How unusual was autumn 2006 in Europe?

TL;DR

The autumn of 2006 in Europe was exceptionally warm, likely due to a combination of rare climate variability and potential underestimation of warming effects by current climate models.

Contribution

This study quantifies the rarity of the 2006 autumn warmth in Europe and evaluates climate model performance in simulating such extreme events.

Findings

2006 autumn was the warmest on record in several European countries.

Estimated return time of such warmth is 200 to 2000 years.

Climate models underestimate the observed temperature rise and do not account for increased probability of warm extremes.

Abstract

The temperatures in large parts of Europe have been record high during the meteorological autumn of 2006. Compared to 1961-1990, the 2m temperature was more than three degrees Celsius above normal from the North side of the Alps to southern Norway. This made it by far the warmest autumn on record in the United Kingdom, Belgium, the Netherlands, Denmark, Germany and Switzerland, with the records in Central England going back to 1659, in the Netherlands to 1706 and in Denmark to 1768. Assuming that the mean of the temperature distribution changes proportional to the global mean temperature, but the shape remains the same includes to first order the effects of global warming. Even under this assumption the autumn temperatures were very unusual, with estimates of the return time of 200 to 2000 years in this region. The lower bound of the 95% confidence interval is more than 100 to 300 years. Climate models that simulate the current atmospheric circulation well underestimate the observed mean rise in autumn temperatures. They do not simulate a change in the shape of the distribution that would increase the probability of warm events under global warming. This implies that the warm autumn 2006 either was a very rare coincidence, or the local temperature rise is much stronger than modelled, or non-linear physics that is missing from these models increases the probability of warm extremes.