Wednesday, January 21, 2009

Scientists attempt to extrapolate temperature trends for Antarctica

A post on the NASA Goddard Institute for Space Studies Web site entitled "Satellites Confirm Half-Century of West Antarctic Warming" discusses a new scientific paper by Eric Steig, et al to appear in Nature entitled "2009: Warming of the Antarctic ice-sheet surface since the 1957 International Geophysical Year" that explains how scientists are using various statistical and modeling techniques to extrapolate a 50-year model of the temperature trends for Antarctica from a somewhat limited actual historical record. The post says that:

The Antarctic Peninsula juts into the Southern Ocean, reaching farther north than any other part of the continent. The southernmost reach of global warming was believed to be limited to this narrow strip of land, while the rest of the continent was presumed to be cooling or stable.

Not so, according to a new analysis involving NASA data. In fact, the study has confirmed a trend suspected by some climate scientists.

"Everyone knows it has been warming on the Antarctic Peninsula, where there are lots of weather stations collecting data," said Eric Steig, a climate researcher at the University of Washington in Seattle, and lead author of the study. "Our analysis told us that it is also warming in West Antarctica."

They summararize their approach:

The finding is the result of a novel combination of historical temperature data from ground-based weather stations and more recent data from satellites. Steig and colleagues used data from each record to fill in gaps in the other and to reconstruct a 50-year history of surface temperatures across Antarctica.

It goes to explain that:

With funding from the National Science Foundation's Office of Polar Programs, Steig and colleagues set out to reconstruct Antarctica's recent past. Ground-based stations have recorded temperatures since 1957, but most of those readings come from the peninsula and areas on the edges of the continent. But at the same time, scientists such as study co-author Joey Comiso of NASA's Goddard Space Flight Center in Greenbelt, Md., have been gathering measurements from a series of Advanced Very High Resolution Radiometer (AVHRR) instruments deployed on satellites since 1981.

To construct the new 50-year temperature record, the team applied a statistical technique to estimate temperatures missing from ground-based observations. They calculated the relationship between overlapping satellite and ground-station measurements over the past 26 years. Next, they applied that correlation to ground measurements from 1957 to 1981 and calculated what the satellites would have observed.

The new analysis shows that Antarctic surface temperatures increased an average of 0.22 degrees F (0.12 degrees C) per decade between 1957 and 2006. That's a rise of more than 1 degree F (0.5 degree C) in the last half century. West Antarctica warmed at a higher rate, rising 0.31 degree F (0.17 degree C) per decade. The results, published Jan. 22 in Nature, confirm earlier findings based on limited weather station data and ice cores.

While some areas of East Antarctica have been cooling in recent decades, the longer 50-year trend depicts that, on average, temperatures are rising across the continent.

They candidly state that they do not have a hard conclusion about the cause of the warming trend in Antarctica:

To identify causes of the warming, the team turned to Drew Shindell of NASA's Goddard Institute for Space Studies in New York, who has used computer models to identify mechanisms driving Antarctica's enigmatic temperature trends.

Previously, researchers focused on Antarctic ozone depletion, which influences large-scale atmospheric fluctuations around the continent -- most notably, the Southern Annular Mode, which speeds up wind flow to isolate and cool the continent.

Shindell compared Steig's temperature data with results from a computer model that can simulate the response of the atmospheric system to changes in land surface, ice cover, sea surface temperatures, and atmospheric composition. He found the ozone-influenced Southern Annular Mode is not necessarily the primary influence on Antarctic climate. Instead, it appears that smaller-scale, regional changes in wind circulation are bringing warmer air and more moisture-laden storms to West Antarctica.

"We still believe ozone depletion can increase wind speeds around Antarctica, further isolating the interior," Shindell said. "But it's clear now that it's not such a dominant influence on temperature trends."

There is a lot of interesting information here, but not enough detail to convince me that their model for extrapolating temperature for a rather large area for a significant number of years is at all accurate enough to reliably fill such a gaping abyss in their limited historical data record. Maybe the extrapolation is accurate, or maybe not. It is hard to say.

Plenty more research is surely needed, and maybe we simply need to collect additional data for more years to come with more real temperature readings rather than extrapolations.

-- Jack Krupansky

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