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

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

Ocean temps predict U.S. heat waves 50 days out, study finds

BOULDER — The formation of a distinct pattern of sea surface temperatures in the middle of the North Pacific Ocean can predict an increased chance of summertime heat waves in the eastern half of the United States up to 50 days in advance, according to a new study led by a scientist at the National Center for Atmospheric Research (NCAR). The pattern is a contrast of warmer-than-average water butting up against cooler-than-average seas. When it appears, the odds that extreme heat will strike during a particular week—or even on a particular day—can more than triple, depending on how well-formed the pattern is.The research is being published in the journal Nature Geoscience."Summertime heat waves are among the deadliest weather events, and they can have big impacts on farming, energy use, and other critical aspects of society," said Karen McKinnon, a postdoctoral researcher at NCAR and the lead author of the study. "If we can give city planners and farmers a heads up that extreme heat is on the way, we might be able to avoid some of the worst consequences."The research was largely funded by the National Science Foundation, NCAR's sponsor. In addition to McKinnon, the research team includes Andrew Rhines, of the University of Washington; Martin Tingley, of Pennsylvania State University; and Peter Huybers, of Harvard University.A fingerprint on the oceanFor the study, the scientists divided the country into regions that tend to experience extreme heat at the same time. The scientists then focused on the largest of the resulting blocks: a swath that stretches across much of the Midwest and up the East Coast, encompassing both important agricultural areas and heavily populated cities. Top: Sea surface temperature anomalies in the mid-latitude Pacific 50 days in advance of June 29, 2012. The pattern inside the green box resembled the Pacific Extreme Pattern, indicating that there would be an increase in the odds of a heat wave in the eastern half of the United States at the end of June. (Image courtesy of Karen McKinnon, NCAR. This image is freely available for media & nonprofit use.)Bottom: June 29, 2012, was the hottest day of the year in the eastern United States. The hot temperatures in late June and early July were part of an extraordinarily hot summer that saw three heat waves strike the country. (Map courtesy of the National Weather Service's Weather Prediction Center.)The research team looked to see if there was a relationship between global sea surface temperature anomalies—waters that are warmer or cooler than average—and extreme heat in the eastern half of the U.S.Right away, a pattern popped out in the middle of the Pacific, above about 20 degrees North latitude. The scientists found that the particular configuration of ocean water temperatures, which they named the Pacific Extreme Pattern, was not only found when it was already hot in the eastern U.S., but that it tended to form in advance of that heat."Whatever mechanisms ultimately leads to the heat wave also leaves a fingerprint of sea surface temperature anomalies behind," McKinnon said.Improving seasonal forecastsTo test how well that fingerprint could predict future heat, the scientists used data collected from 1,613 weather stations across the eastern U.S. between 1982 and 2015, as well as daily sea surface temperatures for the same time period.The scientists defined extreme heat in the eastern U.S. as a summertime day when the temperature readings from the warmest 5 percent of weather stations in the region were at least 6.5 degrees Celsius (11.7 degrees Fahrenheit) hotter than average. The scientists only looked at extreme heat during that region’s 60 hottest days of the year: June 24 through Aug. 22.The researchers "hindcasted" each year in the dataset to see if they could retrospectively predict extreme heat events—or lack of those events—during that year's summer, using only data collected during the other years as a guide.At 50 days out, the scientists were able to predict an increase in the odds—from about 1 in 6 to about 1 in 4—that a heat wave would strike somewhere in the eastern U.S. during a given week. At 30 days out or closer, the scientists were able to predict an increase in the odds—to better than 1 in 2 for a particularly well-formed pattern—that a heat wave would strike on a particular day.This new technique could improve existing seasonal forecasts, which do not focus on predicting daily extremes. Seasonal forecasts typically predict whether an entire summer is expected to be warmer than normal, normal, or cooler than normal.For example, the seasonal forecast issued for the summer of 2012 predicted normal heat for the Northeast and Midwest. But, the summer ended up being especially hot, thanks to three major heat waves that struck in late June, mid-July, and late July.When the science team used the Pacific Extreme Pattern to hindcast 2012, they were able to determine as early as mid-May that there were increased odds of extremely hot days occurring in late June. The hottest day of the summer of 2012, as measured by the technique used for this study, was June 29, when the warmest 5 percent of weather stations recorded temperatures that were 10.4 degrees Celsius (18.7 degrees Fahrenheit) above average."We found that we could go back as far as seven weeks and still predict an increase in the odds of future heat waves," McKinnon said. “What’s exciting about this is the potential for long-range predictions of individual heat waves that gives society far more notice than current forecasts.”Looking aheadScientists do not yet know why the fingerprint on sea surface temperatures in the Pacific predicts heat in the eastern U.S. It could be that the sea surface temperatures themselves kick off weather patterns that cause the heat. Or it could be that they are both different results of the same phenomenon, but one does not cause the other.To learn more about how the two are connected, McKinnon is working with colleagues at NCAR to use sophisticated computer models to try and tease apart what is really happening.The study's findings also point toward the possibility that the Pacific Extreme Pattern, or a different oceanic fingerprint, could be used to forecast other weather events far in advance, including cooler-than-average days and extreme rainfall events.“The results suggest that the state of the mid-latitude ocean may be a previously overlooked source of predictability for summer weather,” McKinnon said.About the articleTitle: Long-lead predictions of eastern United States hot days from Pacific sea surface temperaturesAuthors: Karen McKinnon, Andrew Rhines, Martin Tingley, and Peter HuybersJournal: Nature GeoscienceWriter:Laura Snider, senior science writer and public information officer

