Climate & Climate Change

Curbing carbon: Not enough plant food to go around?

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

How will climate change affect tropical forests?

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

What's behind urban heat islands?

January 13, 2015 | When a team of researchers wanted new insights into why cities tend to be much warmer than surrounding areas, they turned to a specialized and increasingly powerful piece of software that simulates urban environments. The software, part of the NCAR-based Community Earth System Model, focuses on how cities and their residents will be affected by heat waves and other aspects of climate change. The latest version simulates both dense downtowns and more residential areas, as well as atmospheric conditions such as humidity and wind that affect human health during heat waves. New research shows that the comparatively smooth surfaces of buildings, such as these in Dallas, contribute to the urban heat island effect. (Photo by drumguy8800 via Wikipedia Commons.) “More than half the world’s population lives in cities, so it’s important to simulate that environment,” said NCAR scientist Keith Oleson, who works on the urban simulations. “We want to project how climate is likely to change in different types of cities and how that may affect their residents.” Oleson was a co-author on a recent study in Nature, led by Lei Zhao at Yale University and China’s Nanjing University of Information Science and Technology, that quantified the primary reasons that urban areas are generally warmer than surrounding countryside. This phenomenon is known as the urban heat island effect. When moisture evaporates from trees and crops, it helps cool the air slightly. Because of this, scientists have long believed that the main cause of urban heat islands is the lack of evaporation from vegetation-sparse cities. Using the urban model as well as satellite observations, the authors reaffirmed this understanding for nighttime temperatures. However, it's a different story by daylight, as the researchers found that other factors are responsible for keeping cities hotter than rural areas. Those factors differ between dry and moist areas, according to the study, which analyzed 65 cities across North America. In more humid regions, the authors found that a lack of atmospheric turbulence plays a key role in keeping some cities hotter than their surroundings. The smooth surfaces of buildings and other structures are less conducive to removing heat from the surrounding air because they do not trigger as much vertical movement in the atmosphere as do the rougher surfaces of the lush vegetation surrounding these humid cities. The study found that a different process predominated in drier cities, such as those in the U.S. Southwest. In those settings, where surrounding vegetation is shorter and more sparse, rural areas are less effective at dissipating heat. Such results can help urban planners modify urban landscapes in ways that minimize heat stress for residents. For example, while it may not be easy to create more surface roughness to dissipate heat, the study reaffirmed a much-discussed strategy of constructing parking lots, roads, and buildings with lighter colors so they better reflect heat back into space. The next version of the urban climate model will provide even more information for policy makers. It will simulate the extent to which heating and air conditioning systems in buildings affect ambient temperatures. This is an increasingly important issue for megacities in developing countries such as India, where air conditioning units are becoming more common. “This 'waste heat' might exacerbate the urban heat island effect, because you’re essentially moving heat from the inside of a building to the outside,” Oleson said. Other researchers have found that waste heat from cities may be enough to influence temperatures thousands of miles away. Writer/contactDavid Hosansky, NCAR|UCAR Communications    

Not just rain: thunderstorms also pour down ozone

January 7, 2015 | A new study in Geophysical Research Letters offers for the first time unequivocal evidence that large storms move significant amounts of ozone from the stratosphere down to the troposphere, the lowest part of the atmosphere. The finding has implications for global climate because tropospheric ozone is a powerful greenhouse gas as well as a pollutant that affects human health and the environment. The research, led by NCAR scientist Laura Pan, means that scientists may have to re-evaluate climate models with regard to the transport of ozone. Those models generally do not include the role of thunderstorms, as they deal with larger and longer-range phenomena. It was already well established that tropospheric ozone originates in significant measure in the stratosphere. But the transport was primarily attributed to jet streams and other sources of circulation. The new study has its roots in a 2012 field project, known as the Deep Convective Clouds and Chemistry Experiment (DC3), that was based in the Great Plains and focused on the impact of storms on chemical composition of the atmosphere. On the night of May 30, the research aircraft flew through a line of large thunderstorms over Kansas. One of the research aircraft, the NASA DC-8, flew just above the storms in the lower stratosphere, carrying an instrument known as a Differential Absorption Lidar, or DIAL, to measure ozone levels. A rotating supercell thunderstorm moves across northeast Colorado. Thunderstorms such as this move ozone down from the stratosphere into the lower atmosphere. (©UCAR. Photo by Bob Henson.) Pan subsequently discovered that, during that flight, the DIAL instrument recorded a phenomenon that was only hinted before but never observed in an unambiguous fashion. Above the leading edge of the eastward moving storm, DIAL registered a curtain of ozone dipping below the stratosphere, where it was relatively abundant, into the troposphere. On a graph, this ozone-rich air resembled a ram’s horn whose wide end was pushed eastward ahead of the storm and whose narrow end curved westward into the storm. By examining the DIAL data and those from other instruments, the scientists determined that the “ram’s horn,” containing 150 parts per billion by volume (ppbv) of ozone, extended down to an altitude of about 8 km—or about 4 km (~2.5 miles) into the troposphere. At the same altitudes, but away from the storm system, ozone accounted for only 60 to 100 ppbv. In addition, thin filaments of the enhanced ozone extended about 100 km from the cloud’s edge. The researchers then set out to study the ozone transport process by numerically simulating the May 30 storm. Their simulation reproduced the ram’s horn and other observations made during the flight, demonstrating that deep convective storms like the one studied are capable of perturbing the tropopause, normally a stable barrier between stratosphere and troposphere. The authors say that the phenomenon challenges global chemistry climate models, since hundreds of storms like the one observed occur over the United States every summer, adding an as yet undermined quantity of ozone into the troposphere. Further, they say, as storm behavior may change in an evolving climate, it is important to understand and incorporate this process into global chemistry-climate models. Laura L. Pan, Cameron R. Homeyer, Shawn Honomichl, Brian A. Ridley, Morris Weisman, Mary C. Barth, Johnathan W. Hair, Marta A. Fenn, Carolyn Butler, Glenn S. Diskin, James H. Crawford, Thomas B. Ryerson, Ilana Pollack, Jeff Peischl, and Heidi Huntrieser (2014), Thunderstorms Enhance Tropospheric Ozone by Wrapping and Shedding Stratospheric Air, Geophysical Research Letters, 41, 7785-7790, doi: 10.1002/2014GL061921 WriterHarvey Leifert ContactDavid Hosansky, NCAR/UCAR Communications Collaborating institutionsNASA Langley Research CenterNational Oceanic and Atmospheric Administration Earth System Research LaboratoryCooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USAInstitute of Atmospheric Physics, German Aerospace Center FunderNational Science Foundation    

