The Arctic sea ice extent index calculated by a University of Bremen research team led by Dr. Georg Heygster reached a new historical low point of 4.24 million km2 on September 8. The previous one-day minimum was 4.27 million km2 on September 17, 2007 (Figure 1). The usual melt season is not yet over, however, so 2011 extent could decline further. All three graphs in this post will be updated through the end of the season.
The Uni Bremen sea ice index goes back only to the start of satellite observations in 1972, but the researchers suggest that ice cover may not have been this low for thousands of years before that.
The Uni Bremen researchers announced the new ice extent minimum in a press release (in German). An English translation of the text, provided by Dr. Heygster, appears below (or go here for a version with graphics).
Arctic sea ice extent small as never before
Alerting message from the Arctic: The extent the the Arctic sea ice has reached on Sep. 8 with 4.240 million km2 a new historic minimum. Physicists of the University of Bremen now confirm the apprehension existing since July 2011 that the ice melt in the Arctic could further proceed and even exceed the previous historic minimum of 2007. It seems to be clear that this is a further consequence of the man-made global warming with global consequences. Directly, the livehood of small animals, algae, fishes and mammals like polar bears and seals is more and more reduced.
The work group of Georg Heygster at the University of Bremen observes since many years the the sea ice at both poles. With the support of the ESA/GMES project Polar View, daily maps of the sea ice extent are publicly provided at http://www.iup.uni-bremen.de/seaice/amsr/. The retreat of the summerly sea ice since 1972 amount to 50%. For algae and small animals living at the lower side of the ice, less and less living environment remains since they need a certain time to settle there. They are at the beginning of the food chain for fishes, mammals and also man.
The extent of the Arctic sea ice shows a pronounced yearly cycle, with about 15 million km2 in March and five million km2 in September. In 2007 however, it was only 4.267 million km2, the previous smallest value since start of satellite observations in 1972, and most probably since the last climate optimum about 8000 years ago. The current value is 27,000 km2 or 0.6% lower and could even be undercut in the next weeks. The ice maps of the University of Bremen show also that in this year, the Northwest and Northeast passages are simultaneously ice free. This had happed for the first time in 2008, and in 2009 the German shipping company Beluga has traveled it commercially for the first time. Recently, it was crossed in the record time of 8 days only by a tanker, traveling from Huston, Texas to Map Ta Phut, Thailand.
Already in July the new record minimum had been expected because in this month the sea ice extent was minimum, compared with the same month in other years. Due to the high sun elevation and long days in July, the sea ice extent in this period is climatologically more important than that of September. The increased insolation into the open water heat it up, leading to an additional sea ice melt from the bottom and delays formation of new ice in autumn. Moreover, the sea ice retreat can no more be explained with the natural variability from one year to the next, caused e.g. by weather influence. Climate models show rather, that the reduction is related to the man-made global warming which, due to the ice albedo effect, is particular pronounced in the Arctic: an ice area melted by a small temperature increase will then as open water have a much darker surface, absorb more solar radiation as before which causes an additional heating.
In contrast to the minimum in September, the yearly maximum in March decreases less (Figure 3): despite the reduced in in summer, in winter large areas of the arctic ocean freeze up. However, this first-year ice is clearly thinner than multiyear ice having survived one summer. Therefore, in summer first-year ice melts much easier than multiyear ice and after a historic minimum, the ice cover needs even in an unchanged climate several year to fully recover. Observation of the last years show further, that the sea ice thickness reduces, so that the total mass of Arctic sea ice decreases even more drastically than the sea ice extent.
The daily sea ice maps of the University of Bremen are based on observations of the Japanese microwave sensor AMSR-E, in orbit on board the NASA spacecraft Aqua. The institute receives the data from two servers in the US and Japan and produces the maps based on these observations.