Future abrupt loss of Arctic sea ice

We've heard a lot about the melting sea ice in the Arctic. The steady loss of the polar ice cap may endanger the polar bear, but provide new shipping channels and opportunities for commercial exploitation of the Arctic. Since 1979 (the year satellite imagery of the north pole first became available), the areal coverage of the Arctic sea ice has shrunk by about 10% in winter (4% per decade) and 20% in summer (8% per decade). The loss of sea ice, when plotted on a graph (Figure 1), has roughly followed a straight line over time. There are a few noisy ups and downs, reflecting colder and warmer years than average. A trend that approximately follows a straight line is called a "linear" trend. A continued linear summertime 8% per decade loss of sea ice would leave the summertime Arctic Ocean ice-free by 2100. The ocean would still partially freeze in winter, with about 50% of the ocean covered with ice.



Figure 1. Average September Arctic sea ice coverage as observed by satellites between 1979 and 2006. Image credit: NOAA's National Snow and Ice Data Center.

However, there is a distinct possibility that Arctic sea ice loss may show a sudden non-linear decline in coming years. The loss of sea ice with time may no longer follow a nice straight line, but instead suddenly accelerate, allowing the Arctic sea ice to suffer a sudden and complete disintegration in just a decade. The result would be an ice-free Arctic Ocean for the first time since before the last ice age. This possibility was explored in a December 2006 paper (Holland et al.), titled "Future abrupt reductions in the summer Arctic sea ice". The authors ran the Community Climate System Model, one of the top climate models used to formulate the "official word" on climate, the 2007 Intergovernmental Panel on Climate Change (IPCC) report. The model was run for the years 1979-2006, and successfully predicted the 20% loss of summer sea ice during that period. The model then assumed that levels of greenhouse gases would continue to increase, until a doubling of CO2 levels occurred in 2100. This is considered a "middle-of-the-road" scenario, and assumes a reasonable sequence of events will unfold over the coming decades: humans will make some modest efforts to control greenhouse emissions, but not enough to prevent dangerous climate change. The model found that Arctic sea ice continued to decline linearly until about 2024, resulting in about 60% sea ice coverage in September (Figure 2). During this period, the vertical thickness of the sea ice declined from about four meters to one meter. Beginning in 2025, the rate of sea ice loss suddenly tripled, resulting in the total loss of the summertime polar sea ice by 2040. The authors theorize that once the ice reaches a critical thickness--in this case, one meter--the processes that create open water suddenly become more efficient, resulting in a rapid disintegration of the remaining ice.




Figure 2. September Arctic sea ice extent observed in 1979 (yellow line), and 2005 (white area). The predicted coverage of sea ice by Holland et al. (2006) is shown for 2015 (red line) and 2040 (green line). Their model predicts that sea ice in summer by 2040 will occur only in narrow bands along the Canadian Arctic coast. However, there will still be about 50% sea ice coverage in winter. Original image taken from NASA.

The authors tested 11 other models that were also used to formulate the 2007 IPCC report. Six of these 11 models also showed similar sudden losses of the summer sea ice. When these models were run assuming that dramatic efforts to cut greenhouse gas emissions will be made over the coming decades, only 3 of the 15 models tested showed sudden summer sea ice losses.

The authors concluded that "abrupt changes in the summer Arctic sea ice cover are quite likely and can occur early in the 21st century, with the earliest event in approximately 2015". Given that just over 50% of the models tested show such an effect, it is by no means a sure thing that we'll see a total loss of Arctic sea ice by the middle of the century. However, the results should be impetus to drastically cut greenhouse emissions soon, as the probability of an ice-free Arctic increases significantly if we do nothing.

This is the fourth in a series of five blogs on climate change in the Arctic that will appear every Monday and Thursday. Part five is: Why should we be concerned about an ice-free Arctic Ocean? This one might wait a few extra days, as there are other topics I may want to talk about. My next blog will be Monday.

Also, be sure to visit our new Climate Change blog, written by Dr. Ricky Rood of the University of Michigan.

References
Holland, M.M., C.M. Bitz, B. Tremblay, 2006 "Future abrupt reductions in the summer Arctic sea ice", Geophysical Research Letters, 33, L23503, December 2006.