Most of the Earth’s fresh water is locked in the ice that covers Antarctica. As the ocean and atmosphere grow warmer, that ice is with sea levels and global currents changing in response. To understand the potential implications, researchers need to know just how fast the ice is disappearing, and what is driving it back.

The West Antarctic ice sheet, an unstable expanse bordering the Amundsen Sea, is one of the greatest sources of uncertainty in climate projections. Records indicate that it has been steadily shrinking since the 1940s, but key details are missing. Using environmental data gathered from ice samples, tree rings and corals, 91̽�� researchers tailored a climate model to Antarctica and ran simulations to understand how changing weather patterns dictate ice melt.

The results, , were surprising. For years, researchers have hypothesized that westerly winds were ferrying warm water toward the ice sheet, accelerating ice melt. The new study flips the existing narrative on its head, or rather on its side, pointing toward winds from the north instead.

“We know the Earth is warming up on average, but that alone doesn’t explain ice loss in Antarctica,” said , a 91̽��professor of Earth and space sciences. “To understand what’s going to happen in the future, we need to understand the details of what’s happening now, and critically, whether we are connected to it.”

The Antarctic ice sheet covers an area larger than the U.S. and Mexico combined. If the Western-Hemisphere portion were to melt, global sea levels would rise by . The ice sheet is locked in place by ice shelves, fingers of ice that stretch into the sea. Free floating sea ice blankets the surface of the surrounding waters.

To study weather in Antarctica, where there are fewer weather stations than most of the world, scientists use computer simulations that draw from available data sources. Still, these models often lack data that is specific to the region, limiting the accuracy of their outputs.

In the past century, westerly winds blowing over high latitudes of the Southern Hemisphere have grown stronger in response to human-induced climate change. Indirect evidence also suggested that this trend was driving West Antarctic ice loss. But when the researchers dug into that theory, something didn’t add up.

“We thought that we were going to support what the climate models showed, which was that the westerly winds were getting stronger near the coast of Antarctica,” said , lead author and a 91̽��postdoctoral researcher of oceanography. “But there was no evidence of westerly winds strengthening in this part of Antarctica.”

O’Connor’s doctoral research explored how proxy data — historical records from ice cores, trees and coral — can reveal past weather patterns, including wind. Her work showed that the force needed to explain accelerating melt rates was still missing from the equation.

In the new study, researchers conducted a suite of high-resolution ice-ocean simulations to identify what climate patterns were driving ice shelf melting in this critical region of Antarctica. They fed the model a wind pattern for five years at a time, measured how much mass the ice lost, and repeated the process 29 times. Each iteration represented a different wind pattern. Data from the 30 simulations showed that northerly winds consistently exacerbated ice loss. Westerlies did not have the same effect.

The northerly winds, which blow with force in Antarctica, were rearranging the sea ice surrounding Antarctica, capping off small but important gaps called polynyas.

“Sea ice is a really good insulator, it keeps the ocean relatively warm compared to the air,” said a 91̽��professor of oceanography and of atmospheric and climate science. “When northerly winds close the polynyas, it reduces ocean heat loss, which means warmer waters and more melting of ice shelves below the surface.”

Polynyas are like pores on the icy surface of the ocean. When they are blocked, excess heat can’t escape. As the ice shelf melts, fresh water mingles with salty ocean water. A density gradient forms between the fresher, lighter water and the open ocean. This gradient powers a current that pulls in more warm ocean water from miles away, advancing ice shelf melt.

Researchers believe greenhouse gas emissions could be fueling the northerly winds. Early studies suggest that human-induced climate change is decreasing air pressure over the Amundsen Sea. This area hosts an influential low-pressure center that drives many of the Antarctic weather patterns. As it gets even lower, wind speed from the north increases.

“This mechanism provides a connection between West Antarctic ice loss and human-induced climate change, albeit a different mechanism than we previously suspected,” O’Connor said. Which is important, the researchers added, because if emissions are contributing to ice loss, perhaps cutting them could curtail it.

“I think what Gemma has done is going to lead to a complete revolution in the understanding of what drives Antarctic ice loss,” Armour said. “We had all sorts of theories about the winds that blow from west to east, but the northerly winds weren’t even on our radar. We were off by 90 degrees.”