Tropical forests have large appetite for carbon dioxide

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

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

Greenhouse gases linked to past African rainfall

BOULDER — New research demonstrates for the first time that an increase in greenhouse gas concentrations thousands of years ago was a key factor in causing substantially more rainfall in two major regions of Africa. The finding provides new evidence that the current increase in greenhouse gases will have an important impact on Africa’s future climate. The study, led by the National Center for Atmospheric Research (NCAR), is being published this week in Science. “The future impact of greenhouse gases on rainfall in Africa is a critical socioeconomic issue,” said NCAR scientist Bette Otto-Bliesner, the lead author. “Africa’s climate seems destined to change, with far-reaching implications for water resources and agriculture.” The research drew on advanced computer simulations and analyses of sediments and other records of past climate. It was funded by the National Science Foundation, which is NCAR’s sponsor, and the Department of Energy Office of Science. A mysterious period of rain Otto-Bliesner and her co-authors in the United States and China set out to understand the reasons behind dramatic climate shifts that took place in Africa thousands of years ago. Lakes and other water features, such as the Ubari Oasis in southern Libya, were more prevalent across now-dry parts of Africa during past periods of more-plentiful precipitation. (Wikimedia Commons photo by Sfivat.) As the ice sheets that had covered large parts of North America and northern Europe started retreating from their maximum extent around 21,000 years ago, Africa’s climate responded in a way that has puzzled scientists. Following a long dry spell during the glacial maximum, the amount of rainfall in Africa abruptly increased, starting around 14,700 years ago and continuing until around 5,000 years ago. So intense was the cumulative rainfall, turning desert into grasslands and savannas, that scientists named the span the African Humid Period (AHP). The puzzling part was why the same precipitation phenomenon occurred simultaneously in two well-separated regions, one north of the equator and one to the south. Previous studies had suggested that, in northern Africa, the AHP was triggered by a ~20,000-year cyclic wobble in Earth’s orbit that resulted in increased summertime heating north of the equator. (In contrast, the northern hemisphere today is closest to the Sun in winter rather than summer.) That summertime heating would have warmed the land in such a way as to strengthen the monsoon winds from the ocean and enhance rainfall. But Otto-Bliesner said the orbital pattern alone would not explain the simultaneous onset of the AHP in southeastern equatorial Africa, south of the equator, since the wobble in Earth’s orbit led to less summertime heating there rather than more. Instead, the study revealed the role of two other factors: a change in Atlantic Ocean circulation that rapidly boosted rainfall in the region, and a rise in greenhouse gas concentrations that helped enhance rainfall across a wide swath of Africa. Tracing multiple causes of a wetter Africa As Earth emerged from the last Ice Age, greenhouse gases, especially carbon dioxide and methane, increased significantly—reaching almost to pre-industrial levels by 11,000 years ago—for reasons that are not yet fully understood. It was, the authors note, the most recent time during which natural global warming was associated with increases in greenhouse gas concentrations. (Because of feedbacks between the two, greenhouse gas concentrations and global temperature often rise and fall together across climate history.) The end of the last Ice Age also triggered an influx of fresh water into the ocean from melting ice sheets in North America and Scandinavia about 17,000 years ago. The fresh water interfered with a critical circulation pattern that transports heat and salinity northward through the Atlantic Ocean, much like a conveyer belt. The weakened circulation led to African precipitation shifting toward southernmost Africa, with rainfall suppressed in northern, equatorial, and east Africa. When the ice sheets stopped melting, the circulation became stronger again, bringing precipitation back into southeastern equatorial and northern Africa. This change, coupled with the orbital shift and the warming by the increasing greenhouse gases, is what triggered the AHP. To piece together the puzzle, the researchers drew on fossil pollen, evidence of former lake levels, and other proxy records indicating past moisture conditions. They focused their work on northern Africa (the present day Sahel region encompassing Niger, Chad, and also northern Nigeria) and southeastern equatorial Africa (the largely forested area of today’s eastern Democratic Republic of Congo, Rwanda, Burundi, and much of Tanzania and Kenya). In addition to the proxy records, they simulated past climate with the NCAR-based Community Climate System Model, a powerful global climate model developed by a broad community of researchers and funded by the National Science Foundation and Department of Energy, and using supercomputers at the Oak Ridge National Laboratory. By comparing the proxy records with the computer simulations, the study demonstrated that the climate model got the AHP right. This helps to validate its role in predicting how rising greenhouse gas concentrations might change rainfall patterns in a highly populated and vulnerable part of the world. “Normally climate simulations cover perhaps a century or take a snapshot of past conditions,” Otto-Bliesner said. “A study like this one, dissecting why the climate evolved as it did over this intriguing 10,000-year period, was more than I thought I would ever see in my career.” About the article Title: Coherent changes of southeastern equatorial and northern African rainfall during the last deglaciation Authors: Bette L. Otto-Bliesner, James M. Russell, Peter U. Clark, Zhengyu Liu, Jonathan T. Overpeck, Bronwen Konecky, Peter deMenocal, Sharon E. Nicholson, Feng He, Zhengyao Lu Publication: Science, doi: 10.1126/science.1259531

