Climate & Climate Change

Climate change already accelerating sea level rise, study finds

BOULDER, Colo. — Greenhouse gases are already having an accelerating effect on sea level rise, but the impact has so far been masked by the cataclysmic 1991 eruption of Mount Pinatubo in the Philippines, according to a new study led by the National Center for Atmospheric Research (NCAR).Satellite observations, which began in 1993, indicate that the rate of sea level rise has held fairly steady at about 3 millimeters per year. But the expected acceleration due to climate change is likely hidden in the satellite record because of a happenstance of timing: The record began soon after the Pinatubo eruption, which temporarily cooled the planet, causing sea levels to drop.The new study finds that the lower starting point effectively distorts the calculation of sea level rise acceleration for the last couple of decades.The study lends support to climate model projections, which show the rate of sea level rise escalating over time as the climate warms. The findings were published today in the open-access Nature journal Scientific Reports.Mount Pinatubo's caldera on June 22, 1991. (Image courtesy USGS.)"When we used climate model runs designed to remove the effect of the Pinatubo eruption, we saw the rate of sea level rise accelerating in our simulations," said NCAR scientist John Fasullo, who led the study. "Now that the impacts of Pinatubo have faded, this acceleration should become evident in the satellite measurements in the coming decade, barring another major volcanic eruption."Study co-author Steve Nerem, from the University of Colorado Boulder, added: “This study shows that large volcanic eruptions can significantly impact the satellite record of global average sea level change. So we must be careful to consider these effects when we look for the effects of climate change in the satellite-based sea level record."The findings have implications for the extent of sea level rise this century and may be useful to coastal communities planning for the future. In recent years, decision makers have debated whether these communities should make plans based on the steady rate of sea level rise measured in recent decades or based on the accelerated rate expected in the future by climate scientists.The study was funded by NASA, the U.S. Department of Energy, and the National Science Foundation, which is NCAR's sponsor.Reconstructing a pre-Pinatubo worldClimate change triggers sea level rise in a couple of ways: by warming the ocean, which causes the water to expand, and by melting glaciers and ice sheets, which drain into the ocean and increase its volume. In recent decades, the pace of warming and melting has accelerated, and scientists have expected to see a corresponding increase in the rate of sea level rise. But analysis of the relatively short satellite record has not borne that out.To investigate, Fasullo, Nerem, and Benjamin Hamlington of Old Dominion University worked to pin down how quickly sea levels were rising in the decades before the satellite record began.Prior to the launch of the international TOPEX/Poseidon satellite mission in late 1992, sea level was mainly measured using tide gauges. While records from some gauges stretch back to the 18th century, variations in measurement technique and location mean that the pre-satellite record is best used to get a ballpark estimate of global mean sea level.Mount Pinatubo erupting in 1991. (Image courtesy USGS.)To complement the historic record, the research team used a dataset produced by running the NCAR-based Community Earth System Model 40 times with slightly different—but historically plausible—starting conditions. The resulting simulations characterize the range of natural variability in the factors that affect sea levels. The model was run on the Yellowstone system at the NCAR-Wyoming Supercomputing Center.A separate set of model runs that omitted volcanic aerosols — particles spewed into the atmosphere by an eruption — was also assessed. By comparing the two sets of runs, the scientists were able to pick out a signal (in this case, the impact of Mount Pinatubo's eruption) from the noise (natural variations in ocean temperature and other factors that affect sea level)."You can't do it with one or two model runs—or even three or four," Fasullo said. "There's just too much accompanying climate noise to understand precisely what the effect of Pinatubo was. We could not have done it without large numbers of runs."Using models to understand observationsAnalyzing the simulations, the research team found that Pinatubo's eruption caused the oceans to cool and sea levels to drop by about 6 millimeters immediately before TOPEX/Poseidon began recording observations.As the sunlight-blocking aerosols from Mount Pinatubo dissipated in the simulations, sea levels began to slowly rebound to pre-eruption levels. This rebound swamped the acceleration caused by the warming climate and made the rate of sea level rise higher in the mid- to late 1990s than it would otherwise have been.This higher-than-normal rate of sea level rise in the early part of the satellite record makes it appear that the rate of sea level rise has not accelerated over time and may actually have decreased somewhat. In fact, according to the study, if the Pinatubo eruption had not occurred—leaving sea level at a higher starting point in the early 1990s—the satellite record would have shown a clear acceleration."The satellite record is unable to account for everything that happened before the first satellite was launched, " Fasullo said. "This study is a great example of how computer models can give us the historical context that's needed to understand some of what we're seeing in the satellite record."Understanding whether the rate of sea level rise is accelerating or remaining constant is important because it drastically changes what sea levels might look like in 20, 50, or 100 years.“These scientists have disentangled the major role played by the 1991 volcanic eruption of Mt. Pinatubo on trends in global mean sea level,” said Anjuli Bamzai, program director in the National Science Foundation’s Division of Atmospheric and Geospace Sciences, which funded the research.  “This research is vital as society prepares for the potential effects of climate change."Because the study's findings suggest that acceleration due to climate change is already under way, the acceleration should become evident in the satellite record in the coming decade, Fasullo said.Since the original TOPEX/Poseidon mission, other satellites have been launched—Jason-1 in 2001 and Jason-2 in 2008—to continue tracking sea levels. The most recent satellite, Jason-3, launched on Jan. 17 of this year."Sea level rise is potentially one of the most damaging impacts of climate change, so it's critical that we understand how quickly it will rise in the future," Fasullo said. "Measurements from Jason-3 will help us evaluate what we've learned in this study and help us better plan for the future."The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under sponsorship by the National Science Foundation. Any opinions, findings and conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.The graph shows how sea level rises and falls as ocean heat content fluctuates. After volcanic eruptions, the Earth cools and, in turn, the heat content in the ocean drops, ultimately lowering sea level.The solid blue line is the average sea level rise of climate model simulations that include volcanic eruptions. The green line is the average from model simulations with the effect of volcanic eruptions removed, and it shows a smooth acceleration in the rate of sea level rise due to climate change.The blue line between the start of the satellite record and present day makes a relatively straight line — just as we see from actual satellite observations during that time —  indicating that the rate of sea level rise has not accelerated. But in the future, barring another major volcanic eruption, scientists expect sea level to follow the gray dotted line, which is on the same accelerating path as the green line below it. Click to enlarge. (©UCAR. This graph is freely available for media & nonprofit use.) About the articleTitle: Is the detection of sea level rise imminent?Authors: J.T. Fasullo, R. S. Nerem, and B. HamlingtonJournal: Scientific Reports, DOI: 10.1038/srep31245 Funders:  NASANational Science FoundationU.S. Department of Energy Collaborators: Univesity of Colorado Boulder (UCAR member)Old Dominion University (UCAR member)Writer:Laura Snider, Senior Science Writer and Public Information Officer