Other authors include , a 91̽��professor of oceanography; , a 91̽��research scientist of Earth and space sciences; , an assistant professor of engineering at Dartmouth College; Shuntaro Hyogo, a graduate researcher of environmental science at Hokkaido University; and Taketo Shimada, a graduate researcher of environmental science at Hokkaido University

This research was funded by the Washington Research Foundation, the 91̽�� eScience Institute, the U.S. National Science Foundation, a Calvin professorship in oceanography, the Japanese Ministry of Education, Culture, Sports, Science, and Technology, Inoue Science Foundation, NASA Sea Level Change Team, the John Simon Guggenheim Memorial Foundation and JST SPRING.

For more information, contact Gemma O’Connor at goconnor@uw.edu.

The Atlantic Meridional Overturning Circulation, or AMOC, is a system of ocean currents that plays a crucial role in regulating Earth’s climate by transporting heat from the Southern to Northern Hemisphere. Confined to the Atlantic basin, the AMOC modulates regional weather — from mild summers in Europe to monsoon seasons in Africa and India.

Climate models have long predicted that global warming will cause the AMOC to weaken, with some projecting what amounts to a near-collapse relative to the AMOC strength in observations today. But a new study from a team of researchers that included the 91̽�� shows that the AMOC is likely to weaken to a much lesser extent than current projections suggest. The study was published May 29 in .

A severe weakening would have far-reaching consequences, including changes in regional sea level rise, and major shifts in regional climate, such as colder conditions in northern Europe and drier weather in parts of the Amazon and West Africa.

“Our results imply that, rather than a substantial decline, the AMOC is more likely to experience a limited decline over the 21^st century — still some weakening, but less drastic than previous projections suggest,” says , lead author of the study and a 91̽��postdoctoral research fellow in the .

The researchers developed a simplified physical model based on fundamental principles of ocean circulation — specifically, how sea water density differences and the depth of the overturning circulation are related — that also incorporates real-world measurements of the ocean current’s strength. The real-world data was collected over 20 years with monitoring arrays and other observations of the Atlantic basin.

Results show that the AMOC will weaken by around 18-43% by the end of the 21^st century. While this represents some weakening, it’s not the near-collapse that more extreme climate model projections suggest.

Paleoclimate records, like ocean sediments that record past climate conditions, indicate that the AMOC experienced weakening in the past. One example is during the last ice age 20,000 years ago, which led to major swings in the climate that affected North America and Europe.

Contemporary climate models show wide variation in 21^st century projections of AMOC weakening. This study aimed to reconcile these discrepancies by better understanding the physical mechanisms governing the AMOC behavior in climate models. Through this work, researchers shed light on a previously unexplained feature of climate models: the link between the present-day and future strength of the AMOC.

Climate models that simulate a stronger present-day AMOC tend to project greater weakening under climate change. Researchers found that this relationship stems from the depth of the AMOC. A stronger AMOC typically extends to greater depths and allows changes in surface water temperature and salinity properties — caused by global warming and freshwater input — to penetrate deeper into the ocean and drive greater weakening.

A climate model with a stronger and deeper AMOC is less resilient to surface changes and experiences proportionally more weakening than one with a shallower current. Climate models with a shallower present-day AMOC still show weakening under climate change, but to a lesser extent than those with a deeper present-day AMOC.

The researchers used the ocean observations to show that the real-world AMOC is relatively shallow when compared to most climate models. The results indicate that the AMOC will experience only limited weakening, even in the highest emissions scenarios. The study also suggests that much of the previous uncertainty and more extreme weakening projections stemmed from biases in how climate models simulate the ocean’s current state, particularly its density stratification.

“There is immense value in doing basic research,” Bonan said. “It can give us a better indication of what the future might look like, as our study shows.”

Bonan emphasized the need to examine higher-resolution climate models that also include more sophisticated processes. Higher-resolution models might offer deeper insights into AMOC behavior and improve projections of its future changes. The study provides a framework to interrogate and evaluate more sophisticated models.

