July's huge drop in Arctic sea ice extent continued into August 2007, according to figures released this week by the National Snow and Ice Data Center
. August 2007 sea ice extent plunged 31% compared to the average from 1979-2000. As of September 4, 2007, the sea ice extent was a full 17% below the record minimum that occurred on September 20-21, 2005. Although the rate of melting is starting to slow down as the days grow shorter, more melting is expected this month.
The difference in sea ice extent between August 1979 (the beginning of the data record) and August 2007 was a startling 37%. University of Illinois Polar Research Group
presented similar estimates this week. They measure sea ice area--not extent. Sea ice area does not include all the long, narrow cracks in the ice, and so the numbers for sea ice area are different (lower) than for sea ice extent. Their sea ice area estimate for September 5, 2007 (Figure 1) was 42% less than for the same date 28 years ago.Figure 1.
Comparison of sea ice area on September 5, 1979 and September 5, 2007. Sea ice area in early September has declined 42% in the 28 years since 1979. Image credit: University of Illinois Polar Research Group.An ice-free Arctic in just 23 years?
None of our computer climate models predicted that such a huge loss in Arctic ice would occur so soon. Up until this year, the prevailing view among climate scientists was that an ice-free Arctic ocean would occur in the 2070-2100 time frame. The official word on climate change, the February 2007 report from the U.N.-sponsored Intergovernmental Panel on Climate Change (IPCC), warned that without drastic changes in greenhouse gas emissions, Arctic sea ice will "almost entirely" disappear by the end of the century. This projection is now being radically revised. Earlier this year, I blogged
about a new study
that predicted abrupt losses of Arctic sea ice were possible as early as 2015, and that we could see an ice-free Arctic Ocean as early as 2040. Well, the Arctic Ocean has suffered one of the abrupt losses this study warned about--eight years earlier than this most radical study suggested. It is highly probable that a complete loss of summer Arctic sea ice will occur far earlier than any scientist or computer model predicted. In an interview
published yesterday in The Guardian
Dr. Mark Serreze, and Arctic ice expert with the National Snow and Ice Data Center, said: "If you asked me a couple of years ago when the Arctic could lose all of its ice, then I would have said 2100, or 2070 maybe. But now I think that 2030 is a reasonable estimate. It seems that the Arctic is going to be a very different place within our lifetimes, and certainly within our children's lifetimes." While natural fluctuations in wind and ocean circulation are partly to blame for this loss of sea ice, human-caused global warming is primarily to blame. In the words of Dr. Serreze: "The rules are starting to change and what's changing the rules is the input of greenhouse gases. This year puts the exclamation mark on a series of record lows that tell us something is happening."The implications
The melting of the Arctic sea ice will not raise ocean levels appreciably, since the ice is made up of frozen sea water that is floating in the ocean. Sea ice melt does contribute slightly to sea level rise, since the fresh melt water is less dense than the salty ocean water it displaces. According to Robert Grumbine's sea level FAQ
, if all the world's sea ice melted, it would contribute to about 4 millimeters of global sea level rise. This is a tiny figure compared to the 20 feet of sea level rise locked up in the ice of the Greenland ice sheet, which is on land.
The biggest concern about Arctic sea ice loss is the warmer average temperatures it will bring to the Arctic in coming years. Instead of white, reflective ice, we will now have dark, sunlight-absorbing water at the pole, leading to a large increase in average temperature. Warmer temperatures will accelerate the melting of the Greenland ice sheet, which holds enough water to raise sea level 20 feet. The official word on climate, the 2007 IPCC report, predicted only a 0.6-1.9 foot sea level rise
by 2100, due to melting of the Greenland ice sheet and other factors. I believe these estimates will need to be revised sharply upwards in light of the unexpectedly high Arctic sea ice loss this summer.
One more point--global warming skeptics often criticize using computer model climate predictions as a basis for policy decisions. These models are too uncertain, they say. Well, the uncertainty goes both way--sometimes the models will underestimate climate change. We should have learned this lesson
when the ozone hole opened up--another case where the models failed to predict a major climate change. The atmosphere is not the well-behaved, predictable entity the models try to approximate it as. The atmosphere is wild, chaotic, incredibly complex, and prone to sudden unexpected shifts. By pumping large amounts of greenhouse gases into the air, we have destabilized the climate and pushed the atmosphere into a new state it has never been in before. We can expect many more surprises that the models will not predict. Some of these may be pleasant surprises, but I am expecting mostly nasty surprises.
I'll have an update on 99L when it becomes a tropical depression.