The total volume of sea ice in the Arctic is likely to have reached a record low last summer, despite previous reports that the area of ice recovered slightly from the previous year's dramatic decline, leading experts have warned.
The latest alarm about the fate of the Arctic sea ice, due to an unusually high proportion of thinner "first-year" ice, raises the prospect of an acceleration in the loss of ice during the warmer summer months, considered a key indicator of climate change.
Adding to the concern, Nasa and the National Snow and Ice Data Centre (NSIDC)in the US today warned that even in the colder winter months sea ice is failing to recover substantially. The most recent winter maximum, reached on 28 February this year, was the fifth lowest since satellite records began in 1979, and meant the last six years were the sixth lowest on record, the organisations said.
Separately, the journal Science reported a study two weeks ago in which scientists assessed the most reliable 13 from 21 global climate models and claims. Based on these, they have "new confidence" that the most likely date by which summer sea ice will effectively disappear is 2037. Previous estimates ranged from the UN's forecast of the end of this century, to less than five years from now.
The latest volume estimates come from the NSIDC, at the University of Colorado, and are based on a study mapping the age of different ice flows in the Arctic ocean. These show a dramatic loss of the thicker "multi-year" ice in recent years, particularly after the summer of 2007, when the sea ice lost an area the size of Alaska in a single season.
In 2008, the NSIDC reported that summer sea ice area recovered by 9% but was still the second lowest recorded. However, based on the latest data about the much greater area of thin first-year ice and losses of multi-year ice, especially that of five years or more, they believe that in volume terms last summer was the lowest since records began in the 1930s – and probably for at least 700 years and possibly up to 8,000 years, said Walt Meier, a research scientist at the Boulder-based centre. "Our estimate is that it was probably the lowest volume on record," Meier told the Guardian. "Certainly 2007 and 2008 [were] the two lowest [years for] volume and extent."
Loss of volume is an example of climate feedbacks: as more ice melts, thinner ice that replaces it over winter is more vulnerable to melting the following summer and to being blown out of the Arctic into the warmer Atlantic, while thicker multi-year ice melts more quickly when not buffered by frozen water.
Meier said recent sea ice losses were a result of natural variability in clouds, winds and ocean currents and the average rise in global temperatures, which is stronger at the poles: "You can always have natural variability ... but the long-term trend is for a thinner and less extensive sea ice cover."
Vicky Pope, head of climate change advice at the Meteorological Office, said a separate study had shown an increase in older, thicker ice in the Arctic last summer, and urged caution about warnings that the ice will disappear very quickly. "It's very likely the summer sea ice will disappear over the next 60 years, but precisely when I think is still an open question," she said.