Flying lab to investigate Southern Ocean's appetite for carbon

BOULDER -- A team of scientists is launching a series of research flights this month over the remote Southern Ocean in an effort to better understand just how much carbon dioxide the icy waters are able to lock away. The ORCAS field campaign—led by the National Center for Atmospheric Research (NCAR)—will give scientists a rare look at how oxygen and carbon dioxide are exchanged between the air and the seas surrounding Antarctica. The data they collect will help illuminate the role the Southern Ocean plays in soaking up excess carbon dioxide emitted into the atmosphere by humans. "If we want to better predict the temperature in 50 years, we have to know how much carbon dioxide the oceans and terrestrial ecosystems are going to take up," said NCAR scientist Britton Stephens, co-lead principal investigator for ORCAS. "Understanding the Southern Ocean's role is important because ocean circulation there provides a major opportunity for the exchange of carbon between the atmosphere and the vast reservoir of the deep ocean." ORCAS is funded by the National Science Foundation’s Division of Polar Programs. "Building on decades of U.S. Antarctic Program research, new questions of global significance continue to emerge," said Peter Milne, program director of Ocean and Atmospheric Sciences in the Division of Polar Programs. "ORCAS addresses one of those questions: how the Southern Ocean affects global climate by storing, or releasing, carbon dioxide, water vapor, and heat.” Carbon dioxide, the main greenhouse gas contributing to human-caused climate change, is continually transferred back and forth between the atmosphere, plants on land, and the oceans. As more carbon dioxide has been released into the atmosphere by the burning of fossil fuels, oceans have stepped up the amount they absorb. But it's unclear whether oceans have the ability to keep pace with continued emissions. In the Southern Ocean, studies have disagreed about whether the ocean's ability to act as a carbon sink by taking up carbon dioxide is speeding up or slowing down. Measurements and air samples collected by ORCAS—which stands for the O2/N2 Ratio and CO2 Airborne Southern Ocean Study—will give scientists critical data to help clarify what's actually happening in the remote and difficult-to-study region. During the ORCAS campaign, the NSF/NCAR HIAPER research jet will study the air-sea exchange of gases over the Southern Ocean. Click image to enlarge. (Graphic by Alison Rockwell, NCAR. This image is freely available for media & nonprofit use.) Tracking carbon by air The ORCAS field campaign will operate out of Punta Arenas, near the southern tip of Chile. The researchers plan to use the NSF/NCAR HIAPER research aircraft to make 14 flights across parts of the Southern Ocean between Jan. 15 and Feb. 28. A suite of instruments on the modified Gulfstream V jet will measure the distribution of oxygen and carbon dioxide as well as other gases produced by marine microorganisms, plus aerosol and cloud characteristics in the atmosphere. The flights also will observe the ocean color—which can indicate how much and what type of phytoplankton is growing in the water—using NASA's Portable Remote Imaging Spectrometer (PRISM). The addition of the PRISM instrument to the ORCAS campaign was funded by NASA. The science campaign is being led by Stephens and NCAR scientist Matthew Long. Other principal investigators include Elliot Atlas (University of Miami), Michelle Gierach (NASA's Jet Propulsion Laboratory), Ralph Keeling (Scripps Institution of Oceanography), Eric Kort (University of Michigan), and Colm Sweeney (Cooperative Institute for Research in Environmental Sciences). CIRES is a partnership of the National Oceanic and Atmospheric Administration and the University of Colorado Boulder. The management of the field campaign is being handled by NCAR. Logistics include everything from obtaining diplomatic clearances from multiple countries to fly through their airspaces to providing housing and workspace for project scientists in South America. Carbon, oxygen, and phytoplankton Measuring oxygen alongside carbon dioxide can give scientists a clearer picture of the ocean processes affecting carbon dioxide than they would get from measuring carbon dioxide alone. "The air-sea exchange of carbon dioxide is controlled not just by physics but also by biology," Long said. "There's a nice relationship between the fluxes of oxygen and the fluxes of carbon dioxide that can be exploited to gain insight into these processes." Carbon dioxide in the ocean is drawn into a chain of chemical reactions that buffer the impact of biological and physical ocean processes on carbon dioxide in the overlying atmosphere. Oxygen air-sea fluxes, however, are more directly tied to these same biological and physical factors. So if scientists know what's going on with oxygen, they can better understand the processes affecting carbon dioxide as well. The Southern Ocean, which encircles Antarctica, is an especially important carbon sink. The ORCAS field campaign will help scientists better understand whether the Southern Ocean's ability to take up carbon is keeping pace with a continued increase in carbon dioxide emissions by humans. (Photo courtesy of the U.S. Central Intelligence Agency.) Additionally, if scientists know how the concentrations of the two gases change relative to one another with location and time, they can disentangle how biology and physics separately affect the ocean's ability to absorb carbon dioxide. Physics and biology affect the ratio of carbon dioxide to oxygen in the air in different ways. In the austral spring the warmth of the returning Sun drives both carbon dioxide and oxygen out of the Southern Ocean surface and into the atmosphere. But the sunlight also triggers the growth of phytoplankton in the water. As the organisms begin to flourish, they take in carbon dioxide and release oxygen, causing the relative amounts of those two gases in the atmosphere to shift in opposite directions. Observations of these shifts can ultimately tell scientists how much carbon is going where and, more importantly, why. A window into the deep ocean The Southern Ocean is unique among Earth's oceans. Unimpeded by continental landmasses, and driven by a westerly wind, the Southern Ocean is able to form a circular current around Antarctica. This huge flow, the largest current on the planet, connects the adjacent Atlantic, Pacific, and Indian oceans. The complex interactions between this Antarctic Circumpolar Current and currents flowing in from other ocean basins creates an overturning circulation that brings deep water to the surface where it can exchange gases with the atmosphere before it is returned to depth. Once it dives toward the ocean floor, that surface water—and any carbon dioxide it takes with it—can stay sequestered in the deep ocean for hundreds or even thousands of years. Data collected by the ORCAS flights will help determine how much carbon dioxide goes along for the ride. "The Southern Ocean provides a window into the deep ocean, but it's a difficult system to simulate in our Earth system models," Long said. "It's remote, and so there has been a paucity of observations that can be used to improve the models we have." The data generated during the field campaign will be used by the ORCAS team to improve these global computer models so they do a better job representing the complexities of the Southern Ocean. The data set, which will be managed by NCAR, will be publicly available. While the measurements made during the ORCAS campaign will help scientists fine-tune what they know so far about the Southern Ocean, it's possible the project will also bring to light entirely new aspects of how the ocean works. "The Southern Ocean is very inaccessible, and existing measurements are from ships or surface stations that represent only a few tiny dots on a huge map," Stephens said. "The airborne measurements we take will be helpful in terms of understanding the system better. And because we're doing something that no one's ever done before, we're likely to find things that we aren't expecting." The NSF’s Division of Polar Programs manages the U.S. Antarctic Program, through which it funds researchers, coordinates all U.S. government research on the southernmost continent, and provides logistical support needed to make the science possible. WriterLaura Snider, Senior Science Writer and Public Information Officer

