The case for global warming in the Arctic

Attribution of the causes for the recent large changes in the Arctic, including sea ice, temperatures, and biological impacts, is formidable task because it is difficult to separate the signal due to anthropogenic greenhouse gases from the background of large natural climate variability. However, initial observational and modeling evidence of an anthropogenic influence in the Arctic has emerged over the last decade. I address this issue through application of methodological, evidentiary, and performance scientific standards which consist of three parts: is change occurring, which is the most consistent among a range of possible causes, and are these causes predictive? At the end of the 20th and beginning of the 21st century Arctic-wide warming is distinctive from the regional patterns of temperature anomalies that were seen earlier in the 20th century. IPCC model simulation results for the 1930s warm and 1960s cold temperature anomalies are consistent with natural climate variability, but all models show the influence of external forcing from greenhouse gases on warm temperature anomalies in the 1990s, and projections for the future. The recent sea ice reduction of summer 2007 Arctic by 40% compared to its 1980s extent is particularly scary and suggests that other surprises may be in store for the Arctic climate system. Sea ice loss appears to be on a fast track relative to consensus projections by IPCC models. This dramatic sea ice reduction was likely caused by a combination of greenhouse gas increases, fortuitous timing in the natural variability of the atmospheric general circulation, and positive feedbacks associated with a reduction in sea ice.