El Niño, La Niña & ENSO FAQ

Frequently asked questions What is El Niño? What is La Niña? What is ENSO and what does it stand for? How did these terms originate? How are El Niño and La Niña events defined? How do El Niño and La Niña affect weather in other parts of the world? Do El Niño and La Niña influence global temperature? Will climate change influence El Niño and La Niña? Advances in ENSO research Predicting El Niño and La Niña events What forces cause an El Niño to develop? Related stories Here comes El Niño—but what exactly is it?El Niño or La Nada? The great forecast challenge of 2014¡Hola, La Nada! What happens when El Niño and La Niña take a break? What is El Niño? Sea surface temperature and height anomalies in the eastern Pacific Ocean show the influence of expansion and warming during El Niño (November 1997, top) and cooling during La Niña (November 1998, bottom). (Images courtesy NASA.) El Niño is a warming of the central to eastern tropical Pacific that occurs every two to seven years, on average. The term also commonly refers to the atmospheric rearrangements that occur with the oceanic warming. During an El Niño event, sea surface temperatures across a watery expanse often as large as the United States can warm by 1–3°F or more for a period of from a few months to a year or two.    What is La Niña? La Niña, the counterpart to El Niño, is a cooling of the waters across the same region. As with El Niño, the term La Niña typically is used to refer to the associated atmospheric as well as oceanic patterns. It often lasts longer than El Niño, sometimes persisting or recurring for two or more years.  back to top What is ENSO and what does it stand for? El Niño and La Niña are normally accompanied by variations in the tropical Pacific Ocean’s Walker Circulation (see diagram), as well as a vast see-saw in atmospheric pressure—the Southern Oscillation—that modifies the Walker pattern. The term El Niño–Southern Oscillation, or ENSO, refers to the combination of atmospheric and oceanic effects associated with both El Niño and La Niña. El Niño and La Niña can be thought of as the ocean part of ENSO, while the Southern Oscillation can be thought of as the atmospheric component.  Through the atmosphere, teleconnections occur via waves to influence regions far from the tropical Pacific (see below). This diagram shows the Walker Circulation, a vast loop of air above the equatorial Pacific Ocean. See below for an alternate depiction. Click here or on the image to enlarge. (Illustration by Gabriel Vecchi, UCAR.) ENSO is a coupled phenomenon that would not occur without interactions between the atmosphere and ocean. Sometimes an atmospheric shift can occur in the tropical Pacific without the ocean fully responding, or vice versa. However, for an El Niño or La Niña to develop, the atmosphere and ocean must evolve in sync. For example, the usual east-to-west surface trade winds across the tropical Pacific (see diagram at left) tend to weaken during El Niño; this allows warm water to shift toward the eastern tropical Pacific. That warm water supports the eastward development of showers and thunderstorms, and the resulting atmospheric circulations tend to weaken the trade winds even further. Such positive feedbacks are crucial for the development of both El Niño and La Niña events. ENSO is one of the main sources of year-to-year variability in weather and climate around the world. Research to more fully incorporate this interannual variability into computer models is a major focus for improving long-range forecasting.  back to top How did these terms originate? The name El Niño originated in the region where one of the phenomenon’s local effects was first recognized. Fishing people and other coastal residents of Peru had long noticed a strange feature in the eastern Pacific Ocean waters that border their home. This region of tropical yet relatively cool water is host to one of the world’s most productive fisheries and a large bird population. In the first months of each year, a warm southward current usually modified the cool waters. But every few years, this warming started early (in December), was far stronger, and lasted as long as a year or two. Torrential rains fell on the arid land; as one early observer put it, “the desert becomes a garden.” Warm waters flowing south brought water snakes, bananas, and coconuts from equatorial rain forests. However, the same current shut off the deeper, cooler waters that are crucial to sustaining the region’s marine life. Because these strong events were often observed close to Christmas time, the phenomenon was dubbed El Niño (when capitalized, “the little boy” becomes “the Christ Child” in Spanish). Although “la niña” refers to a young girl, the capitalized term La Niña did not originate in the same way; it was adopted by researchers in the 1970s and 1980s to illustrate the relationship between warming and cooling events in the waters of the eastern tropical Pacific. Some researchers suggested “El Viejo” (the old man) or “anti–El Niño” as alternatives, but La Niña has won out as the standard term.  back to top How are El Niño and La Niña events defined? The most widely used definitions of El Niño and La Niña involve sea surface temperatures (SSTs) across the eastern tropical Pacific (see map below). To qualify as an El Niño event, according to the definition used by the National Oceanic and Atmospheric Administration, SSTs must remain at or above 0.5°C (about 1°F) for at least three months across the region labeled Niño3.4. NOAA provides historical records for temperatures in this key region. A “moderate” event would see readings holding at or above 1.0°C, and a “strong” event would be 1.5°C or higher. Sometimes the term “El Niño conditions” is used when SST readings have hit the El Niño threshold but haven’t been in place long enough to qualify as an El Niño event. Sea-surface temperature, averaged across the regions above, is used to track and diagnose El Niño and La Niña conditions. The Niño3.4 region is most commonly used. For reference, the vertical line at 180 degrees longitude represents the International Date Line. (Image courtesy Jan Null, Golden Gate Weather Services.) The Australian Bureau of Meteorology uses a Niño3.4 SST threshold of 0.8°C rather than 0.5°C to define El Niño, because a greater amount of SST warming is needed to produce impacts in Australia. The definitions for La Niña are the same as those for El Niño, except that the sea surface temperatures are below average rather than above. Each region highlighted in the