NCAR develops method to predict sea ice changes years in advance

BOULDER – Climate scientists at the National Center for Atmospheric Research (NCAR) present evidence in a new study that they can predict whether the Arctic sea ice that forms in the winter will grow, shrink, or hold its own over the next several years. The team of scientists has found that changes in the North Atlantic ocean circulation could allow overall winter sea ice extent to remain steady in the near future, with continued loss in some regions balanced by possible growth in others, including in the Barents Sea. "We know that over the long term, winter sea ice will continue to retreat," said NCAR scientist Stephen Yeager, lead author of the new study, which appears in the journal Geophysical Research Letters. "But we are predicting that the rate will taper off for several years in the future before resuming. We are not implying some kind of recovery from the effects of human-caused global warming; it's really just a slow down in winter sea ice loss." The research was funded largely by the National Science Foundation, NCAR's sponsor, with additional support from the National Oceanic and Atmospheric Administration and the U.S. Department of Energy. The researchers tested how well they were able to predict winter sea ice changes by "hindcasting" past decades and then comparing their retrospective predictions to observations of what really happened. This image shows how the model stacked up to real life for the period of 1997–2007. [Enlarge] (©UCAR. This image is freely available for media & nonprofit use.) Yeager is among a growing number of scientists trying to predict how the climate may change over a few years to a few decades, instead of the more typical span of many decades or even centuries. This type of "decadal prediction" provides information over a timeframe that is useful for policy makers, regional stakeholders, and others. Decadal prediction relies on the idea that some natural variations in the climate system, such as changes in the strength of ocean currents, unfold predictably over several years. At times, their impacts can overwhelm the general warming trend caused by greenhouse gases released into the atmosphere by humans. Yeager's past work in this area has focused on decadal prediction of sea surface temperatures. A number of recent studies linking changes in the North Atlantic ocean circulation to sea ice extent led Yeager to think that it would also be possible to make decadal predictions for Arctic winter sea ice cover using the NCAR-based Community Earth System Model. Linking ocean circulation and sea ice The key is accurately representing the Atlantic Meridional Overturning Circulation (AMOC) in the model. AMOC sweeps warm surface waters from the tropics toward the North Atlantic, where they cool and then sink before making a return south in deep ocean currents.  AMOC can vary in intensity. When it's strong, more warm water is carried farther toward the North Atlantic and Arctic oceans, accelerating sea ice loss. When weak, the warm water largely stays farther south, and its effects on sea ice are reversed. The variations in AMOC's vigor—from weak to strong or vice versa—occur over multiple years to decades, giving scientists some ability to predict in advance how it will affect winter sea ice, in particular.  For the decade of 2007‑2017, the research team predicts that there may be some growth of winter sea ice in the Arctic Ocean, particularly on the Atlantic side, where scientists have the most confidence in the model's ability. The image also shows possible sea ice loss in the North Pacific. [Enlarge] For an image of predicted winter sea ice change from 2013‑2023, click here. (©UCAR. This image is freely available for media & nonprofit use.) AMOC now appears to be weakening. Yeager and his co-authors, NCAR scientists Alicia Karspeck and Gokhan Danabasoglu, found in their new study that this change in the ocean is likely to be enough to temporarily mask the impacts of human-caused climate change and stall the associated downward trend in winter sea ice extent in the Arctic, especially on the Atlantic side, where AMOC has the most influence. The limits of a short satellite record The amount of sea ice covering the Arctic typically grows to its maximum in late February after the long, dark winter. The sea ice minimum typically occurs at the end of the summer season, in late September. The new study addresses only winter sea ice, which is less vulnerable than summer ice to variations in weather activity that cannot be predicted years in advance, such as storms