Skewness of Temperature Data Implies an Abrupt Change in the Climate System Between 1985 and 1991

Instrumental records of mean annual temperature extend back to the seventeenth and eighteenth centuries at multiple sites in Europe. For such long time series, we expect and find that histograms of mean annual temperature data become skewed toward higher temperatures with time because of global warming. However, we also find that skewness changed abruptly and started increasing between 1985 and 1991 (95% confidence) at 17 sites. We argue that this finding may imply an abrupt change in the climate system affecting Europe which probably occurred at this time. We investigate possible causes and find Arctic sea ice loss, potentially linked to reduced sulfate aerosol emissions and coupled to temperature by an albedo feedback mechanism, a likely candidate. This is based on good correlations of sea ice extent and sulfate aerosol emissions with skewness of mean annual temperature data. Plain Language Summary In this study we find evidence of a sudden change in the climate that affected Europe and probably occurred between 1985 and 1991. This finding is based on a statistical analysis of temperature data collected since the seventeenth or eighteenth century at 17 sites in Europe. Our analysis shows that the chance of a given year being warmer than average started to rise at all sites at that time. We compared the results of our analysis with possible causes of a change to the climate. The causes we considered were changes of ocean circulation, loss of sea ice from the Arctic Ocean, and a reduction in particle emissions in Europe. Based on this comparison, we were able to show that loss of sea ice, possibly linked to particle emission reductions, was a possible cause of the change we detected. We note that sea ice loss makes the Earth darker, which allows more heat to be absorbed from the Sun, making the Earth warmer. This causes more sea ice to melt and might ultimately result in a complete loss of sea ice in the Arctic Ocean during the summer months. Key Points Skewness is used to search for abrupt changes in temperature time series from Europe An abrupt change is identified at 17 sites between 1985 and 1991 Arctic sea ice loss potentially coupled with sulfate aerosol emission reductions is a possible cause