map can be used to convey the nature of a particular event. Some El Niños show warming focused in the far eastern Pacific (Niño1+2). Others show the greatest oceanic warming across the Niño4 region, with substantially different atmospheric patterns than other El Niño events. This latter type is referred to as “El Niño Modoki” (from the Japanese for “similar but different”). Both El Niño and La Niña are most likely to intensify during northern fall, peak during the winter, and subside in the spring. For this reason, El Niño and La Niña events are often referred to by two adjoining calendar years (e.g., 2005–06). This graphic shows three-month running averages of sea-surface temperature across the Niño3.4 region. (Graph courtesy NOAA.) The strongest El Niño events on record, in 1982–83 and 1997–98, had three-month Niño3.4 averages peaking at 2.2°C and 2.4°C, respectively, according to NOAA’s Oceanic Niño Index table. The strongest La Niña event was in 1973–74, with a three-month Niño3.4 average reaching –2.0°C. An especially prolonged La Niña ran from mid-1998 to early 2001, with the index dipping to –1.7°C.  back to top How do El Niño and La Niña affect weather in other parts of the world? During an El Niño event, as warm surface water in the Pacific is shifted thousands of miles east of its usual location, the showers and thunderstorms nurtured by convection above this warm tropical water also change location. As the rising motion associated with the convection also shifts eastward, it causes other adjustments in atmospheric circulation, well away from the tropical Pacific. These persistent zones of rising and sinking air can shift into new locations for months, causing prolonged wet or dry conditions and related atmospheric heating anomalies. In turn, the anomalous heating sets up planetary-scale waves in the atmosphere that radiate away from the region, especially into the hemisphere experiencing winter. These are “teleconnections”—large-scale, long-lasting shifts in atmospheric circulation that can affect much of the globe. The effects extend throughout the Pacific Rim, across large parts of North America, and on to eastern Africa and other regions. La Niña brings a different set of teleconnections to these and other regions, with some but not all effects roughly opposite to those of El Niño. These maps show the most commonly experienced impacts related to El Niño (“warm episode,” top) and La Niña (“cold episode,” bottom) during the period December to February, when both phenomena tend to be at their strongest. (El Niño and La Niña images courtesy NWS/NCEP Climate Prediction Center.) The maps at right show the most common teleconnections associated with El Niño and La Niña during northern summer and winter. Not every warm- or cool-water event will produce all of these impacts, because other atmospheric features interact with each ENSO event to influence weather and climate around the globe. Weaker events, in particular, may look quite different from the prototypes shown here. El Niño Modoki events (where the warming is concentrated further to the west than usual) show significant differences in teleconnections from other El Niño events. In the United States, a strong El Niño event tends to produce milder- and drier-than-average conditions toward the north and cooler- and wetter-than-average conditions to the south. In California, a strong El Niño very often brings more moisture than usual. However, during the weakest El Niño events, San Francisco and Los Angeles are a bit more likely to be unusually dry than unusually wet. (Meteorologist Jan Null maintains a compilation of additional El Niño “myths and realities.”)  back to top Do El Niño and La Niña influence global temperature? During El Niño, a deep pool of warm water usually restricted to the western tropical Pacific is replaced by a much larger, more shallow pool of warm water that covers most or all of the tropical Pacific. The expanded zone of warm sea surface temperatures allows more heat to be conveyed from the ocean into the atmosphere for months at a time. As a result, globally averaged temperatures often rise by a few tenths of a degree Fahrenheit during the latter stages of a strong El Niño event. Conversely, global temperatures can drop by a similar amount during a La Niña event. NCAR scientist Kevin Trenberth has likened El Niño to a “pressure valve” that releases built-up heat from the oceans into the atmosphere. The oceans cool during El Niño events, while the global atmosphere warms. The red line above shows surface temperatures (over land and ocean), in a running 12-month average, as calculated by NOAA. The vertical, shaded bars indicate El Niño (buff) and La Niña (sky blue) periods, based on Niño3.4 SST (sea surface temperature) anomalies. These anomalies appear in red (warm) and blue (cool) in the lower panel. (Figure 6, “An apparent hiatus in global warming?” Kevin E. Trenberth and John T. Fasullo, Earth’s Future, 2013.)   Scientists often account for ENSO by factoring out these bumps and dips in global temperature when analyzing the long-term trends related to climate change. For example, the first 15 years of this century saw more La Niña than El Niño influence, and global air temperatures showed little rise. Prior to that period, in the 1980s and 1990s, when El Niño events were more frequent, global temperatures rose more sharply. The map at right shows year-to-year fluctuations in global temperature (in red), the long-term trend of rising global temperature (in black), and the starts and stops of all ENSO events (shaded vertical bars). Note that other factors also influence global temperature, such as the eruption of Mt. Pinatubo in 1991. The volcano threw enough sun-blocking material into the atmosphere to cause a drop in global temperatures during 1992, despite the presence of El Niño. Shifts in global temperature, as well as in the likelihood of ENSO events, are closely associated with the state of the Pacific Decadal Oscillation (PDO), a pattern of ocean temperatures that reverses every 20-30 years. More La Niña events tend to be observed when the PDO is negative, and more El Niño events when it is positive. Scientists are not yet sure what prompts the PDO to shift modes.   