Expanding Antarctic sea ice linked to natural variability

BOULDER — The recent trend of increasing Antarctic sea ice extent — seemingly at odds with climate model projections — can largely be explained by a natural climate fluctuation, according to a new study led by the National Center for Atmospheric Research (NCAR). The study offers evidence that the negative phase of the Interdecadal Pacific Oscillation (IPO), which is characterized by cooler-than-average sea surface temperatures in the tropical eastern Pacific, has created favorable conditions for additional Antarctic sea ice growth since 2000. The findings, published in the journal Nature Geoscience, may resolve a longstanding mystery: Why is Antarctic sea ice expanding when climate change is causing the world to warm? The study's authors also suggest that sea ice may begin to shrink as the IPO switches to a positive phase. "The climate we experience during any given decade is some combination of naturally occurring variability and the planet's response to increasing greenhouse gases," said NCAR scientist Gerald Meehl, lead author of the study. "It's never all one or the other, but the combination, that is important to understand." Study co-authors include Julie Arblaster of NCAR and Monash University in Australia, Cecilia Bitz of the University of Washington, Christine Chung of the Australian Bureau of Meteorology, and NCAR scientist Haiyan Teng. The study was funded by the U.S. Department of Energy and by the National Science Foundation, which sponsors NCAR. On Sept. 19, 2014, the five-day average of Antarctic sea ice extent exceeded 20 million square kilometers (about 7.7 million square miles) for the first time since 1979, according to the National Snow and Ice Data Center. The red line shows the average maximum extent from 1979-2014. (Image courtesy NASA's Scientific Visualization Studio/Cindy Starr) Expanding ice The sea ice surrounding Antarctica has been slowly increasing in area since the satellite record began in 1979. But the rate of increase rose nearly five fold between 2000 and 2014, following the IPO transition to a negative phase in 1999. The new study finds that when the IPO changes phase, from positive to negative or vice versa, it touches off a chain reaction of climate impacts that may ultimately affect sea ice formation at the bottom of the world. When the IPO transitions to a negative phase, the sea surface temperatures in the tropical eastern Pacific become somewhat cooler than average when measured over a decade or two. These sea surface temperatures, in turn, change tropical precipitation, which drives large-scale changes to the winds that extend all the way down to Antarctica. The ultimate impact is a deepening of a low-pressure system off the coast of Antarctica known as the Amundsen Sea Low. Winds generated on the western flank of this system blow sea ice northward, away from Antarctica, helping to enlarge the extent of sea ice coverage. “Compared to the Arctic, global warming causes only weak Antarctic sea ice loss, which is why the IPO can have such a striking effect in the Antarctic," said Bitz. "There is no comparable natural variability in the Arctic that competes with global warming.” Sifting through simulations To test if these IPO-related impacts were sufficient to cause the growth in sea ice extent observed between 2000 and 2014, the scientists first examined 262 climate simulations created by different modeling groups from around the world. When all of those simulations are averaged, the natural variability cancels itself out. For example, simulations with a positive IPO offset those with a negative IPO. What remains is the expected impact of human-caused climate change: a decline in Antarctic sea ice extent. But for this study, the scientists were not interested in the average. Instead, they wanted to find individual members that correctly characterized the natural variability between 2000-2014, including the negative phase of the IPO. The team discovered 10 simulations that met the criteria, and all of them showed an increase in Antarctic sea ice extent across all seasons. "When all the models are taken together, the natural variability is averaged out, leaving only the shrinking sea ice caused by global warming," Arblaster said. "But the model simulations that happen to sync up with the observed natural variability capture the expansion of the sea ice area. And we were able to trace these changes to the equatorial eastern Pacific in our model experiments." Scientists suspect that in 2014, the IPO began to change from negative to positive. That would indicate an upcoming period of warmer eastern Pacific Ocean surface temperatures on average, though year-to-year temperatures may go up or down, depending on El Niño/La Niña conditions. Accordingly, the trend of increasing Antarctic sea ice extent may also change in response. "As the IPO transitions to positive, the increase of Antarctic sea ice extent should slow and perhaps start to show signs of retreat when averaged over the next 10 years or so," Meehl said. About the article Title: Antarctic sea-ice expansion between 2000 and 2014 driven by tropical Pacific decadal climate variability Authors: Gerald A. Meehl, Julie M. Arblaster, Cecilia M. Bitz, Christine T. Y. Chung, and Haiyan Teng Publication: Nature Geoscience, DOI: 10.1038/NGEO2751 WriterLaura Snider, Senior Science Writer and Public Information Officer