capable of breaking up the ice crust. Despite their success incorporating AMOC conditions into winter sea ice "hindcasts," the scientists are cautious about their predictions of future conditions. Because satellite images of sea ice extend only back to 1979, the scientists had a relatively short data record for verifying decadal-scale predictions against actual conditions. Additionally, AMOC itself has been measured directly only since 2004, though observations of other variables that are thought to change in tandem with AMOC—such as sea surface height and ocean density in the Labrador Sea, as well as sea surface temperature in the far North Atlantic—go back much farther. "The sea ice record is so short that it's difficult to use statistics alone to build confidence in our predictions," Yeager said. "Much of our confidence stems from the fact that our model does well at predicting slow changes in ocean heat transport and sea surface temperature in the subpolar North Atlantic, and these appear to impact the rate of sea ice loss. So, we think that we understand the mechanisms underpinning our sea ice prediction skill." About the article Title: Predicted slow-down in the rate of Atlantic sea ice lossAuthors: Stephen G. Yeager, Alicia Karspeck, and Gokhan DanabasogluPublication: Geophysical Research Letters WriterLaura Snider, Senior Science Writer and Public Information Officer

Looking at swells in 3D

November 10, 2015 | Scientists have long been interested in studying how winds influence ocean waves.  NCAR Senior Scientist Peter Sullivan wanted to examine the relationship in reverse: How do waves affect the atmosphere? "Most people focus on winds influencing waves because it’s the easiest to study in a laboratory," Sullivan said. "But nature works the other way, too." The result is this striking 3D animation showing the influence of ocean waves on the air above. While strong winds from storms create waves on the ocean surface, those waves don’t just stop. They travel away from the storm, sometimes thousands of miles, to areas with lighter winds. There, in those new areas, the waves, which have become swells, influence the atmosphere. Understanding how this happens provides insight into global weather and climate patterns, as well as into major storms such as hurricanes, which draw energy from the ocean. "With a hurricane, the first thing you see is the fast moving swell created by it," Sullivan said.  Simulating turbulence Sullivan simulated what would happen if a spectrum of small and big waves moved into an area with light winds. He used a technique called large eddy simulation, first developed by NCAR scientists in the 1960s to account for turbulence in computer models. A billion data points were crunched by the NCAR-Wyoming Supercomputing Center’s Yellowstone system. NCAR software engineer Scott Pearse later rendered this two-minute visualization from telling chunks of the simulation. "It’s the data that was actually beautiful," Pearse said. "The ocean wave fields and atmosphere couple, leaving an imprint on each other, and the visualization illustrates some of the complexity of what goes on—in 3D," Sullivan said. Small waves impose a drag on air, while fast and big waves provide thrust, pushing the air forward. "You can see wave signatures in the atmosphere over a large vertical extent," Sullivan said. (Watch for some of these wave-driven winds aloft between the 1:30 and 2:00 mark in the video). Sullivan has made available a simplified version of the new software code, and a number of students are using the code for their graduate work. Pearse created the visualization using a software tool from NCAR called VAPOR, a visualization and analysis platform for ocean, atmosphere, and solar researchers. When he vacationed in Florida after doing initial work on the project, Pearse found he had gained a new perspective on the relationship between the air and ocean. "I saw the waves differently," he said. "Now that I know that atmosphere-ocean interaction is a two-way street, I find myself wondering what the air is doing whenever I see moving water." About the study A journal publication describing the numerical algorithm and further results from the large-eddy simulations can be found at: Sullivan, P.P., J.C. McWilliams and E.G Patton, 2014: Large-eddy simulation of marine boundary layers above a spectrum of moving waves. Journal of the Atmospheric Sciences, doi: 10.1175/JAS-D-14-0095.1 Writer/ContactJeff Smith, Science Writer and Public Information Officer FundersPhysical Oceanography Program at the Office of Naval ResearchNational Science Foundation      