back to top Will climate change influence El Niño and La Niña? Despite steady improvement, it remains difficult for many global climate models to simulate all aspects of ENSO events, so it has been a challenge to determine whether or how El Niño and La Niña will be affected by rising global temperatures. Some models have suggested that the central Pacific could warm enough to produce El Niño–like conditions on a semipermanent basis. The most recent (2013) assessment by the Intergovernmental Panel on Climate Change concluded: There is high confidence that the El Niño-Southern Oscillation (ENSO) will remain the dominant mode of interannual variability in the tropical Pacific, with global effects in the 21st century. Due to the increase in moisture availability, ENSO-related precipitation variability on regional scales will likely intensify. Natural variations of the amplitude and spatial pattern of ENSO are large and thus confidence in any specific projected change in ENSO and related regional phenomena for the 21st century remains low. [See IPCC AR5, Working Group I, PDF, page 21]  back to top Predicting El Niño and La Niña events Since recognizing more than 40 years ago that the oceanic and atmospheric parts of ENSO are strongly linked, scientists have investigated the combined phenomena from a variety of perspectives, gradually building a deeper understanding of this complex, influential atmospheric pattern. When a major El Niño event struck without warning in 1982–83, scientists recognized the event only after it had formed. This was because of limited observations, primitive computer models, the presence of simultaneous effects from a 1982 eruption of Mexico’s El Chichón, and an incomplete recognition of how El Niño development could vary from one event to another. Over the following decade, the United States, Japan, and other nations deployed instrumented buoys across much of the tropical Pacific. Since these buoys can detect the eastward spread of warm water months before an El Niño hits maturity, they serve as an invaluable early-warning tool. A regional model developed at Lamont-Doherty Earth Observatory generated some of the first model-based ENSO predictions starting in the late 1980s. Today, a number of computer models around the world use current ocean temperatures and atmospheric conditions to project the state of ENSO, looking a year or more into the future. Forecasters examine multi-model ensembles, scrutinizing where various models agree or disagree, in order to issue El Niño and La Niña forecasts. This graphic shows the likelihood of El Niño (red vertical bars), La Niña (blue bars), or neutral conditions (green bars) for overlapping three-month periods starting with November 2014–February 2015 (NDJ), with the likelihoods determined by a consensus of experts based on input from multiple computer models. The thin horizontal lines show the likelihood derived from climatology alone (the average occurrence, based on past experience). (Image courtesy International Research Institute for Climate and Society.) Among the leading sources of regular ENSO forecasts are NOAA’s Climate Prediction Center, the International Research Institute for Climate and Society at Columbia University (working with NOAA), and the Australian Bureau of Meteorology. (Note that the Australian BOM uses a higher threshold for El Niño development than the U.S. definition; see above.) Generally, the strongest ENSO events are predicted more accurately than weaker ones. A 2012 analysis in the Bulletin of the American Meteorological Society evaluated ENSO forecasts from 20 different prediction models for the period 2002 to 2011. The study found that the predictive skill of these models actually declined in the 2000s as compared to the 1980s and 1990s—not because of any loss in model quality (the models themselves had actually improved), but because the weaker, more variable ENSO events in this period made forecasting a far greater challenge. For more about the history of ENSO research, see Here comes El Niño—but what exactly is it?  back to top What forces cause an El Niño to develop? To figure out if a strong El Niño is on the way, forecasters must analyze a series of complex events and processes, ranging from the shifting temperature and structure of subsurface waters across the Pacific, to the evolution of trade winds as well as more localized wind bursts. The most intense Kelvin wave ever recorded across the equatorial Pacific Ocean brought a huge area of warmer-than-average water eastward from April to July 2014, but it failed to produce the long-lasting warm surface waters that define an El Niño event. These monthly cross sections show where water temperatures are departing from average within a slice of the equatorial Pacific, with Indonesia on the left and South America on the right. The legend at right is in multiples of 0.5°C. (Image courtesy Australian Bureau of Meteorology.) One ingredient that appears to be crucial is a multiyear buildup of warm water in the western Pacific. Two types of slow-moving disturbances can also push water upward and downward across the Pacific. Rossby waves travel westward, while Kelvin waves move eastward. In both cases, the sea surface rises, then falls, by up to several inches over a period of weeks to months as the wave passes. Often these effects are transient, but sometimes they can help nudge the Pacific into or out of an El Niño pattern. So can the Madden-Julian Oscillation, a pulse of atmospheric energy generated in the Indian Ocean every few weeks. MJO events can push clusters of showers and thunderstorms eastward across the tropical Pacific. In addition, to get an El Niño going, something must slow or reverse the trade winds. Once that happens, the displaced wind and water can then reinforce the El Niño state in a feedback process lasting for months. However, it’s not easy to turn around those persistent east-to-west winds in the first place. Westerly wind bursts can help do the trick. These clumps of west-to-east wind, pushing directly against the trade winds, can span hundreds of miles and can last a few days to several weeks. They can also kick off the Kelvin waves noted above. Although La Niña can develop independently of El Niño, it often materializes immediately after an El Niño event, as Kelvin waves and other phenomena cause the ocean to “bounce back,” overcorrecting the changes initially brought by El Niño. The La Niña pattern involves a strengthening rather than a partial reversal of both trade winds and the larger Walker Circulation. In part because La Niña more closely resembles the neutral state of the Pacific, it is somewhat easier for a La Niña event to last longer (up to 2–3 years) than an El Niño, which rarely persists for more than a year at a time. Scientific adviser: Kevin Trenberth Writer: Bob Henson Last updated : December 2014  back to top    

