| Summary: | Arctic sea ice is a keystone indicator of greenhouse-gas induced global climate change, which is expected to be amplified in the Arctic. Here we directly compare observed variations in arctic sea-ice extent and CO2 since the beginning of the 20th century, identifying a strengthening linkage, such that in recent decades the rate of sea-ice decrease mirrors the increase in CO2, with r ~ –0.95 over the last four decades, thereby indicating that 90 % (r2 ~ 0.90) of the decreasing sea-ice extent is em-pirically “accounted for” by the increasing CO2 in the atmosphere. The author presents an empirical relation between annual sea-ice extent and global atmospheric CO2 concentrations, in which sea-ice reductions are line-arly, inversely proportional to the magnitude of increase of CO2 over the last few decades. This approximates sea-ice changes during the most recent four decades, with a proportionality constant of 0.030 million km2 per ppmv CO2. When applied to future emission scenarios of the Intergovernmental Panel on Climate Change (IPCC), this relationship results in substantially faster ice decreases up to 2050 than predicted by IPCC models. However, depar-tures from this projection may arise from non-linear feedback effects and/or temporary natural variations on interannual timescales, such as the record minimum of sea-ice extent observed in September 2007.
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