Climate Science

An Ice-Free Arctic?

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Science  13 Aug 2004:
Vol. 305, Issue 5686, pp. 919
DOI: 10.1126/science.305.5686.919b

Coupled atmosphere-ice-ocean climate models predict that greenhouse global warming in the Arctic should be greater than the global mean. Two questions spring to mind. First, how much of the changing atmosphere-ice-ocean conditions in the Arctic is a consequence of natural climate processes, and how much is due to external factors such as anthropogenic greenhouse gas forcing? Second, what fraction of the Arctic sea-ice cover will disappear in this century due to anthropogenic factors?

Johannessen et al. have compiled observational data of surface air temperature and sea ice for the quarter century between 1978 and 2003, and compared them with the output of the ECHAM4 and HadCM3 models. They conclude that warming in the early 20th century was due to natural internal climate system variability but that a substantial part of the recent changes is a response to anthropogenic forcing. The area of Arctic sea ice has decreased by 7.4% or 8 × 105 km2 in the past 25 years, with the minimum summer ice coverage occurring in September 2002. Their simulations predict that the Arctic will be almost free of sea ice during the summers toward the end of the this century (for another climate prediction for the late 21st century, see Meehl and Tebaldi, this issue, p. 994). — HJS

Tellus A 56, 328 (2004).

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