Here comes El Niño—but what exactly is it?

December 2, 2014 | It’s been a fascinating journey, watching how scientific and public views of El Niño have evolved over the last several decades. Sometimes you still see El Niño portrayed as a devastating creature, guaranteed to wreak havoc on everything in its supposed path. But the picture has gradually grown more sophisticated as scientists have learned more about the workings of this vast, complex phenomenon and how to communicate its impacts effectively. How did we get from there to here? A few landmarks stand out in the last 50 years of research and communication on El Niño, La Niña, and the El Niño–Southern Oscillation (ENSO).  (For quick definitions of each of these, check out our Weather-Maker Patterns Glossary. For frequentlly asked questions, see the El Niño, La Niña & ENSO FAQ) Linking atmosphere, ocean, and distant weather This diagram shows the Walker Circulation, a vast loop of air above the equatorial Pacific Ocean. (Illustration by Gabriel Vecchi, NOAA/GFDL.) Scientists in the mid-20th century were aware of the tropical Pacific Ocean's Walker Circulation (see diagram), as well as a vast see-saw in atmospheric pressure—the Southern Oscillation—that modified this prototypical pattern. And people living near the coast of Peru and Ecuador knew that the cold upwelling that nourished their fish-packed coastal waters sometimes disappeared for months at a time, especially around Christmas (thus the local name “El Niño,” or “the Christ Child”). These two phenomena were assumed to be unrelated until Jacob Bjerknes published a landmark paper in 1966 (see PDF). The concepts began to mesh further after a major El Niño event in 1972–73, when scientists began to draw firmer links between the oceanic warming of El Niño (which extended much further into the tropical Pacific than researchers had realized) and various atmospheric features that occurred across much of the globe (dubbed (“teleconnections”). The term El Niño–Southern Oscillation (ENSO) soon came into vogue. Pioneering social scientists such as Michael “Mickey” Glantz (then at NCAR, now at the University of Colorado Boulder) delved into the implications of El Niño for people on the ground and the potential usefulness of El Niño predictions for saving lives and livelihoods (see PDF, 1977). Building a detection system Extreme drought across Australia, fostered by a strong El Niño event, fed this wall of dust that swept across Melbourne on February 8, 1983. (Photo by Trevor Farrar, courtesy Australian Bureau of Meteorology and During the northern winter of 1982–83,  the most intense El Niño recorded up to that time brought devastating winter storms to much of California and severe drought and wildfire to Australia, among other widespread impacts. Scientists recognized this event only after it had formed, due to limited observations, primitive computer models, the presence of simultaneous effects from a 1982 eruption of Mexico’s El Chichón, and an incomplete recognition of how El Niño development could vary from one event to another. The next decade saw a concerted effort by the United States, Japan, and other nations to deploy instrumented buoys as part of the Tropical Ocean and Global Atmosphere program (TOGA). Since these buoys can detect the eastward spread of warm water months before an El Niño event hits maturity, they serve as an invaluable early-warning tool. (Unfortunately, it’s been a struggle at times to keep the buoys fully funded and maintained. In early 2014, Nature reported that “nearly half of the buoys in the Tropical Atmosphere Ocean array have failed because of delayed maintenance.”) Dozens of instrumented buoys were moored throughout the equatorial Pacific Ocean in the early 1990s, as part of what’s now called the TAO/TRITON array. The array gathers surface meteorological and oceanographic data and records ocean temperature to a depth of about 1,650 feet. (Photo courtesy NOAA.) Computer models also leapt forward in the 1980s, confirming the ocean-atmosphere linkage and pointing the way toward El Niño forecasts. A regional model developed by Mark Cane and Stephen Zebiak (Lamont-Doherty Earth Observatory) generated some of the first model-based ENSO predictions starting in the late 1980s. Other scientists began to simulate ENSO within global climate models, a task with its own challenges (some of which are still being tackled, as steady progress continues). A landmark field campaign hits the Pacific TOGA mounted one of the largest-ever atmospheric research projects, the Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). Aircraft and ships canvassed the western Pacific tropics for months, and the resulting analyses showed how tightly the ocean and atmosphere components of ENSO were linked. The study is often cited in research papers to this day. El Niño of the century California's Russian River saw damaging floods  in March 1998 in the wake of heavy rain associated with that winter's intense El Niño event. (Photo by Dave Gatley, FEMA.) A massive El Niño, even stronger than the 1982–83 event, took hold in 1997–98. This time, thanks to the TOGA buoy network and more sophisticated satellites, forecasters saw the oceanic precursors well in advance and sounded the alarm ahead of the biggest impacts. These included record rains in California, severe drought in Indonesia, and a deadly tornado outbreak in Florida. The early warning not only helped decision makers charged with public safety get ready: it also piqued the interest of the public, many of whom hadn’t heard the term El Niño before. That label’s pithiness and catchy flair surely helped. Journalists had a field day with “El Niño,” whereas any headline writer would struggle with “the warm phase of the El Niño–Southern Oscillation.” This event’s high-water mark in pop culture came on October 25, 1997, as Chris Farley strode onto Saturday Night Live wearing a pro-wrestling outfit and bellowing, “I am El Niño!” La Niña takes center stage Sea surface temperature and height anomalies in the eastern Pacific Ocean show the influence of expansion and warming during El Niño (November 1997, top) and cooling during La Niña (November 1998, bottom). (Images courtesy NASA.) In surprisingly short order, the intense El Niño conditions of early 1998 segued into a strong La Niña event (1998–99). Over the preceding years, researchers hadn’t been in total agreement on a name for the cool-water counterpart to El Niño—some had proposed “El Viejo” (the old man”), “anti–El Niño,” or “cold event”—but by 1998, La Niña (“the young girl”) had begun to stick. That summer, NCAR’s Glantz brought a diverse group of scientists, journalists, and policy experts to Boulder for the first-ever La Niña Summit (see PDF for executive summary, United Nations University Press for collected papers). Thus far, the 21st century has been a quiet century for El Niño. The last 15 years have been more dominated by La Niña events, part of a Pacific-wide rearrangement of ocean circulation that tends to shift modes every 20–30 years. This phenomenon is referred to as the Pacific Decadal Oscillation or the Interdecadal Pacific Oscillation, depending on which part of the ocean is analyzed. It’s not yet clear what causes the PDO to switch modes from negative (cool sea surface in the central North Pacific) to positive (warm sea surface), or vice versa. But evidence is mounting that the world’s oceans tend to retain more heat deeper down, well below the surface, during the PDO’s negative phase—apparently a big reason why Earth’s atmosphere hasn’t warmed much since the late 1990s. Forecasts, busts, and probabilities Today, a number of computer models around the world use current ocean temperatures and atmospheric conditions to project the state of ENSO, looking a year or more into the future. Forecasters examine multi-model ensembles, scrutinizing where various models agree or disagree, in order to issue El Niño and La Niña forecasts. Among the leading sources of regular ENSO forecasts are NOAA’s Climate Prediction Center, the International Research Institute for Climate and Society at Columbia University, and the Australian Bureau of Meteorology. (Note that the Australian BOM uses a higher threshold for El Niño development than the U.S. definition.) El Niño hype returned in a big way early in 2014, when a huge surge of warm water spread eastward across the tropical Pacific. Some computer models suggested that a major El Niño event could develop by later in the year. But by the autumn of 2014, El Niño hadn’t yet materialized, a no-show that was widely interpreted as a forecast “bust.” Only in November did the signs of an approaching El Niño become more insistent, with sea surface temperatures in the Pacific’s Niño3.4 measurement region finally pushing well above the threshold of 0.5°C above average. Was the forecast really so bad? Northern spring is a notoriously difficult time to predict ENSO evolution. Moreover, the models and forecasters were never in lockstep agreement about what would happen later in 2014. The model consensus posted by the International Research Institute in April (in partnership with NOAA) wasn’t particularly dire, though some models did end up overshooting the mark. As conditions and model solutions evolved, NOAA's predicted likelihood that an El Niño would form by summer 2014 rose from 50% in the March outlook (PDF) to "exceeding 50%" in April (PDF), 65% in May (PDF), and 80% in June (PDF), with that June forecast applying to fall/winter 2014–15 rather than summer 2014. Update: December 2, 2014 | The above paragraph has been revised to incorporate a fuller range of NOAA products issued in early 2014. Lessons from 2014 As of mid-November 2014, the various prediction models examined by the International Research Institute for Climate and Society (IRI) tended to produce a weak to moderate El Niño for winter 2014-15, but with a fair bit of spread among the model solutions. (Image courtesy IRI.) The “missing” El Niño event of mid-2014 reminds tells us that we need to carefully manage our expectations about El Niño prediction. Once the oceanic and atmospheric fingerprints of El Niño begin to take shape in northern fall, as they're doing right now, then we can look more confidently to the impacts likely to occur the following winter, when El Niño events typically peak. However, this isn’t really predicting El Niño itself so much as “nowcasting” the emergence of El Niño’s oceanic warming and predicting the resulting impacts we’re likely to see. The oceanic shift itself remains much more difficult to forecast. The dance between atmosphere and ocean that nurtures and sustains El Niño is simply too complex for any model to capture with total accuracy. That’s why it’s so important to keep probabilities in mind. We can’t expect ironclad certainty on whether or when the eastern Pacific will warm into an El Niño event, but we can use multiple models to help gauge the odds. Similarly, and crucially, since each El Niño is different and since other weather and climate variables are always in the mix, the presence of El Niño doesn’t guarantee that Los Angeles will be doused with midwinter rain or Seattle will be dry and mild. However, it does shift the likelihoods in those directions. In that sense, we might want to think of El Niño less like an appealing buffoon from a comedy sketch—and more like a shrewd oddsmaker from a densely plotted thriller. Dive Deeper El Niño, La Niña & ENSO FAQ (NCAR/UCAR Backgrounder, December 2014) The "Almost" El Niño of 2014 (UCARConnect, December 2014) Writer/contact:Bob Henson, NCAR/UCAR Communications          