Capping warming at 2 degrees

June 27, 2016 | Even if countries adhere to the Paris climate agreement hammered out last fall, capping global warming at 2 degrees Celsius would likely require net zero greenhouse gas emissions by 2085 and substantial negative emissions over the long term, according to an in-depth analysis by scientists at the National Center for Atmospheric Research (NCAR).More than 100 parties to the Paris Agreement submitted pledges to the United Nations Framework Convention on Climate Change outlining their individual commitments to cutting greenhouse gas emissions by 2025 or 2030. The new study finds that, even if all the countries follow through on their commitments, steeper cuts would be necessary after 2030 to stay below 2 degrees of warming. And by the end of the century, total emissions would need to become negative, meaning more greenhouse gases would be removed from the air than are emitted into the atmosphere.These negative emissions would need to reach net minus 15 gigatons of "carbon dioxide equivalent," a measure that tabulates the global warming potential of all types of greenhouse gases in relation to carbon dioxide, according to model simulations created for the study.Worldwide, yearly greenhouse gas emissions now equal about 50 gigatons of carbon dioxide equivalent."The emissions targets in the Paris Agreement are an important first step, and it's known that additional action will be required to meet the goal of limiting warming to 2 degrees," said NCAR scientist Benjamin Sanderson, lead author of the study. "This paper provides details of what the next steps would need to look like in order to actually hit that target."The study, published in Geophysical Research Letters, a journal of the American Geophysical Union, was funded by the U.S. Department of Energy and by the National Science Foundation, NCAR's sponsor. This graph represents eight possible pathways that society could take to have a two-in-three chance of limiting warming to 2 degrees Celsius.  The blue line represents our current emissions trajectory. The red line represents the path that society will be on if countries adhere to the Paris Agreement. The gray lines represent other possibilities, all of which require more stringent emissions cuts in the near term but fewer negative emissions later. Click to enlarge. (©UCAR. This image is freely available for media & nonprofit use.)Small changes now equal big benefits laterEven before the Paris agreement was finished, it was clear that the pledged emissions cuts by 2030 would not be sufficient on their own to meet the target of limiting warming to 2 degrees. This study gives a comprehensive look at the possible paths society could take to have a two-in-three chance of staying below the target."We created a wide range of possible global emissions pathways that would allow us to have a decent shot at limiting warming to two degrees," said Sanderson. "We found that very small increases in the rate at which we cut greenhouse gases now could lead to very large decreases in the amount of negative emissions we need later." Negative emissions in the future will require the massive deployment of technologies that are still hypothetical to draw down greenhouse gases from the atmosphere. That makes it difficult to know how capable society will be to implement large-scale carbon removal in the future.Sanderson and his colleagues, NCAR scientists Brian O'Neill and Claudia Tebaldi, also found that it is still possible to stay below 2 degrees of warming without net negative emissions, but to do so would require near-term cuts that are much more aggressive than those proposed in the Paris agreement.About the articleBenjamin M. Sanderson, Brian C. O’Neill, and Claudia Tebaldi, What would it take to achieve the Paris temperature targets?, Geophysical Research LettersWriter/contact:Laura Snider, Senior Science Writer

