August 12, 2011

Sea Ice Melt May Halt, Reverse In Coming Years

The melting of Arctic sea ice may temporarily stabilize, and the ice may even expand, over the coming years, according to new research by scientists at the National Center for Atmospheric Research (NCAR).

"As we learn more about climate variability, new and unexpected research results are coming to light," said Sarah Ruth, program director in the Division of Atmospheric and Geospace Sciences, which funds NCAR.

"What's needed now are longer-term observations to better understand the effect of climate change on Arctic sea ice."

The computer modeling study reinforces previous findings that the level of Arctic sea ice loss observed in recent decades cannot be explained by natural causes alone, and that the ice will eventually melt away during summer if the climate continues to warm.

However, the current research found that Arctic ice under current climate conditions is as likely to expand as it is to contract in the years ahead.

"One of the results that surprised us all was the number of computer simulations that indicated a temporary halt to the loss of the ice," said lead researcher Jennifer Kay of the NCAR.

"The computer simulations suggest that we could see a 10-year period of stable ice or even a slight increase in the extent of the ice."

"Even though the observed ice loss has accelerated over the last decade, the fate of sea ice over the next decade depends not only on human activity but also on climate variability that cannot be predicted."

Kay explained that variations in atmospheric conditions such as wind patterns could temporarily halt the sea ice loss.

However, the fate of the ice in a warming world remains clear, she said.

"When you start looking at longer-term trends, 50 or 60 years, there's no escaping the loss of ice in the summer."

Kay and her colleagues also ran computer simulations to determine why the Arctic sea ice melted far more rapidly in the late 20th century than projected by computer models.

By analyzing multiple realizations of the 20th century from a single climate model, they attribute about 50 percent of the observed decline to human emissions of greenhouse gases, and the other half to climate variability.

These findings point to climate change and variability working together in equal amounts to accelerate the observed sea ice loss during the late 20th century, she explained.

The extent of summertime Arctic sea ice has shrunk by about one third since 1979, when accurate satellite measurements became available.

And while the ice returns each winter, the extent shrank to a record low in September 2007, and is low again this year.

To simulate what is occurring with the ice, the NCAR team used a newly updated version of one of the world's most powerful computer climate models.

The software, known as the Community Climate System Model, was developed at NCAR in collaboration with scientists at multiple organizations.

The researchers began by evaluating whether the model was a credible tool for the study.

By comparing the computer results with Arctic observations, they verified that, though the model has certain biases, it can capture observed late 20th century sea ice trends and the observed thickness and seasonal variations in the extent of the ice.

Next, Kay and her colleagues conducted a series of future simulations that examined how Arctic sea ice was affected both by natural conditions and by the increased level of greenhouse gases in the atmosphere.

The computer studies indicated that the year-to-year and decade-to-decade trends in the extent of sea ice are likely to fluctuate increasingly as temperatures warm and the ice thins.

"Over periods up to a decade, both positive and negative trends become more pronounced in a warming world," said NCAR scientist Marika Holland, a co-author of the study.

The simulations also indicated that Arctic sea ice is just as likely to expand as to contract over short time periods of time under the climate conditions of the late 20th and early 21st century.

However, the researchers warned that more modeling studies and longer-term observations are needed to better understand the impacts of climate change and weather variability on Arctic ice.

Furthermore, it is difficult to disentangle the variability of weather systems and sea ice patterns from the ongoing impacts of human emissions of greenhouse gases, they said.

"The changing Arctic climate is complicating matters," said Kay.

"We can't measure natural variability now because, when temperatures warm and the ice thins, the ice variability changes and is not entirely natural."

The study, which was funded by the National Science Foundation and the Department of Energy, appears this week in the journal Geophysical Research Letters (GRL).


Image 1: Scientists are finding some surprising results about sea ice in the Arctic. Credit: NOAA

Image 2: Arctic sea ice may hold together for longer than thought, at least in the short-term. Credit: NOAA

Image 3: Greenhouse gas mitigation can reduce the loss of Arctic sea ice. Credit: NOAA


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