, professor of atmospheric & climate science and oceanography at the UW, was a co-author.  Other co-authors are Tapio Schneider and Andrew Thompson of the California Institute of Technology, Laure Zanna of New York University and Shantong Sun of Laoshan Laboratory in Qingdao, China. The study was funded by the National Science Foundation, the David and Lucile Packard Foundation and Schmidt Sciences LLC.

This story is adapted from a release by the California Institute of Technology.

For more information, contact Bonan at dbonan@uw.edu.

As carbon dioxide accumulates in the atmosphere, the Earth will get hotter. But exactly how much warming will result from a certain increase in CO2 is under study. The relationship between CO2 and warming, known as climate sensitivity, determines what future we should expect as CO2 levels continue to climb.

New research led by the 91̽�� analyzes the most recent ice age, when a large swath of North America was covered in ice, to better understand the relationship between CO2 and global temperature. It finds that while most future warming estimates remain unchanged, the absolute worst-case scenario is unlikely.

The open-access was published April 17 in Science Advances.

“The main contribution from our study is narrowing the estimate of climate sensitivity, improving our ability to make future warming projections,” said lead author , a 91̽��doctoral student in atmospheric sciences. “By looking at how much colder Earth was in the ancient past with lower levels of greenhouse gases, we can estimate how much warmer the current climate will get with higher levels of greenhouse gases.”

The new paper doesn’t change the best-case warming scenario from doubling CO2 — about 2 degrees Celsius average temperature increase worldwide — or the most likely estimate, which is about 3 degrees Celsius. But it reduces the worst-case scenario for doubling of CO2 by a full degree, from 5 degrees Celsius to 4 degrees Celsius. (For reference, CO2 is currently at 425 ppm, or about 1.5 times preindustrial levels, and unless emissions drop is headed toward double preindustrial levels before the end of this century.)

As our planet heads toward a doubling of CO2, the authors caution that the recent decades are not a good predictor of the future under global warming. Shorter-term climate cycles and atmospheric pollution’s effects are just some reasons that recent trends can’t reliably predict the rest of this century.

“The spatial pattern of global warming in the most recent 40 years doesn’t look like the long-term pattern we expect in the future — the recent past is a bad analog for future global warming,” said senior author , a 91̽��associate professor of atmospheric sciences and of oceanography.

Instead, the new study focused on a period 21,000 years ago, known as the Last Glacial Maximum, when Earth was on average 6 degrees Celsius cooler than today. Ice core records show that atmospheric CO2 then was less than half of today’s levels, at about 190 parts per million.

“The paleoclimate record includes long periods that were on average much warmer or colder than the current climate, and we know that there were big climate forcings from ice sheets and greenhouse gases during those periods,” Cooper said. “If we know roughly what the past temperature changes were and what caused them, then we know what to expect in the future.”

Researchers including co-author , a 91̽��professor of atmospheric sciences, have created new statistical modeling techniques that allow paleoclimate records to be assimilated into computer models of Earth’s climate, similar to today’s weather forecasting models. The result is more realistic temperature maps from previous millennia.

For the new study the authors combined prehistoric climate records — including ocean sediments, ice cores, and preserved pollen — with computer models of Earth’s climate to simulate the weather of the Last Glacial Maximum. When much of North America was covered with ice, the ice sheet didn’t just cool the planet by reflecting summer sunlight off the continents, as previous studies had considered.

By altering wind patterns and ocean currents, the ice sheet also caused the northern Pacific and Atlantic oceans to become especially cold and cloudy. Analysis in the new study shows that these cloud changes over the oceans compounded the glacier’s global cooling effects by reflecting even more sunlight.

In short, the study shows that CO2 played a smaller role in setting ice age temperatures than previously estimated. The flipside is that the most dire predictions for warming from rising CO2 are less likely over coming decades.

“This paper allows us to produce more confident predictions because it really brings down the upper end of future warming, and says that the most extreme scenario is less likely,” Armour said. “It doesn’t really change the lower end, or the average estimate, which remain consistent with all the other lines of evidence.”