Climate modeling 101: Explanations without equations

A new book breaks down climate models into easy-to-understand concepts. (Photo courtesy Springer.) June 21, 2016 | Climate scientists tell us it's going to get hotter. How much it rains and where it rains is likely to shift. Sea level rise is apt to accelerate. Oceans are on their way to becoming more acidic and less oxygenated. Floods, droughts, storms, and other extreme weather events are projected to change in frequency or intensity.  But how do they know what they know? For climate scientists, numerical models are the tools of the trade. But for the layperson — and even for scientists in other fields — climate models can seem mysterious. What does "numerical" even mean? Do climate models take other things besides the atmosphere into account?How do scientists know if a model is any good? * Two experts in climate modeling, Andrew Gettelman of the National Center for Atmospheric Research and Richard Rood of the University of Michigan, have your answers and more, free of charge. In a new open-access book, "Demystifying Climate Models," the pair lay out the fundamentals. In 282 pages, the scientists explain the basics of climate science, how that science is translated into a climate model, and what those models can tell us (as well as what they can't) — all without using a single equation. *Find the answers on pages 8, 13, and 161, respectively, of the book. AtmosNews sat down with Gettelman to learn more about the book, which anyone can download at   NCAR scientist Andrew Gettelman has written a new book on climate modeling with Richard Rood of the University of Michigan. (Courtesy photo. This image is freely available for media & nonprofit use.) What was the motivation to write this book? There isn't really another book that sets out the philosophy and structure of models. There are textbooks, but inside you'll find a lot of physics and chemistry: information about momentum equations, turbulent fluxes — which is useful if you want to build your own model. And then there are books on climate change for the layperson, and they devote maybe a paragraph to climate modeling. There's not much in the middle. This book provides an introduction for the beginning grad student, or someone in another field who is interested in using model output, or anyone who is just curious how climate works and how we simulate it. What are some of the biggest misperceptions about climate models that you hear? One is that people say climate models are based on uncertain science. But that's not true at all. If we didn't know the science, my cellphone wouldn't work. Radios wouldn't work. GPS wouldn't work. That's because the energy that warms the Earth, which radiates from the Sun, and is absorbed and re-emitted by Earth's surface — and also by greenhouse gases in the atmosphere — is part of the same spectrum of radiation that makes up radio waves. If we didn't understand electromagnetic waves, we couldn't have created the technology we rely on today. The same is true for the science that underlies other aspects of climate models. (Learn more on page 38 of the book.) But we don't understand everything, right? We have understood the basic physics for hundreds of years. The last piece of it, the discovery that carbon dioxide warms the atmosphere, was put in place in the late 19th, early 20th century. Everything else — the laws of motion, the laws of thermodynamics — was all worked out between the 17th and 19th centuries. (Learn more on page 39 of the book.) We do still have uncertainty in our modeling systems. A big part of this book is about how scientists understand that uncertainty and actually embrace it as part of their work. If you know what you don't know and why, you can use that to better understand the whole climate system. Can we ever eliminate the uncertainty? Not entirely. In our book, we break down uncertainty into three categories: model uncertainty (How good are the models at reflecting how the Earth really works?), initial condition uncertainty (How well do we understand what the Earth system looks like right now?), and scenario uncertainty (What will future emissions look like?) To better understand, it might help to think about the uncertainty that would be involved if you had a computer model that could simulate making a pizza. Instead of trying to figure out what Earth's climate would look like in 50 or 100 years, this model would predict what your pizza would look like when it was done.  The first thing you want to know is how well the model reflects the reality of how a pizza is made. For example, does the model take into account all the ingredients you need to make the pizza, and how they will each evolve? The cheese melts, the dough rises, and the pepperoni shrinks. How well can the model approximate each of those processes? This is model uncertainty. The second thing you'd want to know is if you can input all the pizza's "initial conditions" into the model. Some initial conditions — like how many pepperoni slices are on the pizza and where — are easy to observe, but others are not. For example, kneading the pizza dough creates small pockets of air, but you don’t know exactly where they are. When the dough is heated, the air expands and forms big bubbles in the crust. If you can't tell the model where the air pockets are, it can't accurately predict where the crust bubbles will form when the pizza is baked. The same is true for a climate model. Some parts of the Earth, like the deep oceans and the polar regions, are not easy to observe with enough detail, leaving scientists to estimate what the conditions there are like and leading to the second type of uncertainty in the model results.  Finally, the pizza-baking model also has to deal with "scenario uncertainty," because it doesn't know how long the person baking the pizza will keep it in the oven, or at what temperature. Without understanding the choices the human will make, the model can't say for sure if the dough will be soft, crispy, or burnt. With climate models, over long periods of time, like a century, we've found that this scenario uncertainty is actually the dominant one. In other words, we don't know how much carbon dioxide humans around the world going to emit in the years and decades to come, and it turns out that that's what matters most.  (Learn more about uncertainty on page 10 of the book.) Any other misperceptions you frequently hear? People always say, "If we can't predict the weather next week, how can we know what the climate will be like in 50 years?" Generally speaking, we can't perfectly predict the weather because we don't have a full understanding of all the current conditions. We don't have observations for every grid point on a weather model or for large parts of the ocean, for example. But climate is not concerned about the exact weather on a particular day 50 or 100 years from now. Climate is the statistical distribution of weather, not a particular point on that distribution. Climate prediction is focused on the statistics of this distribution, and that is governed by conservation of energy and mass on long time scales, something we do understand. (Learn more on page 6 of the book. Read more common misperceptions at Did you learn anything about climate modeling while working on the book? My background is the atmosphere. I sat down and wrote the whole section on the atmosphere in practically one sitting. But I had to learn about the other aspects of models, the ocean and the land, which work really differently. The atmosphere has only one boundary, a bottom boundary. We just have to worry about how it interacts with mountains and other bumps on the surface. But the ocean has three hard boundaries: the bottom and the sides, like a giant rough bathtub. It also has a boundary with the atmosphere on the top. Those boundaries really change how the ocean moves. And the land is completely different because it doesn't move at all. Writing this book really gave me a new appreciation for some of the subtleties of other parts of the Earth System and the ways my colleagues model them. (Learn more on page 13 of the book.) What was the most fun part of writing the book for you? I think having to force myself to think in terms of analogies that are understandable to a variety of people. I can describe a model using a whole bunch of words most people don't use every day, like "flux." It was a fun challenge to come up with words that would accurately describe the models and the science but that were accessible to everyone.