Burning questions about winter cold

November 10, 2014 | Old Man Winter seems to have gone maverick in the Northern Hemisphere over the last few years. Take 2014 as an example. It's on track to be the warmest globally in more than a century of record-keeping, with May, June, August, and September all setting world heat records for those particular months. Yet February only managed to tie for 21st warmest globally, mainly because of two regions of prolonged cold across North America and central Eurasia (see map below). Much of North America and central Eurasia were plastered with unusually cold air (blue regions) during February 2014, while most other land areas on Earth ran warmer than average. (Image courtesy National Climatic Data Center.) As people who've endured some of the worst cold and snow in decades try to reconcile that experience with the picture of a warming planet, they're also bracing for what 2014–15 might bring. A highly anticipated shift toward colder-than-average weather over the next few days across the central and eastern United States might be the first volley. With a weak to perhaps moderate El Niño now expected to take shape, the U.S. winter outlook from NOAA calls for temperatures across the Midwest and Northeast to be near or above average, in line with what's most common during El Niño events. However, AccuWeather is calling for recurrent bouts of cold and snow in roughly the same area. These outlooks aren't directly comparable—for example, AccuWeather doesn't specify whether the northeastern cold and snow would be worse than in an average winter—but the contrasting tones do suggest differences of opinion about what's most likely to steer our upcoming winter. Two takes on our upcoming winter: seasonal outlooks issued in October by NOAA (top) and AccuWeather (bottom).  Likewise, there's major disagreement among scientists on exactly what might be causing the more general tendency toward colder winters in places like the central and eastern United States, Europe, and Russia. One factor is the Arctic, where depleted sea ice may be playing a role. Another is the tropical Pacific, where a tendency toward cooler-than-average sea-surface temperatures may also be involved. The debate—one of the liveliest in weather and climate science today—is largely about which factor is most important. The role of the seas, from equator to pole The question of what's put the chill on northern midlatitude winters is tightly linked to debate over the "global warming hiatus," the widely publicized leveling of global temperatures that's run from the late 1990s into the early 2010s. As we've noted before, multiple studies indicate that the world's oceans have mopped up more heat than usual in this period and stored it at great depths, thus accounting for much of the hiatus. Scientists are still exploring which oceanic regions are most involved. 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 in An apparent hiatus in global warming? Earth's Future, doi:10.1002/2013EF000165.) One fascinating aspect of the hiatus is that it's been concentrated in the months of December through February (DJF)—the period known as meteorological winter in the Northern Hemisphere. This interval has seen a slight drop in global temperature since the late 1990s, while the other nine months have held fairly steady (see map at right). "The strongest pause is in the northern winter," said NCAR's Kevin Trenberth, who recently analyzed seasonal aspects of the hiatus with three NCAR colleagues in the journal Nature Climate Change. While places like Chicago, New York, Berlin, and Moscow have had a few severe winters lately, they've also had some very mild ones. A much larger, more sustained region of cooling lies across the central and eastern tropical Pacific Ocean, where El Niño and La Niña play out (see map below). As it happens, La Niña—the periodic cooling of surface water across the eastern tropical Pacific—has predominated over El Niño since the late 1990s. Both El Niño and La Niña events tend to be at their strongest in DJF. Where it's really cool: The map above was produced by subtracting average November-through-March temperatures for 1976–1998 from the readings for 1998–2012. The most prominent cooling can be found over the northeast Pacific and the eastern equatorial Pacific. (Figure 3(f) from Trenberth et al., Seasonal aspects of the recent pause in surface warming, Nature Climate Change, doi:10.1038/NCLIMATE2341.) La Niña events typically last just a year or two, but a similar pattern can persist across the North Pacific for periods of 20 or 30 years—then flip to an opposite mode, more akin to El Niño, for another 20-30 years. This is the Pacific Decadal Oscillation, and it appears to have worked in concert with specific La Niña events to reinforce the cooler-than-normal waters at the equatorial surface since around 1998. During La Niña, stronger-than-usual trade winds keep warmer equatorial waters shunted toward the western Pacific. Such long-lasting rearrangements can goose the atmosphere into persistent responses that extend over a great distance. Modeling experiments carried out for the Trenberth et al. paper suggest that, especially in DJF, the unusually warm equatorial waters toward the western Pacific near the equator have led to a train of rising and sinking air pockets, called quasi-stationary Rossby waves. Analyzed at the jet-stream level (about 6 miles high), this wave train arcs northward into Alaska, eastward across the Canadian Arctic, then back south over the North Atlantic. Trenberth and colleagues argue that the wave train they identified could be the main culprit disrupting the "polar vortex" (the band of high-altitude winds that normally encircles polar regions), thus allowing for more southward intrusions of Arctic air and poleward surges of mild air. However, another prominent view is that the topsy-turvy pattern might be primarily caused not by the tropical ocean but by the Arctic itself. Some of the leading researchers in this camp include Jennifer Francis (Rutgers University), Judah Cohen (Atmospheric and Environmental Research, or AER), and James Overland (NOAA Pacific Marine Environmental Laboratory). The Arctic-as-driver viewpoint has gotten major play in media coverage over the last several