The research was funded by the National Science Foundation, the Department of Defense’s National Defense Science and Engineering Graduate Fellowship, the Alfred P. Sloan Foundation, the National Oceanic and Atmospheric Administration and the European Union’s Horizon 2020 program. Other co-authors are Jessica Tierney at the University of Arizona; Matthew Osman at the University of Cambridge in the U.K.; Cristian Proistosescu and Philip Chmielowiec at the University of Illinois Urbana-Champaign; Yue Dong at the University of Colorado; Natalie Burls at George Mason University; Timothy Andrews at the U.K. Met Office Hadley Centre; Daniel Amrhein and Jiang Zhu at the NSF National Center for Atmospheric Research in Boulder; Wenhao Dong at the University Corporation for Atmospheric Research in Boulder and Geophysical Fluid Dynamics Laboratory; and Yi Ming at Boston College.

For more information, contact Cooper at vcooper@uw.edu or Armour at karmour@uw.edu.

Forecasters are predicting a “” this year. This will be the third winter in a row that the Pacific Ocean has been in a La Niña cycle, something that’s happened only twice before in records going back to 1950.

New research led by the 91̽�� offers a possible explanation. The , recently published in Geophysical Research Letters, suggests that climate change is, in the short term, favoring La Niñas.

“The Pacific Ocean naturally cycles between El Niño and La Niña conditions, but our work suggests that climate change could currently be weighing the dice toward La Niña,” said lead author , a 91̽��research scientist in atmospheric sciences. “At some point, we expect anthropogenic, or human-caused, influences to reverse these trends and give El Niño the upper hand.”

Scientists hope to predict the direction of these longer-term El Niño-like or La Niña-like climate trends in order to protect human life and property.

“This is an important question over the next century for regions that are strongly influenced by El Niño, which includes western North America, South America, East and Southeast Asia and Australia,” Wills said.

El Niño and La Niña events have , affecting patterns of rainfall, flooding and drought around the Pacific Rim. A winter tends to be cooler and wetter in the Pacific Northwest and hotter and drier in the U.S. Southwest. Other worldwide effects include drier conditions in East Africa, and rainier weather in Australia, Indonesia, Malaysia and the Philippines.

Knowing what to expect in the future helps communities prepare for potential weather in the coming season and in years to come.

Global warming is widely expected to favor El Niños. The reason is that the cold, deep water rising to the sea surface off South America will meet warmer air. Anyone who’s sweated knows that evaporation has a cooling effect, so the chillier ocean off South America, which has less evaporation, will warm faster than the warmer ocean off Asia. This decreases the temperature difference across the tropical Pacific and lightens the surface winds blowing toward Indonesia, the same as occurs during El Niño. Past climate records confirm that the climate was more El Niño-like during warmer periods.

But while Earth’s atmosphere has warmed in recent decades, the new study shows a surprising trend in the tropical ocean. The authors looked at temperatures at the surface of the ocean recorded by ship-based measurements and ocean buoys from 1979 to 2020. The Pacific Ocean off South America has actually cooled slightly, along with ocean regions farther south. Meanwhile, the western Pacific Ocean and nearby eastern Indian Ocean have warmed more than elsewhere. Neither phenomenon can be explained by the natural cycles simulated by climate models. This suggests that some process missing in current models could be responsible.

The upshot of these changes on either side of the tropical Pacific is that the temperature difference between the eastern and western Pacific has grown, surface winds blowing toward Indonesia have strengthened, and people are experiencing conditions typical of La Niña winters. The study focuses on temperature patterns at the ocean’s surface. Thirty years of data is too short to study the frequency of El Niño and La Niña events.

“The climate models are still getting reasonable answers for the average warming, but there’s something about the regional variation, the spatial pattern of warming in the tropical oceans, that is off,” Wills said.

The researchers aren’t sure why this pattern is happening. Their current work is exploring tropical climate processes and possible links to the ocean around Antarctica. Once they know what’s responsible, they may be able to predict when it will eventually switch to favor El Niños.

“If it turns out to be natural long-term cycles, maybe we can expect it to switch in the next five to 10 years, but if it is a long-term trend due to some processes that are not well represented in the climate models, then it would be longer. Some mechanisms have a switch that would happen over the next few decades, but others could be a century or longer,” Wills said.

The study was conducted before this year’s potential triple La Niña was announced. But Wills is cautious about declaring victory.

“These year-to-year changes are very unpredictable and it’s important not to get too hung up on any individual year — it doesn’t add a lot of statistical weight,” Wills said. “But I think it’s something that we should watch for in the next few years.”

