at least every six hours, and possibly—with the use of additional specialized techniques—as frequently as every hour. Using both the dynamical and statistical approaches also will enable the team to better understand the uncertainties around future climate as well as evaluate the relative strengths of the techniques."Transportation systems are critical to the U.S. economy, and they represent some of the largest investments of our tax dollars," Mearns said. "We want to make sure that they'll hold up to a future climate."FunderNational Science FoundationPartnersCarnegie Mellon UniversityRAND CoroporationWriter/contactDavid Hosansky, Manager of Media Relations

Scientists link recent California droughts and floods to distinctive atmospheric waves

BOULDER, Colo. — The crippling wintertime droughts that struck California from 2013 to 2015, as well as this year's unusually wet California winter, appear to be associated with the same phenomenon: a distinctive wave pattern that emerges in the upper atmosphere and circles the globe.Scientists at the National Center for Atmospheric Research (NCAR) found in a recent study that the persistent high-pressure ridge off the west coast of North America that blocked storms from coming onshore during the winters of 2013-14 and 2014-15 was associated with the wave pattern, which they call wavenumber-5. Follow-up work showed that wavenumber-5 emerged again this winter but with its high- and low-pressure features in a different position, allowing drenching storms from the Pacific to make landfall. "This wave pattern is a global dynamic system that sometimes makes droughts or floods in California more likely to occur," said NCAR scientist Haiyan Teng, lead author of the California paper. "As we learn more, this may eventually open a new window to long-term predictability." The high- and low-pressure regions of wavenumber-5 set up in different locations during January 2014, when California was enduring a drought, and January 2017, when it was facing floods. The location of the high and low pressure regions (characterized by anticylonic vs. cyclonic upper-level air flow) can act to either suppress or enhance precipitation and storms. The black curves illustrate the jet streams that trap and focus wavenumber-5. (Image by Haiyan Teng and Grant Branstator, ©UCAR. This image is freely available for media & nonprofit use.)  The finding is part of an emerging body of research into the wave pattern that holds the promise of better understanding seasonal weather patterns in California and elsewhere. Another new paper, led by NCAR scientist Grant Branstator, examines the powerful wave pattern in more depth, analyzing the physical processes that help lead to its formation as well as its seasonal variations and how it varies in strength and location.The California study was published in the Journal of Climate while the comprehensive study into the wave patterns is appearing in the Journal of the Atmospheric Sciences. Both papers were funded by the National Science Foundation, which is NCAR's sponsor, as well as by the Department of Energy, the National Oceanic and Atmospheric Administration, and NASA.The new papers follow a 2013 study by Teng and Branstator showing that a pattern related to wavenumber-5 tended to emerge about 15-20 days before major summertime heat waves in the United States.Strong impacts on local weather systemsWavenumber-5 consists of five pairs of alternating high- and low-pressure features that encircle the globe about six miles (10 kilometers) above the ground. It is a type of atmospheric phenomenon known as a Rossby wave, a very large-scale planetary wave that can have strong impacts on local weather systems by moving heat and moisture between the tropics and higher latitudes as well as between oceanic and inland areas and by influencing where storms occur.The slow-moving Rossby waves at times become almost stationary. When they do, the result can be persistent weather patterns that often lead to droughts, floods, and heat waves. Wavenumber-5 often has this stationary quality when it emerges during the northern winter, and, as a result, is associated with a greater likelihood of persistent extreme events.To determine the degree to which the wave pattern influenced the California drought, Teng and Branstator used three specialized computer models, as well as California rainfall records and 20th century data about global atmospheric circulation patterns. The different windows into the atmosphere and precipitation patterns revealed that the formation of a ridge by the California coast is associated with the emergence of the distinctive wavenumber-5 pattern, which guides rain-producing low-pressure systems so that they travel well north of California.Over the past winter, as California was lashed by a series of intense storms, wavenumber-5 was also present, the scientists said. But the pattern had shifted over North America, replacing the high-pressure ridge off the coast with a low-pressure trough. The result was that the storms that were forced north during the drought winters were, instead, allowed to make landfall.Clues to seasonal weather patternsForecasters who predict seasonal weather patterns have largely looked to shifting sea surface temperatures in the tropical Pacific, especially changes associated with El Niño and La Niña. But during the dry winters of 2013-14 and 2014-15, those conditions varied markedly: one featured the beginning of an El Niño while the sea surface temperatures during the other were not characteristic of either El Niño or La Niña.The new research indicates that the wave pattern may provide an additional source of predictability that sometimes may be more important than the impacts of sea surface temperature changes. First, however, scientists need to better understand why and when the wave pattern emerges.In the paper published in Journal of the Atmospheric Sciences, Branstator and Teng explored the physics of the wave pattern. Using a simplified computer model of the climate system to identify the essential physical processes, the pair found that wavenumber-5 forms when strong jet streams act as wave guides, tightening the otherwise meandering Rossby wave into the signature configuration of five highs and five lows."The jets act to focus the energy," Branstator said. "When the jets are present, the energy is trapped and cannot escape." But even when the jets are present, the wavenumber-5 pattern does not always form, indicating that other forces requiring study are also at play.The scientists also searched specifically for what might have caused the wave pattern linked to the severe California drought to form. In the paper published in the Journal of Climate, the pair found that extremely heavy rainfall from December to February in certain regions of the tropical Pacific could double the probability that the extreme ridge associated with wavenumber-5 will form. The reason may have to do with the tropical rain heating parts of the upper atmosphere in such a way that favors the formation of the wavenumber-5 pattern.But the scientists cautioned that many questions remain."We need to search globally for factors that cause this wavenumber-5 behavior," Teng said, "Our studies are just the beginning of that search."About the articlesTitle: Causes of Extreme Ridges That Induce California DroughtAuthors: Haiyan Teng and Grant BranstatorJournal: Journal of Climate, DOI: 10.1175/JCLI-D-16-0524.1
Title: Tropospheric Waveguide Teleconnections and Their SeasonalityAuthors: Grant Branstator and Haiyan TengJournal: Journal of the Atmospheric Sciences, DOI: 10.1175/JAS-D-16-0305.1Writer:David Hosansky, Manager of Media Relations