years, and research on it continues to accumulate. How melting ice could lead to big snow Cohen began staking out theoretical territory starting more than a decade ago. Through a series of studies with colleagues at AER and the Massachusetts Institute of Technology, he found that greater-than-usual snowfall extent over Siberia during October correlates strongly with unusual cold and snow several months later, during the heart of winter, across the middle and higher latitudes of North America and Eurasia. The idea, as AER's website explains, is that heavy autumn snow encourages the build-up of deep, cold air masses over Russia, accompanied by a persistent trough, or dip, in the jet stream. As autumn moves toward winter, Rossby waves propagate upward from the trough into the stratosphere, disrupting the polar vortex. This would encourage increased "waviness" in the jet stream and help foster southward-plowing Arctic outbreaks. It would also help explain why the atmospheric hiatus has been focused in DJF, as noted by Cohen and colleagues in 2012 and more recently analyzed in the Trenberth paper. Arctic sea ice normally melts back to its minimum extent in September before regrowing in winter. The last 35 years show an unmistakable drop in September sea-ice extent across the Arctic. (Graph courtesy National Snow and Ice Data Center.) Why would this chain of events occur more often today than, say, 20 or 30 years ago? The biggest change in recent years at high northern latitudes has been the dramatic loss of summer and autumn sea ice, especially since 2007. Cohen hypothesizes that moisture from the Barents and Kara seas—which are now largely unfrozen well into autumn—is helping pump up snowfall totals over Siberia, located just to the southeast. Hunting for a middle ground While these two groups of scientists are often portrayed as being at loggerheads, it's possible that both of their hypothesized processes are at play. And perhaps it's too soon to tell. A review paper by Cohen, Francis, Overland, and eight other researchers, which appeared in Nature Geoscience in August, aims to identify what's generally accepted and what remains in question. "We certainly do not discount a tropical influence on the Arctic or on midlatitude wave patterns," said coauthor James Screen (University of Exeter) in an email to AtmosNews. "Indeed, this has been shown in a number of studies." Led by Cohen, the review paper focuses on Arctic amplification—the well-documented tendency of warming to be strongest at high latitudes, toward the North Pole—and its potential role in midlatitude weather extremes, especially during winter. This is a critical relationship to understand, especially since researchers agree that Arctic amplification is likely to continue. The current lack of consensus on the role of the Arctic in midlatitude weather, they suggest, "may help explain the media portrayal of a polarized view among scientists." When compared to the 1981–2010 mean value, snowfall was more extensive than usual for October (blue and purple areas, in percent above average) across much of the Northern Hemisphere last month. In Eurasia, the amount of snow cover was the second largest (behind only 1976) for any October since satellite monitoring began in 1967. Researchers have connected October snowfall in Eurasia to an increased risk of midlatitude cold outbreaks in the winter. (Map courtesy Rutgers Snow Lab.) The paper notes that both observational analyses and modeling studies lend some support to Cohen's hypothesis involving autumn snow cover in Eurasia. The review also suggests that, along with helping to produce increased snow cover, the delayed freeze-up of nearby sea ice could itself work in tandem with the Siberian cold and snow to displace the jet stream in autumn and produce a more variable polar vortex by early winter. One analysis published in September outlines the physics behind this chain of events, and another recent study suggests that the odds of severe winters in central Eurasia have roughly doubled because of the reduced Barents and Kara sea ice. "I think we can now say that this mechanism has transitioned from a hypothesis to a theory," said Jennifer Francis in an email to AtmosNews. On the other hand, Trenberth argues that the tropical oceans, though distant from the Arctic, hold a vastly larger store of energy, and thus any rearrangement of tropical water ought to have an outsized impact on mid- and high-latitude winter weather. Arctic amplification "is a consequence, not a cause" of the unusual jet-stream patterns, according to a recent essay by Trenberth. He joined John Wallace (University of Washington) and several other leading researchers in a letter published in Science last winter urging caution in attributing Arctic amplification and midlatitude extremes to sea-ice loss. Given enough time, this now-cooking debate could prove moot. A number of scientists expect that by later in the century, overall global warming—amplified in the Arctic—should predominate. A three-year project led by NCAR's Clara Deser and Lantao Sun is now working to quantify the longer-term picture. Their approach: Using two climate models (from NCAR and the UK Met Office) that extend up through the stratosphere, vary the amount of Arctic sea ice, and then look for the effects on mid- and high-latitude climate toward the latter part of this century. Initial results are now being finalized for a soon-to-be published paper. As for the coming winter, snow cover extent across Eurasia in October came in as the second most extensive on record—perhaps a chilly portent. Cohen will be posting a winter outlook in an annually updated report on the NSF website by the end of November. Meanwhile, according to a post by Mashable's Andrew Freedman, NOAA is considering a shift in its own winter outlook in a scheduled November 20 update, in light of the ample Siberian snowpack, El Niño's lackluster onset, and other factors. If global warming does eventually make the roughest winters increasingly rare, it's easy to envision today's youngsters telling their own grandkids about what conditions were like during the legendary cold and snow of the 2010s. Writer/contactBob Henson, NCAR/UCAR Communications    


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