Coral reveals long-term link between Pacific winds, global climate

BOULDER — New research indicates that shifts in Pacific trade winds played a key role in twentieth century climate variation, a sign that they may again be influencing global temperatures. The study, led by scientists at the National Center for Atmospheric Research (NCAR) and the University of Arizona (UA), uses a novel method of analyzing chemical changes in coral to show that weak tropical Pacific trade winds coincided with globally warming temperatures early in the twentieth century. When the natural pattern shifted and winds began to strengthen after 1940, the warming slowed. The finding gives support to the theory that strong Pacific trade winds are currently helping to prevent global temperatures from climbing, even as society continues to emit carbon dioxide and other greenhouse gases. When the winds weaken as part of a natural cycle, warming will likely resume once again, the authors say. A coral core drawn from these equatorial atolls of western Kiribati has revealed the influence of Pacific winds on global temperatures. New research shows that weak trade winds from about 1910 to 1940 contributed to warmer temperatures worldwide, while stronger winds from about 1940 to 1970 helped lead to more stable temperatures. (Photo by J. Warren Beck, University of Arizona.) “Strong winds in the tropical Pacific are playing a role in the slowdown of warming over the past 15 years,” said lead author Diane Thompson, a postdoctoral scientist at NCAR. “When the winds inevitably change to a weaker state, warming will start to accelerate again.” “Mother Nature is always going to inject little ups and downs along our path to a warmer world,” said University of Arizona professor Julia Cole, a co-author. “We’re trying to understand how those natural variations work so that scientists can do a better job of predicting the actual course of climate change into the future. “ The study is being published this week in Nature Geoscience. It was funded by the National Science Foundation, NCAR’s sponsor, as well as by the National Oceanic and Atmospheric Administration, University of Arizona, Philanthropic Education Organization, U.K. Natural Environment Research Council, and U.S. Department of Energy. Where is the heat going? Despite increases in greenhouse gases, global surface temperatures have not risen significantly since 2001. This pause in global warming, often called the hiatus, has become the focus of research by climate scientists who are trying to track the missing heat. By using climate models and observations, scientists are finding evidence that the heat is going into the subsurface ocean, perhaps as a result of changes in atmospheric circulation. A study earlier this year in Nature Climate Change, by an international team of climate scientists, pointed to unusually strong trade winds along the equator in the Pacific Ocean that are driving heat into the ocean while bringing cooler water to the surface. This is leaving less heat in the air, thereby temporarily offsetting warming from increasing greenhouse gases. The study by Thompson and her colleagues indicates that this process has happened before, and in the opposite direction: weaker winds allowed warming to accelerate. The research team focused on the early twentieth century – a time when a third of the century’s global warming took place, even though major accumulations of greenhouse gases were not yet occurring. Some previous research suggested that rising sea-surface temperatures in the Atlantic Ocean were to blame. That warming did not begin until the mid-1920s, however, when the global atmospheric warming was already well underway. As it turned out, the researchers had access to an important piece of evidence. Sitting in a UA lab was an old core drawn from a coral skeleton near a western tropical Pacific island. It had been chemically analyzed in the 1990s and then largely forgotten. Thompson, while working on her doctoral dissertation, realized that the core could reveal tropical Pacific wind patterns during the period from 1894 to 1982 when it had grown just outside of the island’s lagoon. The reason has to do with the effects of wind on water chemistry. When strong bursts of wind came in from the west, they stirred up manganese in the sediment at the bottom of the lagoon. The local corals took up the manganese in their skeletons as they grew. Such wind bursts from the west occur more commonly when the trade winds, which normally blow from the east, are weak. Chemical analysis of the yearly banded coral skeleton showed a comparatively high occurrence of spikes in the manganese-to-calcium ratio from about 1910 to 1940—the same period when Earth experienced significant warming. However, the number of these events dropped between the 1940s and 1970s, when temperatures leveled off. Thompson and her co-authors compared the ratio to wind observations since 1960, when observations became more reliable, and verified that the high manganese-to-calcium ratio correlated with weaker trade winds. They also combed through more scattered records before 1960 and again found a correlation of the chemical ratio to wind strength. Thompson stressed that the winds are just one contributor to changes in global climate. Another reason that temperatures leveled off in mid-century likely has to do with increased industrialization and emissions of particles that block sunlight and exert a cooling influence. Later in the century, increased emissions of greenhouse gases played a dominant role. “This research shows that the influence of winds on climate is not anything new. These mechanisms have been at work earlier,” Thompson said. “We believe this is a significant contribution to understanding the role of natural processes in modulating global temperature change.” About the Article Title: Early 20th century global warming linked to tropical Pacific wind strength  Authors: Diane M. Thompson, Julia E. Cole, Glen T. Shen, Alexander W. Tudhope, Gerald A. Meethl  Journal: Nature Geoscience, doi:10.1038/ngeo2321

Why do La Niña events linger?

