Monday, April 15, 2013

West Antarctic Ice Cores find changes at upper bound of normal

A new ice core study from the West Antarctic Ice Sheet (WAIS) concludes that current dramatic and unusual warming changes on the West Antarctic Ice Sheet may still be in the bounds of natural decadal variability.

In other words, the warming trend signal is still not strong enough to distinguish it above the noise of peak warming periods in multi-decade long cycles involving the Tropical Pacific atmospheric circulation, sea surface temperatures and the El Niño (ENSO) cycle.

While acknowledging the rapid warming over the West Antarctic Ice Sheet the authors conclude that "It is unknown whether these changes are part of a longer-term trend."

Related: Global warming in Antarctica: Thwaites and Pine Island Glaciers | Waking the giant: Global Warming in the Weddell Sea | Southern Ocean warming impact on Antarctic Ice Sheet and global sea level | Antarctic Paradox: ocean warming melting ice shelves causing sea ice expansion

The study was published in Nature Geosciences as - Recent climate and ice-sheet changes in West Antarctica compared with the past 2,000 years

Lead author of the study Eric Steig from the University of Washington identified that the majority of Antarctic warming came during the 1990s in response to El Niño conditions in the tropical Pacific Ocean. The researchers suggest the changes in the 1990s was similar to changes in other decades with temperature spikes and identified the 1830s and 1940s.

"If we could look back at this region of Antarctica in the 1940s and 1830s, we would find that the regional climate would look a lot like it does today, and I think we also would find the glaciers retreating much as they are today," said Steig in a media release.

The data for this study comes from an ice core from the West Antarctic Ice Sheet Divide that provides a record of the last 2000 years, along with a number of shorter ice cores dating back 200 years. The divide marks the highest point of the West Antarctic Ice Sheet where ice flows toward either the Weddell or Ross seas.

The abstract for the study puts forward:

"Changes in atmospheric circulation over the past five decades have enhanced the wind-driven inflow of warm ocean water onto the Antarctic continental shelf, where it melts ice shelves from below. Atmospheric circulation changes have also caused rapid warming over the West Antarctic Ice Sheet, and contributed to declining sea-ice cover in the adjacent Amundsen-Bellingshausen seas. It is unknown whether these changes are part of a longer-term trend.....We conclude that the uncertain trajectory of tropical climate variability represents a significant source of uncertainty in projections of West Antarctic climate and ice-sheet change."
Steig said the changes in West Antarctica were at the upper bound of normal. He commented, "The magnitude of unforced natural variability is very big in this area, and that actually prevents us from answering the questions, 'Is what we have been observing exceptional? Is this going to continue?'"

Study co-author Dr Ailie Gallant from Monash University commented to the ABC that West Antarctic decadal variations in temperature are linked to wind circulation patterns in the tropical Pacific Ocean and sea surface temperatures. She said that the study highlights the need to better understand processes in the tropical Pacific that influence the global climate including the West Antarctic Ice Sheet.