Co-authors of the study are and at the UW; , a postdoctoral researcher at the Lamont-Doherty Earth Observatory who did the work as part of her 91̽��doctoral research; and at the University of Illinois at Urbana-Champaign. The study was funded by the National Science Foundation, the National Oceanic and Atmospheric Administration and the Alfred P. Sloan Foundation.

For more information, contact Wills at rcwills@uw.edu. Note: Wills is currently based in Colorado.

Countries around the world pledged in the Paris Agreement to limit warming to 1.5 degrees Celsius, or, at most, 2 degrees Celsius. As emissions rates gradually begin to decline, countries are looking at how many greenhouse gases can still be emitted while remaining below these temperature targets, which are deemed the upper limits to avoid the most catastrophic impacts to the climate system.

New led by the 91̽�� calculates how much warming is already guaranteed by past emissions. While previous research has explored this question for carbon dioxide, the new work includes related emissions such as methane, nitrogen oxide and aerosols, like sulfur or soot.

Under a moderate future emissions scenario, by 2029 the planet has a two-thirds chance of at least temporarily exceeding warming of 1.5 degrees Celsius, even if all emissions cease on that date, the study finds. If humans continue on a moderate emissions pathway, by 2057 there’s a two-thirds chance that the planet will at least temporarily exceed warming of 2 degrees Celsius. The study was published June 6 in Nature Climate Change.

“It’s important for us to look at how much future global warming can be avoided by our actions and policies, and how much warming is inevitable because of past emissions,” said lead author , a 91̽��doctoral student in oceanography. “I think that hasn’t been clearly disentangled before – how much future warming will occur just based on what we’ve already emitted.”

The authors used a climate model to study what would happen to Earth’s temperature if all emissions were to suddenly stop in each year from 2021 to 2080, along eight different emissions pathways. While it’s not realistic to suddenly turn off all human-generated emissions, authors say that it’s an absolute “best-case scenario” that establishes a lower limit for future warming.

Earlier studies of this type looked at emissions of carbon dioxide and found little to no “warming in the pipeline” once emissions cease. The new study, however, includes shorter-lived greenhouse gases, such as methane and nitrogen oxide, as well as particulate pollution like sulfur and soot.

Different emissions can either warm or cool the planet. Particulate pollution reflects sunlight and has a slight cooling effect, offsetting global warming. These particles settle out of the atmosphere much more quickly than heat-trapping greenhouse gases. Stopping all human emissions simultaneously thus produces a temporary bump of about 0.2 degrees Celsius that begins abruptly when emissions stop and lasts for about 10 to 20 years.

“This paper looks at the temporary warming that can’t be avoided, and that’s important if you think about components of the climate system that respond quickly to global temperature changes, including Arctic sea ice, extreme events such as heat waves or floods, and many ecosystems,” said co-author , a 91̽��associate professor of atmospheric sciences and of oceanography. “Our study found that in all cases, we are committed by past emissions to reaching peak temperatures about five to 10 years before we experience them.”

The paper finds that if countries aim to achieve their goals of staying below 2 degrees Celsius of warming, then the total amount of carbon that humans can still emit, the remaining “carbon budget,” is significantly smaller than previous estimates.

“Our findings make it all the more pressing that we need to rapidly reduce emissions,” Dvorak said.

Other co-authors are and at the UW; at the University of Illinois at Urbana-Champaign; and at the University of Leeds. The study was funded by the National Science Foundation, the National Oceanographic and Atmospheric Administration, the Alfred P. Sloan Foundation and the U.K. Natural Environment Research Council.

For more information, contact Dvorak at mtdvorak@uw.edu or Armour at karmour@uw.edu. Note: Dvorak is traveling and her responses may be delayed.

Grants: NSF: AGS-1752796, AGS-1665247, NOAA: UWSC12184, Alfred P. Sloan Foundation: FG-2020-13568, NERC: NE/T009381/1

About twice each decade, the United Nations’ , or IPCC, looks at what is known about the science of climate change, the extent to which human activities are changing the Earth’s climate, and what risks these changes pose to human and natural systems. Organized into three working groups, each assessment is a years-long international effort that lays out the current understanding, projections for change over this century and options to manage the challenges ahead.