August 12, 2014 | For millions of people, the onset of El Niño or La Niña in northern autumn indicates whether they’re likely to face unusually warm, cold, wet, or dry conditions over the coming winter. A new modeling study pins down the process that apparently determines why La Niña events often last twice as long as typical El Niño events—a result with major implications for seasonal predictions extending more than a year out. “Our work suggests that we can make significant progress on predicting La Niña events and determining how long they might last,” said NCAR scientist Clara Deser. “Because of the disproportionate impact of La Niña on drought throughout the world, this could help lead to concrete social benefits.” Deser recently coauthored a paper with Pedro DiNezio (University of Hawaii) that is now in early release in the Journal of Climate. La Niña conditions were in place during the northern winters of 2010–11 and 2011–12, with major drought impacts observed in the United States. This 10-day average of ocean height across the tropical Pacific, collected by the U.S–French Ocean Surface Topography Mission/Jason-2 satellite, spans a period centered on September 3, 2010, when the La Niña event had already reached moderate levels. Lower ocean heights centered on the equator (blue and purple) indicate cooler-than-average conditions there. (Image courtesy NASA.) Although they’re often portrayed as a pair of opposites, El Niño and La Niña aren’t mirror images of each other. El Niño events—which involve warmer-than-usual water overspreading the eastern tropical Pacific—tend to last no longer than one year. However, in recent decades, almost half of La Niña events—which lead to strong trade winds and cooler-than-usual surface water—have returned for a second consecutive northern winter. This was the case during 2010–11 and 2011–12, two La Niña years that were both associated with devastating U.S. droughts. DiNezio and Deser examined the duration of La Niña in climate simulations performed with the Community Climate System Model (CCSM4). Computer models have long struggled to realistically simulate El Niño and La Niña, but CCSM4 is one of a small number of newer models in which El Niño and La Niña events are produced at a realistic pace and strength, making it suitable for the new research. Oceanic clues to La Niña's staying power Longstanding theories predict that the stronger trade winds associated with La Niña kick off very slow-moving ocean waves that move westward across the Pacific, bounce off the coast, and then, a year later, return to the central Pacific. These waves depress the thermocline—the boundary between frigid deep waters and warmer surface waters—by several dozen feet, which allows the ocean surface to warm and effectively ends La Niña. The team scrutinized this mechanism in 252 La Niña events produced by the CCSM4 in a simulation of 1,300 years of climate. In one out of every three simulated La Niña events, the waves did not succeed in ending the event, and La Niña persisted into a second year. As La Niña becomes stronger, the magnitude of this effect does not increase proportionally—a result observed in the model as well as in observations. This disproportionate, nonlinear behavior occurs because as the thermocline deepens, the resulting warming effect on surface waters reaches a limit. Thus, the depressed thermocline may need to persist for a second year in order to bring a strong La Niña event to an end. This result has implications for predicting two-year La Niña events. Notably, the team found that the depth of the thermocline six months before a La Niña begins was correlated with sea-surface temperature anomalies a year and a half later. “This could provide an 18-month lead time for predicting the return of La Niña conditions for a second year,” said Deser. Pedro N. DiNezio and Clara Deser, Nonlinear controls on the persistence of La Niña, Journal of Climate (2014), doi:http://dx.doi.org/10.1175/JCLI-D-14-00033.1 Writer/contactBob Henson, NCAR/UCAR Communications Collaborating institutionsNCARInternational Pacific Research Center, University of Hawaii FundersJapan Agency for Marine-Earth Science and TechnologyNASANational Oceanic and Atmospheric AdministrationNational Science FoundationUniversity of Hawaii In Graphic Terms La Niña (left), neutral or "normal" conditions (middle), and El Niño (right) each have a distinctive signature of oceanic and atmospheric processes, as seen in this cross section of the tropical Pacific Ocean extending from Australia and Indonesia (left side of each panel) to the Americas (right side of each panel). Warm sea surface temperatures (reds and oranges) are shunted to the west during an El Niño event but extend eastward during a La Niña event. These warmer surface waters tend to be collocated with showers and thunderstorms and a deeper thermocline, the oceanic boundary separating warmer surface waters from colder, deeper waters. New research relates the depth of the thermocline in the central Pacific to the duration of a subsequent La Niña event. (Diagrams courtesy NOAA Pacific Marine Environment Laboratory.)        

