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Climate in Context: Arctic Sea Ice Melt Season Winding Down; Irrigating the Climate

By Michael D. Lemonick and Andrew Freedman

Daily Arctic sea ice extent as of September 6, 2010, along with daily ice extents for years with the four lowest minimum extents. Credit: National Snow and Ice Data Center.

Arctic Sea Ice Melt Season Winding Down, Ice Extent Now Third Lowest in Satellite Record

The National Snow and Ice Data Center (NSIDC) in Boulder, Colo. announced today that Arctic sea ice extent had declined to the third lowest in the satellite record, surpassing last year's seasonal minimum. However, sea ice extent is not as low as it was in 2007, when the seasonal minimum shattered records, and with just about two weeks remaining in the melt season, a new record low is not expected this year. According to the NSIDC, ice extent during the month of August was the second lowest in the satellite record, after 2007. 

Notably, both the famed Northwest Passage and the Northern Sea Route are currently described as "largely free of ice," and the NSIDC reports that there are at least two expeditions that are attempting to take advantage of these conditions to accomplish a historical feat: circumnavigating the Arctic Ocean. One of the ships is helmed by the Norwegian explorer Borge Ousland, while the other – the Peter I yacht – hails from Russia.

According to NSIDC:

Average ice extent for August was 5.98 million square kilometers (2.31 million square miles), 1.69 million square kilometers (653,000 square miles) below the 1979 to 2000 average, but 620,000 square kilometers (240,000 square miles) above the average for August 2007, the lowest August in the satellite record. Ice extent remained below the 1979 to 2000 average everywhere except in the East Greenland Sea near Svalbard.

The minimum ice extent for the year will probably occur in the next two weeks. 

The rate and severity of Arctic sea ice melt can vary significantly from one part of the vast region to another, and one summer to the next, depending on weather patterns and ocean currents during the melt season. In August, a weather pattern known as the "dipole anomaly," which contributed to the record-breaking melt in 2007, returned to the area. This setup brought higher than average air pressure over the northern Beaufort Sea, and lower than average air pressure over the Siberian side of the Arctic. 

The NSIDC cited both the dipole anomaly and "rotten ice" for causing an abnormally rapid decline in ice extent in the Beaufort and Chukchi Seas during August. This was offset somewhat, by slower ice loss rates in the East Siberian Sea and the Central Arctic, as well as the Laptev and Kara Seas. "The reason for slow ice loss in the Kara Sea, however, is that there was already very little ice in that region at the beginning of August," the NSIDC noted.

Another Lesson in Climate Complexity: Irrigation-Induced Cooling?

Climate scientists know that while human-generated greenhouse gases are generally warming the Earth, the warming can slow or even briefly reverse on short timescales – as it did for several decades during the mid-20th century, for example, when air pollution blocked sunlight and cooled the planet, or in the early 1990s, when similar (but natural) air pollution from the eruption of Mt. Pinatubo did the same.

Now a new paper in the Journal of Geophysical Research has identified another temporary cooling factor that works at a far more local scale. In places such as California’s Central Valley and the Indus Valley in South Asia, farmers irrigate their fields to grow crops – and when that water evaporates from the soil and from the crops themselves, it cools the land and the air above, just as evaporating perspiration cools the human body. The extra moisture in the air, in turn, leads to more rain downwind of the irrigated spots. In some spots though, cooling from irrigated crops can actually lead to less rain by altering wind patterns.

According to the study, on a global scale the cooling effect is minimal – about a tenth of a degree Celsius, on average. But in specific regions the cooling can be as much as 3 degrees Celsius, compared with less than 1 degree C of average global warming over the past century, say authors Michael Puma and Benjamin Cook, of Columbia University and the Goddard Institute for Space Studies.

However, the groundwater farmers use for irrigation isn’t unlimited. When it runs out, not only will hundreds of millions of people face food shortages, but they could also face a spate of significant local warming as the places they live catch up to the rest of the globe in an uncomfortable hurry.

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