New estimate of ocean heat finds more warming

March 10, 2017 | The oceans may be storing 13 percent more heat than previously estimated, according to a new study co-authored by scientists at the National Center for Atmospheric Research (NCAR).The finding, published in the journal Science Advances, is based on a new analysis of how ocean temperatures have changed since 1960. The research team, led by Lijing Cheng of the Chinese Academy of Sciences, compared their results to estimates published in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change in 2013."In other words, the planet is warming quite a lot more than we thought," said NCAR scientist Kevin Trenberth, a study co-author.The vast majority of excess heat trapped on Earth by greenhouse gas emissions — about 90 percent — is stored in the oceans, but measuring how the heat content of the oceans has changed over time has been a challenge due to sparse observations.The deployment of an Argo float off a research vessel. Data collected from Argo floats were used to validate a new estimate of ocean heat content. (Image courtesy of the Australian Commonwealth Scientific and Industrial Research Organization.)Historically, the temperature of ocean waters was measured by a variety of ships, but this limited observations to areas where ships traveled. In more recent decades, measurements of ocean heat have increased, thanks to new observational techniques. In 2000, scientists began deploying a network of thousands of floats called Argo to profile conditions in the top layer of the ocean extending down 2,000 meters (6,562 feet). Argo achieved near global coverage in 2005, though some remote regions are still not sampled.To fill the large gaps in the historical ocean temperature record, the research team used a combination of statistical techniques and model output to determine how useful a single observation can be for inferring information about the surrounding area, as well as how the temperatures in different parts of the world's oceans relate to one another. They found that, in most regions, a single ocean observation could provide valuable information about conditions as far as 2,000 kilometers (1,243 miles) away.To check if they were correct, they used Argo observations. At first, they chose data from only a small number of floats in the network to mimic the scarcity of observations that would have been available in the mid-20th century. Then they used their new technique to create an entire ocean temperature map based on those few observations. When they checked their map against the full complement of Argo observations, they found that their reconstruction tracked closely with reality. "The results were remarkable," Trenberth said. "They give us much more confidence about what the ocean heat content was, stretching back to the late 1950s."The results allowed the team to estimate the total warming between 1960 and 2005 to be 337 zettajoules (a measure of energy). They also found that changes were small until 1980, when the amount of heat stored in the oceans began to steadily increase. Since 1990, significant amounts of heat have begun to seep deeper into the ocean layers. NCAR scientist and co-author John Fasullo said the study also highlights the impact of improved observations and models, which are giving scientists important insights into what the world once looked like."Science not only looks toward the future, but is also continually trying to make sense of the past," he said. "This work is an example of how advances in technology have enabled an improved understanding of past changes in the ocean, where variability has always been a bit of an enigma due to its vastness and depth. The insights associated with this work change not only our understanding of past climate but also how future changes might unfold."The other co-authors are Tim Boyer, of the National Oceanic and Atmospheric Administration; John Abraham, of the University of St. Thomas; and Jiang Zhu, of the Chinese Academy of Sciences.About the articleTitle: Improved estimates of ocean heat content from 1960 to 2015Authors: Lijing Cheng, Kevin Trenberth, John Fasullo, Tim Boyer, John Abraham, and Jiang ZhuJournal: Science Advances, DOI: 10.1126/sciadv.1601545Writer/contactLaura Snider, Senior Science Writer and Public Information Officer FundersChinese Academy of SciencesNational Science Foundation of ChinaU.S. Department of EnergyNational Science FoundationNASA CollaboratorsChinese Academy of SciencesNational Oceanic and Atmospheric AdministrationUniversity of St. Thomas

