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New views of Antarctica

Ice cores help explain model overheating, tropical connections

by David Hosansky

Antarctica is clearly a cold place—but just how much has its climate varied in the last century, and how much will it warm in the future? Two studies recently published by university- and NCAR-based scientists are helping answer these questions more fully. One paper finds that coupled climate models have consistently overstated warming across Antarctica as a whole, while the other finds that warming across West Antarctica in the last century was on par with worldwide warming, with some notable variability along the way.

“Over the past century, most of Antarctica has not undergone the fairly dramatic warming that has affected the rest of the globe,” says NCAR scientist Andrew Monaghan, the lead author of a study published on 5 April in Geophysical Research Letters. Monaghan’s coauthors were David Schneider (NCAR) and David Bromwich (Ohio State University).

Funded by NSF and the U.S. Department of Energy, this study marks the first time that scientists have been able to compare records of the past 50 to 100 years of Antarctic climate with simulations run on computer models.

antarctic temperature trends

 

andrew monaghan

This map of Antarctica shows the approximate boundaries of areas that have warmed or cooled over the past 35 years. The map is based on temperatures in a recently constructed data set by NCAR scientist Andrew Monaghan, right, and colleagues. The data combine observations from ground-based weather stations, which are few and far between, with model background fields to reveal past temperatures. (photo by Carlye Calvin, illustration by Steve Deyo, UCAR.)

Monaghan and colleagues compared recently constructed temperature data sets from Antarctica, based on data from ice cores and ground weather stations, to 20th-century model simulations. While the observed Antarctic temperatures rose by about 0.4°F (0.2°C) over the past century, the climate models showed increases of 1.4°F (0.75°C). The error appeared to be caused by models overestimating the amount of water vapor, perhaps due to the cold Antarctic atmosphere handling moisture differently than the atmosphere over warmer regions.

The study delivered a mixed verdict on Antarctica’s potential impact on sea-level rise. The Intergovernmental Panel on Climate Change has estimated that sea-level rise could amount to 18–59 centimeters (7–23 inches) this century, in part because of melting glaciers worldwide. The GRL paper suggests that warming in Antarctica over the next century could offset that by about 5 cm (2 in) if the continent warms by 5.4°F (3°C), as computer models have indicated. The reason is that the warmer air over Antarctica would hold more moisture and generate more snowfall, thereby locking up additional water in the continent’s ice sheets.

But the authors caution that model projections of future Antarctic climate may be unreliable. “The research clearly shows that you can actually slow down sea-level rise when you increase temperatures over Antarctica because snowfall increases, but warmer temperatures also have the potential to speed up sea-level rise due to enhanced melting along the edges of Antarctica,” says Monaghan, who did some of his research at Ohio State before coming to NCAR. “Over the next century, whether the ice sheet grows from increased snowfall or shrinks due to more melt will depend on how much temperatures increase in Antarctica, and potentially on erosion at the ice sheet edge by the warmer ocean and rising sea level.”

From tropical Pacific to frozen Antarctic

While the work by Monaghan and colleagues focused on net temperature changes over all of Antarctica, a new analysis of ice cores by scientists at NCAR and the University of Washington (UW)estimates temperature changes over West Antarctica, which comprises about 25% of Antarctica’s surface area and contains enough ice to raise sea level by about 5 meters (16 feet).

Published online in August in the Proceedings of the National Academy of Sciences, the work shows that dramatic interannual temperature swings and a century-long warming trend across West Antarctica are connected to conditions in the tropical Pacific Ocean. This connection is illustrated by the exceptional 1939–42 El Niño event. Other research has linked the century-long warming trend in the tropics to human-produced greenhouse gases, a link especially evident since the 1970s.

southern ocean

When a strong El Niño develops across the tropical Pacific, it can influence weather and climate as far away as the southern polar region. This occurs via a “wave train” of areas with unusually high or low pressure in the upper atmosphere (H’s and L’s) that leads to warmer-than-normal temperatures in West Antarctica. Bright reds near the equator show the unusually warm sea-surface temperatures (SSTs) associated with an El Niño during 1940-41. There are no SST data for that period for the portions of the Southern Ocean shown here. Analysis of ice cores drilled in West Antarctica (red dots) reveals that air temperatures there warmed by as much as 10°F as this three-year-long El Niño unfolded, then dropped by as much as 13°F afterward. (Illustration by Steve Deyo, UCAR.)

“As the tropics warm, so too will West Antarctica,” says NCAR’s Schneider, who conducted much of the research with UW’s Eric Steig before moving to NCAR as a postdoctoral researcher. “These ice cores reveal that West Antarctica’s climate is influenced by atmospheric and oceanic changes thousands of miles to the north.”

Schneider and Steig estimate that West Antarctica warmed about 1.6°F (0.9°C) over the 20th century. That is slightly more than the global average of about 1.3°F (0.7°C). Because of the large swings in annual temperature during the 1930s and 1940s, there is a considerable margin of uncertainty in the century-long estimate, says Schneider. He notes that, since 1950, there is increased confidence in the warming trend, which has averaged about 0.8°F (0.4°C) per decade.

“Schneider and Steig’s work underscores the important regional differences in the response to the tropical, ozone, and greenhouse forcing that all affect Antarctic temperature changes on century timescales,” says Monaghan, who did not participate in the study. “The results add to a growing body of evidence suggesting that the West Antarctic Ice Sheet is much more susceptible to the climate warming occurring at lower latitudes than its East Antarctic counterpart.”

david schneider

David Schneider (photo by Carlye Calvin)

The new set of cores comes from a relatively snowy part of the continent. This allows for enough detail for scientists to infer year-to-year temperature changes. The data show that the Antarctic climate is highly responsive to changes in the Pacific. Although the heart of El Niño’s oceanic warming is based in the tropical Pacific, it often fosters a circulation pattern that pushes relatively mild, moist air toward West Antarctica, where it can temporarily displace much colder air (see graphic). For example, during the 1939–42 El Niño, temperatures in West Antarctica rose by about 6–10°F (3–6°C), and then dropped by an estimated 9–13°F (5–7°C) over the next two years.

To reconstruct climate trends over the last century, Schneider and Steig analyzed ice cores collected from eight locations across West Antarctica. They measured heavy and light stable isotopes of oxygen and hydrogen, the elements that make up the ice itself. During warm episodes, the heavy isotopes are more common because of a number of processes, such as a reduction in condensation that would otherwise remove them.

Most of the ice cores for the study were collected during the U.S. International Trans-Antarctic Scientific Expedition, which Schneider and Steig participated in, under sponsorship from NSF’s Office of Polar Programs.

“These results help put Antarctica’s recent climate trends into a global context,” says Schneider. Steig adds that while the influence of tropical climate on West Antarctic climate was not unknown, “these results are the first to demonstrate that we can unambiguously detect that influence in ice core records.”

Schneider notes that although coupled climate model simulations like those evaluated by Monaghan and colleagues do resolve El Niños, they are very unlikely to produce them with the same timing, strength, and duration as the real El Niño events observed during the 20th century.

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