Days of our lives are getting longer

June 16, 2016 | With the days at their longest this time of year, consider this: They're lingering a tiny bit more because of climate change. That's because the runoff from melting ice sheets and glaciers is indirectly slowing the planet's rotation. As the water makes its way to the tropics, it redistributes a fraction of Earth's mass from the poles to the equator. The gradual shifting of mass to the planet's widest circumference, where it has to be moved more than 20,000 miles for a full rotation, has the effect of almost imperceptibly slowing Earth's spin. Figure skaters use the same approach, lifting their arms above their heads to accelerate into a spin and extending them outward to slow down. An iceberg floats in Disko Bay, near Ilulissat, Greenland, on July 24, 2015. Runoff from melting ice sheets and glaciers is shifting the mass of the planet and slightly slowing Earth's rotation. (Photo by Saskia Madlener, NASA.) To be sure, this change in Earth's rotation is not something that most of us would notice, adding no more than about a couple of thousandths of a second to the length of a day over the course of a century. And it's just one contributor to a long-term slowdown in Earth's spin, which is also affected by the moon's gravitational influence on tides. The 24-hour day may be something we take for granted, but it averaged only about 23 hours during the time of the dinosaurs. Still, the slowing of Earth's rotation is a sign of climate change's far-reaching and sometimes surprising impacts on our environment.  "The components of our Earth system are connected in ways that are not obvious at first glance," said NCAR climate scientist John Fasullo. "The idea that the warming of the surface can be connected to the length of the day can seem very esoteric. But when you think about it, the strong physical ties become apparent. "One of the great challenges for scientists is to make these connections and understand how aspects of our environment that seem so different are, in fact, connected with each other," he added. Indeed, much of the research around NCAR has highlighted unexpected connections among different parts of our physical world. Some examples: The heat generated by everyday activities in large cities alters the jet stream, affecting temperatures across thousands of miles Proliferating populations of pine beetles that are devastating Rocky Mountain forests may also be influencing local temperature and precipitation patterns The opening and closing of the Bering Strait may have helped drive global climate patterns during ice ages over the past 100,000 years The peak of the solar cycle and its aftermath appear to have impacts on Earth's climate that resemble La Niña and El Niño events, influencing weather around the world Fast-growing coastal cities alter weather patterns, making it easier for air pollutants to accumulate during warm summer weather instead of being blown out to sea. "Understanding how these seemingly disparate processes are connected with each other is critically important to advancing Earth system science," said NCAR Director Jim Hurrell. "Equally important, however, is that this improved understanding is vital for anticipating events that can have enormous consequences for society." Writer/contact:David Hosansky, Manager of Media Relations

Future summers could regularly be hotter than the hottest on record

BOULDER — In 50 years, summers across most of the globe could regularly be hotter than any summer experienced so far by people alive today, according to a study by scientists at the National Center for Atmospheric Research (NCAR).  If climate change continues on its current trajectory, the probability that any summer between 2061 and 2080 will be warmer than the hottest on record is 80 percent across the world's land areas, excluding Antarctica, which was not studied. If greenhouse gas emissions are reduced, however, that probability drops to 41 percent, according to the study. "Extremely hot summers always pose a challenge to society," said NCAR scientist Flavio Lehner, lead author of the study. "They can increase the risk for health issues, but can also damage crops and deepen droughts. Such summers are a true test of our adaptability to rising temperatures." The study, which is available online, is part of an upcoming special issue of the journal Climatic Changethat will focus on quantifying the benefits of reducing greenhouse gas emissions. The research was funded by the U.S. National Science Foundation (NSF) and the Swiss National Science Foundation. If greenhouse gas emissions remain unabated. virtually every summer between 2061-2080 could be hotter than any in the historical record. (Image is in the public domain.) Simulating a range of summers The research team, which includes NCAR scientists Clara Deser and Benjamin Sanderson, used two existing sets of model simulations to investigate what future summers might look like. Both had been created by running the NCAR-based Community Earth System Model 15 times, with one assuming that greenhouse gas emissions remain unabated and the other assuming that society reduces emissions. The Community Earth System Model is funded by NSF and the U.S. Department of Energy. The simulations were run on the Yellowstone system at the NCAR-Wyoming Supercomputing Center. By using simulations that were created by running the same model multiple times, with only tiny differences in the initial starting conditions, the scientists could examine the range of summertime temperatures we might expect in the future for the "business-as-usual" and reduced-emissions scenarios. "This is the first time that the risk of record summer heat and its dependence on the rate of greenhouse gas emissions has been so comprehensively evaluated from a large set of simulations with a single state-of-the-art climate model," Deser said. The scientists compared the results to summertime temperatures recorded between 1920 and 2014 as well as to 15 sets of simulated summertime temperatures for the same historic period. By simulating past summers — instead of relying solely on observations — the scientists established a large range of temperatures that could have occurred naturally under the same conditions, including greenhouse gas concentrations and volcanic eruptions. "Instead of just comparing the future to 95 summers from the past, the models give us the opportunity to create more than 1,400 possible past summers," Lehner said. "The result is a more comprehensive and robust look at what should be considered natural variability and what can be attributed to climate change." Emissions cuts could yield big benefits The scientists found that between 2061 and 2080, summers in large parts of North and South America, central Europe, Asia, and Africa have a greater than 90 percent chance of being warmer than any summer in the historic record if emissions continue unabated. This means that virtually every summer would be as warm as the hottest to date. In some regions, the likelihood of summers being warmer than any in the historical record remained less than 50 percent, but in those places — including Alaska, the central U.S., Scandinavia, Siberia, and continental Australia — summer temperatures naturally vary a great deal, making it more difficult to detect the impact of climate change. Reducing emissions would lower the global probability that future summers will be hotter than any in the past, but the benefits would not be spread uniformly. In some regions, including the U.S. East Coast and large parts of the tropics, the probability would remain above 90 percent, even if emissions were reduced. But it would be a sizable boon for other regions of the world. Parts of Brazil, central Europe, and eastern China would see a reduction of more than 50 percentage points in the chance that future summers would be hotter than the historic range. Since these areas are densely inhabited, a large part of the global population would benefit significantly from climate change mitigation. “We've thought of climate change as 'global warming'; among what matters is how this overall warming affects conditions that hit people where they live,” said Eric DeWeaver, program director in NSF’s Division of Atmospheric and Geospace Sciences, which funds NCAR.  “Extreme temperatures pose risks to people around the globe. These scientists show the power of ensembles of simulations for understanding how these risks depend on the level of greenhouse gas emissions.” Lehner recently published another study looking at the overlay of population on warming projections. “It's often overlooked that the majority of the world's population lives in regions that will see a comparably fast rise in temperatures," he said.  About the article Title: Future risk of record-breaking summer temperatures and its mitigation Authors: Flavio Lehner, Clara Deser, and Benjamin M. Sanderson Publication: Climatic Change, DOI: 10.1007/s10584-016-1616-2 Writer:Laura Snider, Senior Science Writer