Inside the warming hiatus

Bob Henson • January 8, 2014 | The globally averaged surface air temperature hasn’t risen much in the last 15 years. But as a fresh batch of research has made clear, there’s been ample heating of Earth—which becomes evident when looking at certain times of the year and in particular locations, including deep in the ocean. One of the latest examinations of this topic comes from NCAR scientists Kevin Trenberth and John Fasullo in Earth’s Future, the new open-access journal from the American Geophysical Union. The title of their December 5 paper—“An apparent hiatus in global warming?”—hints at the frustration many scientists (as well as some policy makers) have felt over how the global pause in surface temperature has sometimes been interpreted. As Trenberth and Fasullo put it, “Global warming has not stopped; it is merely being manifested in different ways.” Regional and seasonal variations help drive home this point. This graphic shows globally averaged temperature anomalies (deviations from a 30-year average) in degrees Celsius since 1970 as compiled by NOAA. Most of the hiatus in atmospheric warming since the early 2000s has occurred during the months December through February (DJF, the orange trace above). (Graph adapted from Figure 3, "An apparent hiatus in global warming?", Earth's Future, doi:10.1002/2013EF000165.) Most of the surface temperature hiatus has been in the period from December to February. Global temperatures have trended downward since the early 2000s during northern winter more than in other seasons, as shown by the graphic at right. The last few years have brought several high-profile outbreaks of cold and snow to North America and Eurasia, including one just this week. Most of the hiatus has been in the subtropics and midlatitudes. The Arctic has warmed dramatically over the last 15 years, with record depletions of summer sea ice and record amounts of melt over the Greenland ice sheet. And the tropics have continued to warm (except for the eastern tropical Pacific—see below).  These two factors imply that what’s been seen as a global pause in warming isn’t really a worldwide phenomenon. The Pacific Decadal Oscillation leaves a footprint of anomalies in sea surface temperature (lower map) that closely matches the pattern of recent global temperature trends (top map, derived from NASA/GISS data for the period 1999–2012 as compared to 1976–1998). (Images adapted from Figures 8 and 9, "An apparent hiatus in global warming?", Earth's Future, doi:10.1002/2013EF000165.)  The top map at left brings out the spatial variation even more clearly. It compares air temperatures during the hiatus period (1999 up to 2012) with those in the preceding 23 years (1976–1998), when relatively rapid warming was under way. What jumps out right away is the band of blue, indicating cooling, that hugs the entire west coast of the Americas, from Alaska to Chile, with a large spike extending west into the central tropical Pacific. “It is the central and eastern Pacific, more than anywhere else, that has not warmed in the past decade or so,” notes Trenberth. As it turns out, the pattern of observations shown in this map closely resembles the signature of the Pacific Decadal Oscillation (PDO), as shown in the bottom map at left.  The PDO-ENSO dance Like an oceanic cousin of the El Niño/Southern Oscillation (ENSO), the PDO switches back and forth between warm (positive) and cool (negative) modes. However, it changes modes only about every 20 to 30 years, rather than every year or two like ENSO. When the PDO is positive, El Niños tend to be stronger and more frequent; when it’s negative, La Niñas typically predominate. That’s a critical point, because it’s well known that El Niño events serve as a giant release valve for ocean heat. During El Niño, shifts in ocean circulation allow the surface waters of the eastern tropical Pacific to warm dramatically. That can pump enough heat into the global atmosphere to boost temperatures by up to 0.4°C (0.7°F) for the better part of a year. Likewise, during a strong La Niña, oceans as a whole retain more heat than usual, and the global atmosphere cools down for a few months. The Pacific Decadal Oscillation, or PDO, tends to alternate between positive (warm) and negative (cool) phases every 20 to 30 years, with cool conditions predominating in the last 15 years. (Graph courtesy NOAA National Climatic Data Center.)  The blockbuster 1997–98 El Niño helped produce the record-warm year of 1998. Only a year later, the PDO entered a negative (cool) phase that’s persisted ever since, with no major El Niños but several significant La Niñas. It’s thus understandable that we haven’t seen a dramatic new record in global surface air temperature, even though both 2005 and 2010 nudged just past 1998 in NOAA’s long-term data. The PDO is believed to be driven mainly by natural variations, and it’s not yet known what causes it to switch phases. Trenberth and Fasullo speculate that the record-strong El Niño of 1997–98 released so much heat from the ocean that the PDO’s switch to a heat-storing negative mode may have been some type of compensating response. If so, the mechanism behind it isn’t yet clear, and it’s quite possible that computer models of Earth’s climate aren’t capturing it. According to Trenberth, the negative PDO might itself be setting the stage for a major compensation. The trade winds are a key player here, as changes in atmospheric pressure create the "sloshing water in a bathtub" effect that is a hallmark of both ENSO and the PDO. In the tropical Pacific, the tendency toward stronger-than-normal trade winds since the PDO’s last