UCAR staff add climate storybook to Elementary GLOBE's line-up

March 2, 2017 | In a new illustrated storybook, a group of school children travel with a scientist to Greenland and the Maldives to learn about tools used to study climate change and its impacts. After seeing the challenge of melting glaciers and rising seas, the students come back with ideas on how to reduce their own greenhouse emissions.What in the World is Happening to Our Climate? introduces new material to a series of children's adventure science books published by Elementary GLOBE (part of the Global Learning and Observations to Benefit the Environment program).The newest storybook, funded by NASA Langley Research Center, is the product of a partnership between staff in two University Corporation for Atmospheric Research programs: the GLOBE Implementation Office and the UCAR Center for Science Education, or SciEd. SciEd supports the education and outreach efforts of the National Center for Atmospheric Research (NCAR), which UCAR manages with sponsorship by the National Science Foundation.The climate book is available for download at no charge:Becca Hatheway, SciEd's manager of teaching and learning, said NASA asked UCAR a couple of years ago to create educational resources for children in advance of the installation of the Sage III instrument on the International Space Station to measure ozone and aerosols in Earth’s atmosphere. (Sage III was installed last month).The result was What's Up in the Atmosphere: Exploring Colors in the Sky, a storybook featuring children who learn about the colors of the sky and their relationship to air quality through observations and photos. Hatheway and Kerry Zarlengo, a former elementary school teacher and literacy coach, wrote the book in 2015.During discussions about the air quality project, "we pitched the idea of doing a climate change book as well, and NASA was supportive," Hatheway said. "We've always wanted to do one on this topic — it's in the NCAR wheelhouse."UCAR's Elementary GLOBE's new climate storybook is geared to children in grades K-4. (©UCAR. Illustration by Lisa Gardiner. This image is freely available for media & nonprofit use.)Hatheway co-wrote the text for the climate book with Diane Stanitski, a deputy director at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder. The Elementary GLOBE series, which now numbers seven storybooks, is aimed at introducing K-4 students to Earth system science. The first five books focus on clouds, water, phenology, soils, and the Earth system. NASA is funding an update of those books, some of which are more than a decade old.Books are field tested by teachers, and the modules come with learning activities and a teacher's guide and glossary. The idea is that younger children will be guided in the reading and activities, while older children can learn more independently.Most of the storylines focus on a group of school children who go on adventures to learn and collect data about a topic.Lisa Gardiner, whose role at UCAR includes developing educational resources, has illustrated all of the books in the series. She said the climate book holds special meaning for her."It's at the root of what we do at SciEd," Gardiner said. "A lot of young kids want to know about climate change, but there aren't that many resources for their age group."Gardiner said she tries to make her illustrations as realistic as possible. To learn more about the Maldives, Gardiner asked Alison Rockwell of NCAR's Earth Observing Laboratory for photos from a field campaign several years ago. "I wanted to know what the houses looked like, what the people were wearing."The activities are realistic, too. The climate book's activities include building a model of a coastal community, predicting which features would be at risk of flooding, and then "flooding" the model to see the results.Children learning about wind energy in the new Elementary GLOBE climate storybook. (©UCAR. Illustration by Lisa Gardiner. This image is freely available for media & nonprofit use.)Julie Malmberg, a GLOBE project manager, said the storybooks and learning activities can be downloaded for free, or educators can purchase a hard copy of the entire module for the cost of the printing and binding. She has heard from school officials, such as one in a West Virginia district, using the resources for grade-school teacher training.Most educators, Malmberg said, download the materials. Between 2012-2016, GLOBE recorded 42,533 storybook downloads and 54,197 downloads of learning activities. Do You Know Clouds Have Names, co-authored with NCAR Senior Scientist Emerita Peggy LeMone, is the most popular storybook, while the most popular learning activities are connected to a book called The Scoop on Soils.Hatheway said SciEd plans to provide copies of the climate change and sky color books to teachers who attend its professional development workshops or programs at the Mesa Lab, as well as at conferences SciEd staffers attend. NOAA plans to distribute the climate book at the National Science Teachers Association conference this spring.While the storybooks were developed for the educational community in the U.S., some have been translated into other languages and distributed by GLOBE partners in other countries.The GLOBE Program is an international science and education program that provides students and the public worldwide with the opportunity to participate in the scientific process and contribute to understanding of the Earth system and global environment.Writer/contactJeff Smith, Science Writer and Public Information Officer   

