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  • Räisänen, J., and L. Ruokolainen, 2008: Ongoing global warming and local warm extremes: a case study of winter 2006-2007 in Helsinki, Finland. Geophysica, 44, 45-65.

December 2006 and March 2007 were both record warm in large parts of northern Europe. Here we focus on temperatures observed in Helsinki, Finland, and study whether these mild winter months can be interpreted just as an extreme of natural variability or whether they should be regarded as a symptom of the ongoing global warming. A regression analysis suggests that atmospheric circulation conditions affected the warmth of both months, with a particularly large contribution in December. Yet, the regression model is not accurate enough to reliably tell how large a part of the warm anomalies should be attributed to factors other than circulation, including potentially a circulation-independent contribution of global warming. We therefore also apply a frequentist approach. Making use of the observed global warming and assuming that climate models correctly simulate the geographical pattern of forced temperature change, we reach the estimate that the temperatures observed in December 2006 and March 2007 should have a return period of about 60-80 years in the actual present-day climate. By contrast, observations for 1901-2005 suggest return periods of the order of several hundred years. Thus, the warming observed this far appears already to be sufficient to cause a several-fold increase in the probability of extremely high monthly mean temperatures. In the future, with continued global warming, this probability will further increase.

Full Article: http://www.geophysica.fi/pdf/geophysica_2008_44_1-2_045_raisanen.pdf

Last Updated: 2009-09-18

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