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 

A CO2 milestone in Earth's history

(Illustration by Eric Morgan, Scripps Institution of Oceanography.) May 12, 2016 | Earth’s atmosphere is crossing a major threshold, as high levels of carbon dioxide (CO2)—the leading driver of recent climate change—are beginning to extend even to the globe's most remote region. Scientists flying near Antarctica this winter captured the moment with airborne CO2 sensors during a field project to better understand the Southern Ocean's role in global climate. This illustration shows the atmosphere near Antarctica in January, just as air masses over the Southern Ocean began to exceed 400 parts per million of CO2. The 400 ppm level is regarded as a milestone by climate scientists, as the last time concentrations of the heat-trapping gas reached such a point was millions of years ago, when temperatures and sea levels were far higher. The field project, led by the National Center for Atmospheric Research (NCAR) and known as ORCAS, found that there is still air present in the Southern Hemisphere that has less than 400 ppm of CO2—but just barely. In the north, the atmosphere had first crossed that threshold in 2013, as shown by observations taken at Mauna Loa, Hawaii, by the National Oceanic and Atmospheric Administration and Scripps Institution of Oceanography. Most fossil fuels are burned in the Northern Hemisphere, and these emissions take about a year to spread across the equator. As CO2 increases globally, the concentrations in the Southern Hemisphere lag slightly those further north. "Throughout humanity, we have lived in an era with CO2 levels below 400 ppm," said Ralph Keeling, director of the CO2 Program at the Scripps Institution of Oceanography and a principal investigator on ORCAS. "With these data, we see that era drawing to a close, as the curtain of higher CO2 spreads into the Southern hemisphere from the north. There is no sharp climate threshold at 400 ppm, but this milestone is symbolically and psychologically important." The air found by ORCAS with less than 400 ppm of CO2 was located in a wedge at lower altitudes. At higher altitudes, the air had already exceeded 400 ppm. This pattern is mostly a consequence of the way the air circulates in the region. At these southerly latitudes, the air arrives from the Northern Hemisphere at higher elevations and then mixes downward. Emissions of CO2 have been increasing since the 19th century. The measurements were taken by instruments operated by NOAA, NCAR, Scripps, Harvard University, and the University of Michigan. Scripps scientist Eric Morgan created this illustration. ORCAS was funded by the National Science Foundation. "This is the last we'll see of sub-400 ppm CO2 in the Southern Hemisphere, unless we're able to some day achieve negative emissions," said NCAR scientist Britton Stephens, co-lead principal investigator for ORCAS. "While 400 is just a number, for someone who was born when the atmosphere held 327 ppm of CO2, it’s certainly a reminder of our steadily increasing emissions and failure thus far to do enough to reduce them." About the image The illustration was created by interpolating 20 profiles measured on Feburuary 5 and 8, 2016. The vertical axis has been increased for better visibility. The image is freely available for nonprofit and media use. Please credit Eric Morgan, Scripps Institution of Oceanography. Writer/contact:David Hosansky, Manager of Media Relations

