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Sunday, January 25, 2009

A Brief Exegesis on Arctic Sea Ice

Arctic sea ice reaches 2008 minimum

In a comment on yesterday's post Carbon Emissions Curbs in Time for Copenhagen?, where some discussion ensued in the comments on carbon taxes vs. carbon cap-and-trade, reader Ted Clayton wrote,

Going for a Carbon Tax not only stiffs the President, but can be expected to harden public attitudes which already show little interest in the Anthropogenic Global Warming hypothesis. The best test of this hypothesis will come this summer, as we watch the Arctic Ocean icepack to see if it continues rebuilding. [[Emphasis mine.]]

This is a good opportunity to talk about what the overall trends in Arctic sea ice tell us about the state of the climate.

The quick hit: Polar sea ice is decreasing -- faster than expert scientists expected. The extent of Arctic summer sea ice hit a record low in 2007, the lowest since satellite measurements began in 1979. 2008 saw the second-lowest extent on record. Since the Arctic is more sensitive to changes in the climate than other environments on the globe, the melt of the Arctic is probably an early indicator of growing climate instability.

In the summer, Arctic ice melts, and recedes. In the winter, it forms and advances. What happens over the course of a year in terms of how much melting and freezing occur, or a couple years, is much less significant as an indication of what's going on with the climate, than the trends over longer periods of time.

Remember -- natural variation is occurring at the same time human-propelled global warming is destabilizing the climate. Interpreting trends over time is a question of signal to noise: The longer the period of time we look at, the more "noise" filters out -- in this case natural shifts in the climate system, such as El Nino/La Nina years -- and the stronger the signal that emerges.

What's emerged is that over the past 30 years,the extent of Arctic ice has decreased about 4.2 percent per decade, according to research cited by the National Snow and Ice Data Center. Below is the NSIDC's chart showing the overall trend -- that polar sea ice is decreasing at a fast rate -- since 1979, the year satellite measurements began. The gray line charts the average extent of Arctic sea ice during the summer melt from 1979 to 2000. The dark blue line shows the melt in the summer of 2008, and the light blue line shows the extent during the summer melt in 2007, when summer sea ice hit a record low:

September polar ice, 1979-2008

Per the NSIDC: "Extent comparisons: This graph compares 5-day running means for Arctic sea ice extent (area of ocean with ice concentration of at least 15 percent) for the long-term mean (1979-2000), the record low (2007) and the extent for 2008. Although Arctic sea ice retreated more slowly in June and July 2008 than it had the previous year, it experienced a record loss in August 2008." [[Emphasis mine.]]

In September of 2007, the extent of Arctic sea ice was 4.67 million square kilometers, the lowest on record -- 39.2 percent below the 1979-2000 average. In September of 2008, the extent was 4.67 million sq. km., "only 9 percent above 2007, despite cooler summer conditions," per the NSIDC.

Four of the past eight years have seen record-breaking loss of Arctic sea ice. Prior to 2007 and 2008's melts, the record lows were in 2002 (5.96 billion sq. km., a 15.3 percent drop from the 1979-2000 average) and 2005 (5.57 billion sq. km., 20.9 percent below the 1979-2000 average).

Sea ice thickness, has also decreased substantially over the past half-century or so, by about 1.3 meters between the 1950s and the 1990s, below shown by data on sea ice draft (the amount of ice below the water's surface):

Decrease in Arctic sea ice draft, 1958 to 1997.

Per the NSIDC: "Mean sea ice draft: Decrease in Arctic Sea Ice Draft for 1958 to 1997. Graph derived from Rothrock et al. 1999."

This 2003 article at the NASA Earth Observatory web site has a good overview of how overall warming of temperatures at the Arctic is accelerating.

Image: "This map shows the extent of sea ice on September 17, 2008, measured by the Advanced Microwave Scanning Radiometer (AMSR-E) on NASA’s Aqua satellite. Percent ice coverage is in shades of light blue to white. The gray line traces the boundary of the area normally covered by ice at the summer minimum based on data from 1979-2000. The line represents the median minimum ice extent; to qualify as "normally ice covered" an area has to meet the center’s criteria of at least 15 percent ice cover in at least half of the years in the record. The extent on September 17 was clearly much smaller than normal. Although this is not the first year the Northwest Passage has been navigable, it is the first year on record that both the Northwest Passage and the Northern Sea Route, on the opposite side of the Arctic, were both open." Source: NASA Earth Observatory

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