Arctic sea ice smaller than ever, melting faster than predicted, satellite images show

Arctic sea ice smaller than ever, melting faster than predicted, satellite images show

Jane Kay, Chronicle Environment Writer


The Arctic Ocean sea ice area was smaller last month than any other April since NASA starting taking satellite images nearly 30 years ago, climate scientists said.

The National Snow and Ice Data Center uses the daily satellite data to continually measure the vast floating pack ice, and is releasing the April findings today.

"It's safe to say that this April will be a new record low. Up until now, last year had been the lowest,'' said Walt Meier, a research scientist at the University of Colorado's snow and ice center.

What happens in the Arctic affects the rest of the planet because the sea ice provides a cooling effect as it reflects sunlight back into space.

Between 1979 and 2006, the summertime icepack shrank 9 percent each decade, according to the satellite data. It is at its smallest each year in September, which is the end of summer in the Arctic. The ice is largest in March. Although it is also getting smaller each year during winter, those changes aren't happening nearly as quickly as they are during the summer.

Sea ice could disappear during the summertime between 2050 and 2100, leaving the polar bear, walrus, ring seals and other Arctic creatures without habitat, according to estimates of the U.N. Intergovernmental Panel on Climate Change.

But in a new study published Tuesday, scientists at the National Snow and Ice Data Center and the National Center for Atmospheric Research conclude that the shrinking summertime Arctic pack ice is about 30 years ahead of the climate model projections.

In the online edition of Geophysical Research Letters, they reported that observations going back to 1953 show that the sea ice is retreating more rapidly than estimated by the 18 computer models used by the IPCC.

"If we look at the satellite era, which is our most reliable period of observations, the observed trend is that the summertime sea ice is declining at a rate of 9.1 percent per decade," said Julienne Stroeve, lead author and Arctic climate scientist at the snow-and-ice center. "This compares with the average of the IPCC models, which show a summertime decline of 4.3 percent per decade."

The authors didn't put a specific time frame on when the Arctic Ocean would be ice-free in the summertime, saying there's a danger in extrapolating because the trend is not likely to remain linear.

Things are changing in the Arctic, and there is much uncertainty over the effect of the influx of warmer waters and changing winds.

Water from the Pacific and Atlantic oceans is speeding the melt of the sea ice, although it's difficult for scientists to predict how precisely that will affect the quality of the sea ice -- namely, the thickness and stability.

Another uncertainty is how the melting ice will affect global warming.

According to Stroeve, new snow reflects about 90 percent of the energy from the sun. Not all of the sea ice is snow-covered, so it reflects about 70 percent of the solar radiation. In contrast, open water absorbs about 93 percent and increases climate temperatures.

There are a number of reasons why the computer models don't reflect the full impact of global warming from the increased carbon dioxide and other greenhouse gases in the atmosphere, said the center's Meier.

Historically, among most of the climate models, the sea ice component has been less well developed than other components, including changes in atmospheric and ocean circulation that transport heat to polar regions, he said.

"There are things going on in the physical system of the sea ice that the models don't quite capture," Meier said. Thickness is one of them.

"We don't have good measurements of ice thickness. We can do a reasonable job with models but it's an estimate based on limited observation. There is a fair amount of uncertainty.''

But if the best guess describes the ice as thicker than it really is, it underestimates the potential for melting, he said.

The main point of the paper is to focus on ways to improve the models, Meier said. One of the authors, Marika Holland, a scientist at the snow-and-ice center, prepared one of the models for the IPCC. Hers was the closest overall to the observed trend, Meier said.

The IPCC's third report on global climate change is to be released Friday.
Online resources

View the satellite image of the sea ice:

nsidc.org/data/seaice_index

E-mail Jane Kay at jkay@sfchronicle.com.