The most recent IPCC report, released in 2013, included from several 91̽�� faculty members. Several 91̽��faculty members also contributed to the fourth IPCC report, which in 2007 shared the Nobel Peace Prize.

Now four members of the 91̽��community will be among the sixth assessment’s and review editors, announced in April by the Geneva-based organization. The document is expected to be completed in three years, and a synthesized version will be available in early 2022.

, assistant professor of Oceanography and of Atmospheric Sciences, is a lead author for Working Group 1, which assesses the physical science basis for a changing climate. He will work on the chapter for “Earth’s energy budget, climate feedbacks, and climate sensitivity.”

, a scientist at the National Oceanographic and Atmospheric Administration and an affiliate faculty member in Oceanography, is also a review editor for Working Group 1, contributing to the chapter focused on carbon and other biogeochemical cycles.

, a professor of Global Health and of Environmental and Occupational Health Sciences, is a review editor for Working Group II, which focuses on climate impacts, adaptation and vulnerability. Ebi will review the chapter on the “point of departure and key concepts” for the impacts of climate change.

Dr. , an associate professor of Emergency Medicine, of Environmental and Occupational Health Sciences and of Global Health, is a lead author in that same working group. He will be a lead author for chapter 7, looking at “health, wellbeing and the changing structure of communities.”

More than 720 experts from 90 countries were selected as either coordinating lead authors, lead authors or review editors. They were chosen from 2,858 experts representing 105 countries.

“The Sixth Assessment Report will update our knowledge on climate change, its impacts and risks, and possible response options,” IPCC chair Hoesung Lee said in a . “These author teams, drawn from the hundreds of excellent nominations the IPCC was fortunate to receive, provide us with the necessary expertise across a range of disciplines to conduct the assessment.”

###

It is also one of the most hotly debated numbers in climate science. Observations in the past decade seem to suggest a value that is lower than predicted by models. But a 91̽�� study shows that two leading methods for calculating how hot the planet will get are not as far apart as they have appeared.

In climate science, the climate sensitivity is how much the surface air temperature will increase if you double carbon dioxide from pre-Industrial levels and then wait a very long time for the Earth’s temperature to fully adjust. Recent observations predicted that the climate sensitivity might be less than that suggested by models.

The , published April 17 in , focuses on the lag time in Earth’s response. According to most models of climate change, during the early stages of global warming the sensitivity to greenhouse gas emissions is relatively small. As the ocean catches up and feedbacks kick in, however, the sensitivity increases and the warming rate speeds up. The new study shows that when this difference is factored in, the observations and climate models are in agreement, with recent observations supporting a previously accepted long-term climate sensitivity of about 2.9 degrees Celsius.

“The key is that you have to compare the models to the observations in a consistent way,” said author , a 91̽��assistant professor of oceanography and atmospheric sciences. “This apples-to-apples approach — where you factor in how long the planet has been adjusting to a change in its atmosphere — shows that climate sensitivity in the models is actually in line with what has been seen in the recent observations.”

The planet’s temperature takes thousands of years to fully adjust to a shift in the makeup of its atmosphere — the warming Earth has experienced to date is just a taste of what is in store. Early climate studies suggested that if the amount of carbon dioxide in the atmosphere doubled from pre-Industrial levels (we’re now about 1.4 times) the planet would eventually warm by about 3 degrees C, with possible values as high as 5 or 6 degrees C.

But recent of warming so far and emissions to date have suggested that climate sensitivity may be just under 2 degrees Celsius, with a maximum possible value of 4 degrees C.

“If true, this really would be a shift in our understanding of the long-term climate sensitivity,” Armour said.

For the new study, Armour looked at 21 leading global climate models run with increasing carbon dioxide. He focused on the warming rate compared to carbon dioxide levels, or climate sensitivity, in the early stages compared with in the late stages. The late-stage sensitivity across all the models was an average of 26 percent higher than the early-stage values. When factoring in that today’s observations are for the early stages of warming, the recent observations support a climate sensitivity of 2.9 degrees Celsius.

