March 5, 2014
Arctic Sea Ice Is Being Lost At A Rate Of Five Days Every Ten Years
April Flowers for redOrbit.com - Your Universe Online
According to new research from the University College London, the melt season across the Arctic is getting longer by five days every ten years.Julienne Stroeve, Professor of Polar Observation and Modeling at UCL Earth Sciences, led the team in analyzing satellite data. The analysis, published in Geophysical Research Letters, revealed that the Arctic Ocean is absorbing even more of the sun’s energy in summer, leading to an even later appearance of sea ice in the autumn. The autumn freeze-up is happening up to 11 days per decade later than normal in some regions.
The new findings have implications for tracking climate change, and possible practical applications for shipping and the resource industry in the arctic regions.
“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.”
Temperatures have been rising during all calendar months, however trends in melt onset are significantly smaller than that of autumn freeze-up. Despite this smaller trend, the timing of melt onset greatly influences how much of the Sun's energy is absorbed by the ice and sea, which in turn is affected by how reflective the surface is. Surfaces that are highly reflective, such as ice, are said to have a high albedo — meaning they reflect most of the incoming heat back into space. Surfaces that are less reflective, such as liquid water, have a low albedo and absorb most of the solar heat directed at them.
The team says that even a small change in the spring extent of sea ice can cause vastly more heat to be absorbed over the summer. This leads to a significant delay in the autumn onset of ice. Multi-year ice, which is ice that survives through the summer without melting, has a higher albedo than single year ice, which only covers the sea in winter. The proportion of the Arctic winter ice composed of multi-year ice has been dropping from around 70 percent in the 1980s to 20 percent today.
The team set out to study feedback effects such as these, in which small changes in atmospheric temperature and sea ice lead to large changes in heat absorption. They analyzed over 30 years of satellite imagery of the Arctic region. The imagery breaks the entire region into 15.5 mile x 15.5 mile squares. The researchers analyzed the albedo for each of these squares for each month in which they had data. They were able to update trends and add an additional 6 years onto the most recent analysis of its kind. The team observed that the new data continues the trend towards longer ice-free periods.
“The headline figure of five days per decade hides a lot of variability. From year to year, the onset and freeze-up of sea ice can vary by about a week. There are also strong variations in the total length of the melt season from region to region: up to 13 days per decade in the Chukchi Sea, while in one, the Sea of Okhotsk, the melt season is actually getting shorter.”
Though relatively small changes, the amounts of energy involved are enormous — hundreds of megajoules of extra energy accumulated in every square meter of sea. For every square kilometer, this is the equivalent of several times the energy released by the atom bomb at Hiroshima.
A truly sophisticated understanding of when the sea ice will freeze is essential for organizations such as oil drillers operating in the Arctic. The findings — which are closely in line with prior work, giving added confidence in the veracity of models of the Arctic — of this study will also affect climate scientists by helping them better understand the feedback mechanisms inherent in the Arctic climate.