Widespread loss of ocean oxygen to become noticeable in 2030s

BOULDER — A reduction in the amount of oxygen dissolved in the oceans due to climate change is already discernible in some parts of the world and should be evident across large regions of the oceans between 2030 and 2040, according to a new study led by the National Center for Atmospheric Research (NCAR). Scientists know that a warming climate can be expected to gradually sap the ocean of oxygen, leaving fish, crabs, squid, sea stars, and other marine life struggling to breathe. But it's been difficult to determine whether this anticipated oxygen drain is already having a noticeable impact. "Loss of oxygen in the ocean is one of the serious side effects of a warming atmosphere, and a major threat to marine life," said NCAR scientist Matthew Long, lead author of the study. “Since oxygen concentrations in the ocean naturally vary depending on variations in winds and temperature at the surface, it's been challenging to attribute any deoxygenation to climate change. This new study tells us when we can expect the impact from climate change to overwhelm the natural variability." The study is published in the journal Global Biogeochemical Cycles, a publication of the American Geophysical Union. The research was funded by the National Science Foundation, NCAR's sponsor. Deoxgenation due to climate change is already detectable in some parts of the ocean. New research from NCAR finds that it will likely become widespread between 2030 and 2040. Other parts of the ocean, shown in gray, will not have detectable loss of oxygen due to climate change even by 2100. (Image courtesy Matthew Long, NCAR. This image is freely available for media & nonprofit use.)   Cutting through the natural variability The entire ocean—from the depths to the shallows—gets its oxygen supply from the surface, either directly from the atmosphere or from phytoplankton, which release oxygen into the water through photosynthesis. Warming surface waters, however, absorb less oxygen. And in a double whammy, the oxygen that is absorbed has a more difficult time traveling deeper into the ocean. That's because as water heats up, it expands, becoming lighter than the water below it and less likely to sink. Thanks to natural warming and cooling, oxygen concentrations at the sea surface are constantly changing—and those changes can linger for years or even decades deeper in the ocean. For example, an exceptionally cold winter in the North Pacific would allow the ocean surface to soak up a large amount of oxygen. Thanks to the natural circulation pattern, that oxygen would then be carried deeper into the ocean interior, where it might still be detectable years later as it travels along its flow path. On the flip side, unusually hot weather could lead to natural "dead zones" in the ocean, where fish and other marine life cannot survive. To cut through this natural variability and investigate the impact of climate change, the research team—including Curtis Deutsch of the University of Washington and Taka Ito of Georgia Tech—relied on the NCAR-based Community Earth System Model, which is funded by the National Science Foundation and the U.S. Department of Energy. The scientists used output from a project that ran the model more than two dozen times for the years 1920 to 2100 on the Yellowstone supercomputer, which is operated by NCAR. Each individual run was started with miniscule variations in air temperature. As the model runs progressed, those tiny differences grew and expanded, producing a set of climate simulations useful for studying questions about variability and change. Using the simulations to study dissolved oxygen gave the researchers guidance on how much concentrations may have varied naturally in the past. With this information, they could determine when ocean deoxygenation due to climate change is likely to become more severe than at any point in the modeled historic range. The research team found that deoxygenation caused by climate change could already be detected in the southern Indian Ocean and parts of the eastern tropical Pacific and Atlantic basins. They also determined that more widespread detection of deoxygenation caused by climate change would be possible between 2030 and 2040. However, in some parts of the ocean, including areas off the east coasts of Africa, Australia, and Southeast Asia, deoxygenation caused by climate change was not evident even by 2100. Picking out a global pattern The researchers also created a visual way to distinguish between deoxygenation caused by natural processes and deoxygenation caused by climate change. Using the same model dataset, the scientists created maps of oxygen levels in the ocean, showing which waters were oxygen-rich at the same time that others were oxygen-poor. They found they could distinguish between oxygenation patterns caused by natural weather phenomena and the pattern caused by climate change.  The pattern caused by climate change also became evident in the model runs around 2030, adding confidence to the conclusion that widespread deoxygenation due to climate change will become detectable around that time. The maps could also be useful resources for deciding where to place instruments to monitor ocean oxygen levels in the future to get the best picture of climate change impacts. Currently ocean oxygen measurements are relatively sparse. "We need comprehensive and sustained observations of what's going on in the ocean to compare with what we're learning from our models and to understand the full impact of a changing climate," Long said. About the article Title: Finding forced trends in oceanic oxygenAuthors: Matthew C. Long, Curtis Deutsch,and Taka ItoJournal: Global Biogeochemical Cycles Writer:Laura Snider, Senior Science Writer and Public Information Officer