“There have been a lot of other papers that looked at the reasons for the changes in climate sensitivity over time,” Armour said. “This paper was the first attempt to quantify the effect across all the comprehensive models we use for climate prediction.”

The situation can be likened to pressing the gas pedal on a car, but the mass of the vehicle takes a while to get rolling. If the driver floors the gas pedal, it can be tricky to calculate the car’s final speed based on its initial reaction.

In the Earth system, the ocean temperatures around Antarctica and in the eastern Pacific Ocean have not risen in recent decades. Armour’s previous research showed that mean seawater touched by climate change will take centuries to reach the surface of the Southern Ocean. Similar but less extreme, currents reaching the eastern tropical Pacific from below the surface have also not seen daylight for decades.

Eventually, water touched by a warmer atmosphere will reach the eastern tropical Pacific and later the Southern Ocean. Warming in these regions will then activate feedbacks that will kick the planet’s warming into a higher gear.

“Currently we don’t have any evidence that the models are too sensitive compared to the observations,” Armour said. “The models appear to be in line with the observed range of warming.”

The various climate models show a wide range of values between the early-stage and late-stage sensitivities. Armour and students are exploring why these differences between the models exist, in order to improve them and better model how climate sensitivity shifts over time.

###

For more information, contact Armour at 206-221-4402 or karmour@uw.edu.

The resolves a scientific conundrum, and an inconsistent pattern of warming often seized on by climate deniers. Observations and climate models show that the unique currents around Antarctica continually pull deep, centuries-old water up to the surface – seawater that last touched Earth’s atmosphere before the machine age, and has never experienced fossil fuel-related climate change. The paper is published May 30 in .

“With rising carbon dioxide you would expect more warming at both poles, but we only see it at one of the poles, so something else must be going on,” said lead author , a 91̽��assistant professor of oceanography and of atmospheric sciences. “We show that it’s for really simple reasons, and ocean currents are the hero here.”

Gale-force westerly winds that constantly whip around Antarctica act to push surface water north, continually drawing up water from below. The Southern Ocean’s water comes from such great depths, and from sources that are so distant, that it will take centuries before the water reaching the surface has experienced modern global warming.

Other places in the oceans, like the west coast of the Americas and the equator, draw seawater up from a few hundred meters depth, but that doesn’t have the same effect.

“The Southern Ocean is unique because it’s bringing water up from several thousand meters [as much as 2 miles],” Armour said. “It’s really deep, old water that’s coming up to the surface, all around the continent. You have a lot of water coming to the surface, and that water hasn’t seen the atmosphere for hundreds of years.”

The water surfacing off Antarctica last saw Earth’s atmosphere centuries ago in the North Atlantic, then sank and followed circuitous paths through the world’s oceans before resurfacing off Antarctica, hundreds or even a thousand years later.

Delayed warming of the Antarctic Ocean is commonly seen in global climate models. But the culprit had been wrongly identified as churning, frigid seas mixing extra heat downward. The study used data from and other instruments to trace the path of the missing heat.

“The old idea was that heat taken up at the surface would just mix downward, and that’s the reason for the slow warming,” Armour said. “But the observations show that heat is actually being carried away from Antarctica, northward along the surface.”

In the Atlantic, the northward flow of the ocean’s surface continues all the way to the Arctic. The study used dyes in model simulations to show that seawater that has experienced the most climate change tends to clump up around the North Pole. This is another reason why the Arctic’s ocean and sea ice are bearing the brunt of global warming, while Antarctic waters are largely oblivious.

“The oceans are acting to enhance warming in the Arctic while damping warming around Antarctica,” Armour said. “You can’t directly compare warming at the poles, because it’s occurring on top of very different ocean circulations.”

Knowing where the extra heat trapped by greenhouse gases goes, and identifying why the poles are warming at different rates, will help to better predict temperatures in the future.

“When we hear the term ‘global warming,’ we think of warming everywhere at the same rate,” Armour said. “We are moving away from this idea of ‘global warming’ and more toward the idea of regional patterns of warming, which are strongly shaped by ocean currents.”

The research was funded by the National Science Foundation and NASA. Co-authors are and at the UW, and and at the Massachusetts Institute of Technology.

###

For more information, contact Armour at karmour@uw.edu or 206-221-4402.