Slower snowmelt in a warming world

BOULDER, Colo. — As the world warms, mountain snowpack will not only melt earlier, it will also melt more slowly, according to a new study by scientists at the National Center for Atmospheric Research (NCAR).The counterintuitive finding, published today in the journal Nature Climate Change, could have widespread implications for water supplies, ecosystem health, and flood risk."When snowmelt shifts earlier in the year, the snow is no longer melting under the high sun angles of late spring and early summer," said NCAR postdoctoral researcher Keith Musselman, lead author of the paper. "The Sun just isn't providing enough energy at that time of year to drive high snowmelt rates."Snowpack in the Colorado Rockies as seen from the NSF/NCAR C-130 research aircraft. (©UCAR. Photo by Carlye Calvin. This image is freely available for media & nonprofit use.)The study was funded by the National Science Foundation, NCAR's sponsor.The findings could explain recent research that suggests the average streamflow in watersheds encompassing snowy mountains may decline as the climate warms — even if the total amount of precipitation in the watershed remains unchanged. That's because the snowmelt rate can directly affect streamflow. When snowpack melts more slowly, the resulting water lingers in the soil, giving plants more opportunity to take up the moisture. Water absorbed by plants is water that doesn't make it into the stream, potentially reducing flows.Musselman first became interested in how snowmelt rates might change in the future when he was doing research in the Sierra Nevada. He noticed that shallower, lower-elevation snowpack melted earlier and more slowly than thicker, higher-elevation snowpack. The snow at cooler, higher elevations tended to stick around until early summer — when the Sun was relatively high in the sky and the days had grown longer — so when it finally started to melt, the melt was rapid.Musselman wondered if the same phenomenon would unfold in a future climate, when warmer temperatures are expected to transform higher-elevation snowpack into something that looks much more like today's lower-elevation snowpack. If so, the result would be more snow melting slowly and less snow melting quickly. To investigate the question, Musselman first confirmed what he'd noticed in the Sierra by analyzing a decade's worth of snowpack observations from 979 stations in the United States and Canada. He and his co-authors — NCAR scientists Martyn Clark, Changhai Liu, Kyoko Ikeda, and Roy Rasmussen — then simulated snowpack over the same decade using the NCAR-based Weather Research and Forecasting (WRF) model.Once they determined that the output from WRF tracked with the observations, they used simulations from the model to investigate how snowmelt rates might change in North America around the end of the century if climate change continues unabated."We found a decrease in the total volume of meltwater — which makes sense given that we expect there to be less snow overall in the future," Musselman said. "But even with this decrease, we found an increase in the amount of water produced at low melt rates and, on the flip side, a decrease in the amount of water produced at high melt rates."While the study did not investigate the range of implications that could come from the findings, Musselman said the impacts could be far-reaching. For example, a reduction in high melt rates could mean fewer spring floods, which could lower the risk of infrastructure damage but also negatively affect riparian ecosystems. Changes in the timing and amount of snowmelt runoff could also cause warmer stream temperatures, which would affect trout and other fish species, and the expected decrease in streamflow could cause shortages in urban water supplies."We hope this study motivates scientists from many other disciplines to dig into our research so we can better understand the vast implications of this projected shift in hydrologic patterns," Musselman said.About the articleTitle: Slower snowmelt in a warmer worldAuthors: Keith N. Musselman, Martyn P. Clark, Changhai Liu, Kyoko Ikeda, and Roy RasmussenJournal: Nature Climate Change, DOI: 10.1038/nclimate3225WriterLaura Snider, Senior Science Writer and Public Information OfficerFunderNational Science Foundation


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