shift has pushed increasing volumes of water from east to west. Sea levels in and near the Philippines have risen by more than eight inches relative to the eastern tropical Pacific, which only added to the storm surge inflicted by Supertyphoon Haiyan in November 2013. Not just Supertyphoon Haiyan: The Philippine coastline near Real, east of Manila, suffered major damage following several tropical cyclones in late 2004. Sea levels are rising more quickly in the western tropical Pacific than in many other parts of the globe, in part because of a persistent negative phase of the Pacific Decadal Oscillation. (Wikimedia Commons photo by Lance Corporal Joel Abshier, U.S. Marine Corps.) “Eventually this difference in sea level becomes unsustainable, and the water sloshes back,” asserts Trenberth. “At some point, perhaps because of a fairly random weather event, we will likely have an El Niño event that leads us back into a positive PDO.” At depth Perhaps the most intriguing place where the hiatus isn’t a uniform entity is beneath the surface of the sea. The amount of ocean heat stored at deeper levels (below 700 meters, or 2300 feet) has increased markedly during the atmospheric hiatus. Trenberth and Fasullo examined changes in ocean heat content using a newly released reanalysis called ORAS-4, produced by the European Centre for Medium-Range Weather Forecasts. Working with the ECMWF’s Magdalena Balmaseda, who led an in-depth analysis, they found that almost all of the net heat storage from 1980 to the early 2000s was in the upper 700 meters. Things changed after that, as the deeper oceans began absorbing heat at an ever-increasing rate. By 2010, more than a third of the added ocean heat from the previous decade had been tucked away below 700 meters. It’s not obvious why the heat storage has evolved this way, but a major analysis published in July by the UK Met Office supports the general picture. In section 2 (see PDF), the report finds that the upper ocean has been out of radiative balance (a function of heat exchange) with the atmosphere since 2000, implying that heat is being stored in the deep ocean. The report also found that the post-2000 pattern in vertical heat exchange resembles what was observed from 1950 to 1965—a period that also featured a negative PDO and a flat trend in global temperatures. Other possible causes Some scientists have found possible explanations beyond the oceans. In the current draft of its 2014 assessment, Working Group I of the Intergovernmental Panel on Climate Change addresses the gamut of possible causes (see PDF of Chapter 9, pp. 26–30). NASA's Solar Dynamics Observatory recorded this X2-class solar flare on September 29, 2013. The current peak in the 11-year cycle of solar activity follows one of the longest and most dramatic downturns in the cycle observed in the past century. (Image courtesy NASA.) The IPCC attributes the hiatus in roughly equal measure to A cooling trend from natural variability (including the oceanic factors discussed above) Changes in Earth’s radiative balance since 2000 The latter are mainly due to a series of weak volcanic eruptions and a prolonged downturn in the natural 11-year cycle that controls sunspots and other facets of solar activity. However, the IPCC expressed low confidence in its ability to determine quantitatively how much each of these factors contributed to the hiatus, and some of the most recent modeling studies suggest that an atmospheric heating pause can emerge through natural variability alone. In calculations for a forthcoming paper, Trenberth and colleagues estimate that radiative changes might account for about 20% of the current hiatus, leaving ocean storage as the biggest player by far.  Climate contrarians have long observed that computer models of 21st-century climate didn’t “predict” the hiatus. However, these analyses overlook the fact that century-long trends are known to be masked by natural variability on decadal and shorter timescales. Moreover, global temperatures over the last 20 years are still within the range of model projections for an atmosphere that’s warming over the long term. (See "Modeling the pause," below, for several success stories.) Looking ahead, long-range computer models from NOAA and other centers are already hinting that an El Niño event may be on the horizon in the latter part of 2014. If this happens, we might see a new global temperature record in 2014—which, in turn, might help put the much-belabored hiatus to rest. Modeling the pause Some climate models that link ocean and atmosphere have been able to simulate pauses in atmospheric warming lasting a decade or more. In these cases, an uptick in deep-ocean heat storage appears to account for the “missing heat”, as found by NCAR’s Gerald Meehl and colleagues in a 2011 study. Researchers have also been able to replicate the current hiatus, and show its intimate link to the oceans, by putting sea-surface temperatures (SSTs) from the 1990s and 2000s into a model and letting the atmosphere run free. A group led by Virginia Guemas (Catalan Institute of Climate Science, Spain) found that models keying off global SSTs from the year 2000 suggested that surface warming would be tamped down over the next five years (which, in fact, it was). Even if a model is given the observed SSTs only for the eastern tropical Pacific—less than 10% of the globe’s surface area—it can still do a surprisingly good job of simulating the ups and downs of global temperature over the last 50 years. That was the finding of Yu Kosaka and Shang-Ping Xie (Scripps Institution of Oceanography) in a paper published this past fall in Nature.


Subscribe to Oceans