Rising Voices melds indigenous, western science perspectives

March 24, 2016 | Indigenous people around the world are often among the first to experience the consequences of extreme weather and climate change. The effects on their lives and livelihoods of sea level rise, changes in farming and fishing seasons, and other environmental impacts often are dramatic. Yet their perspectives are rarely considered in public policy discussions. In many tribal communities, climate change exacerbates a situation already marked by economic hardship, resource loss, and discrimination. Now in its fourth year, a program hosted by NCAR called Rising Voices brings social and physical scientists and engineers together with Native American community members to build bonds that lead to collaboration on research proposals and projects. The premise is that indigenous peoples experience and understand the changes occurring in their communities, while scientists can provide insight on the underlying causes and how those changes might be managed. "We need to appreciate the experience and knowledge that has been transferred from generation to generation to generation in Native American communities," said Bob Gough, a founding member of Rising Voices, an attorney, and a descendant of the Leni Lenape tribe of Delaware. Rising Voices co-founder Bob Gough (far right), speaks at a Rising Voices workshop in Boulder. An attorney and descendant of the Leni Lenape tribe of Delaware, Gough has been involved in Native American and climate change issues for decades. (Photo by Craig Elevitch.) For NCAR Director Jim Hurrell, indigenous knowledge systems are critical to understanding the current and future impacts of climate variability and change, and "they are especially central to discussions around adaptation strategies. Rising Voices has been tremendously successful in bringing the indigenous and scientific communities together on these issues, and the collaborative efforts that are emerging are going to pay tremendous dividends." Many of the indigenous communities involved in Rising Voices are already contending with significant impacts. In January, the U.S. Department of Housing and Urban Development announced it would fund a proposal to resettle the Isle de Jean Charles Band of the Biloxi-Chitimacha-Choctaw tribe, a Louisiana Bayou community that has lost virtually all its land due to rising sea levels and to erosion caused by extreme weather as well as human activities such as oil and gas development. This is believed to be the first resettlement in the United States related to climate change. Rising Voices co-founder and NCAR scientist Heather Lazrus. (Photo by Kat Barr.) A Native American village in Kivalina, Alaska, is expected to soon face a similar fate, while many tribes in the Southwest are struggling with severe drought and scarce water. Members of both the Isle de Jean Charles and Kivalina tribes participate in Rising Voices. Bull Bennett, an ecologist, Mi'kmaq tribal member in North Dakota, and Rising Voices participant gave a vivid example of just one problem facing cold-climate communities during a video interview last summer for a new climate exhibit at NCAR's Mesa Lab. "Imagine you carve out your cellar in the permafrost and that's how you store your meat in the lean times," Bennett said. "And now imagine the permafrost thaws and your basement is full of water and the structure isn't supported and it falls in. That's what communities in Alaska are dealing with in the interior, with profound permafrost thaw. And it's only going to get worse." UN panel urges scientists to tap indigenous knowledge Rising Voices comes at a time of increasing recognition of the role indigenous people play worldwide in climate issues. In 2014, the United Nation's Intergovernmental Panel on Climate Change highlighted how indigenous knowledge and practice, including the "holistic view of community and environment, are a major resource for adapting to climate change." Building bonds, respecting cultural protocols Eileen Shea, former director of climate services at the National Oceanic and Atmospheric Administration and a participant in Rising Voices since the beginning, said establishing common ground through cultural ceremonies is a critical element of building trust between indigenous people and scientists. She still remembers a NOAA workshop she helped coordinate in Hawaii in 1998 that opened with a hula chant and dance. Far from being a tourist gimmick, the chants, when translated into English, described how the winds would periodically change direction and bring warmer water near the shoreline, negatively affecting fishing. "You could hear scientists in the back say, That must have been an El Niño," Shea recalled. When another chant talked about changes over a longer period that affected plants, water resources, and fish, "you could hear scientists in the back say, They are talking about the PDO.” The Pacific Decadal Oscillation is a recurring pattern of ocean-atmosphere climate variability over the mid-latitudes of the Pacific Ocean. Such protocols help to build mutual respect and trust, Shea said. "It puts everyone on a level playing field." In the case of Rising Voices, "over the years people have felt more comfortable sharing their stories of the weather and storms and ice breaks," Shea said. And, as indigenous people recount how their seasonal weather, along with their hunting and fishing calendars, have changed over the years, "you begin to see some alignment with Western science and history." The Rising Voices program grew out of a hallway coffee conversation three years ago between Gough and Heather Lazrus, an NCAR environmental anthropologist. At the time, Gough was involved in a project to improve wind-energy predictions and map Indian reservations for potential renewable energy projects. Initially intended as a one-time workshop, Rising Voices received additional funding for subsequent workshops which have been organized by Lazrus, Gough, and Julie Maldonado of the Livelihoods Knowledge Exchange Network. The NCAR Director's Office is the primary funder. Rising Voices has grown from 45 participants at the first workshop to more than 110 at the third annual workshop last July. (NCAR hosted a similar meeting in 2008). Gough, who grew up clamming and fishing on former tribal homelands on the New Jersey Coast, has been involved in tribal climate and energy issues for several decades. He said that while there are academic efforts to include indigenous people, Rising Voices fills a niche as a community-oriented group that connects tribes to each other and to scientists. Participants have also come from the U.S. National Climate Assessment and the Department of Interior's Climate Science Centers. In a survey of last summer's workshop participants, nearly two-thirds of respondents said they came away with a stronger appreciation of cultural protocols and knowledge required for partnerships in key areas, including water, relocation, climate cycles, and health and livelihood hazards. More than three-quarters said the workshop supported collaborative scientific-indigenous partnerships "extremely well" or "a lot." Lazrus said the ultimate goal is for indigenous perspectives to inform science. For example, Rising Voices is a formal partner in NCAR's Engineering for Climate Extremes Partnership, which is developing tools that help communities adapt and build resilience to extreme weather events. But while outcomes are important, the primary benefits of Rising Voices right now are to encourage connections and collaboration, and to support indigenous science students and early-career scientists. "In those respects," Lazrus said, "Rising Voices is succeeding."   Writer/contactJeff Smith, Science Writer and Public Information Officer FundersNCAR DirectorateNorth Central Climate Science Center Colorado State University CollaboratorsIntertribal Council on Utility Policy Kiksapa Consulting LLCIndigenous People's Climate Change Working Group


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