Arctic ice autumn freeze-up starting later -
According to new research to be published in the forthcoming issue of Geophysical Research Letters, the ice-free season in the Arctic is growing by five days per decade.
Analysis of satellite data by a team of researchers headed by Julienne Stroeve indicate increasing absorption of solar energy in the summer Arctic ocean is pushing back the start of the autumn freeze. The data shows the autumn freeze beginning as much as 11 days later per decade in some regions.
Average temperatures are increasing year-round throughout the Arctic, but the trends of the start of seasonal melt are appreciably smaller than changes in Autumn freeze. The research shows that even small changes in the onset of melt has a big impact on how much solar energy is absorbed by sea and ice. This relatively small change in the timing of ice melt affects albedo, or the reflectivity of surface area, leading to substantially more energy being absorbed in summer and an ever later start of Autumn freeze. In other words, small changes have big impacts.
“The extent of sea ice in the Arctic has been declining for the last four decades,” says Stroeve, “and the timing of when melt begins and ends has a large impact on the amount if ice lost each summer. With the Arctic region becoming more accessible for long periods of time, there is a growing need for improved prediction of when the ice retreats and reforms in winter .”
Decrease in multi-year ice
Another trend coming from changes in albedo and heat absorption is the precipitous drop in the amount of “old ice” or multi-year ice that survives the summer melt. Older ice has a higher albedo and thus reflects more solar energy back into space than does single-year ice. The proportion of multi-year winter ice in the Arctic has decreased from around 70 percent in the 1980′s to only about 20 percent today.
Determining the effect of small feedbacks from atmospheric temperature and sea ice on large changes in heat absorption was the focus of the research. Analyzing 30 years of satellite data over the Arctic region, Stroeve and her team broke the data down into 25X25 kilometer squares, analyzing the albedo for each square for each month of available data. The research confirms the trend of longer ice-free periods, supporting and appending previous observations.
An atomic bomb’s worth of energy
The energy involved in these changes of absorption and albedo are immense – totaling hundreds of megajoules of extra energy per square meter of sea. Put another way, the extra energy accumulated amounts to several times the energy released in the atomic bomb dropped over Hiroshima heralding the end of World War II.
Image credit: NASA Goddard Space Flight Center, courtesy flickr