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More heat waves: increase of extremes due to climate change

10/24/2011 - The Moscow heat wave last year was, with high probability, the result of climate change – contrary to what some have assumed. With a likelihood of 80 percent, it was not natural short-term climatic variability but the long-term warming trend that caused the temperature record in the region surrounding the Russian capital in July 2010, according to scientists of the Potsdam Institute for Climate Impact Research (PIK). They developed a formula for calculating how frequently weather extremes occur in a changing climate. This week their findings are published in the Proceedings of the National Academy of Sciences.
More heat waves: increase of extremes due to climate change

The red dots indicate temperature peaks in Moscow. Image: PIK

“In many countries unprecedented weather extremes were observed during the last decade, while at the same time global mean temperature is rising steeply,” says lead-author Stefan Rahmstorf. “We looked at how these things are connected.” The researchers have quantified how many additional weather records are caused by climate change. Without climatic warming, natural fluctuations would also lead to new records, but they would do so less often. In this study, the researchers apply their method to heat records, but in the future, other extremes will be investigated. "For temperature, we show that climate change overall leads to more extremes," says Rahmstorf. "This is in many cases harmful to people."

"In many cases harmful to people"

The very hot summer of 2003 in Europe, often referred to as the summer of the century, resulted in tens of thousands of deaths. The record heat wave in 2010 even topped the summer of 2003, causing massive crop failure and forced Russia to ban wheat exports temporarily. In addition to this, huge wildfires plagued the country.

The steeper the climatic warming trend, the greater the number of heat records will be. In contrast, larger year-to-year temperature fluctuations lead to fewer records. At first this seems counter-intuitive as for a single event, it is of course a particularly high peak that scores the record. Such a peak, however, makes subsequent records less likely, so that variability overall reduces the number of records. It is the ratio of the climatic warming trend to the variability that determines the expected number of new records. Observational data supports this and is explained by this theoretical insight.


Only small decrease of cold extremes

Extreme cold makes people suffer just as extreme heat does. "Unfortunately, our analysis shows that the increase of heat extremes is not at all compensated by a decrease in cold extremes," says co-author Dim Coumou. This decrease in fact is found to be quite small. "In total, the frequency of monthly temperature records has already multiplied."


Article: Rahmstorf, S., Coumou, D. (2011): Increase of extreme events in a warming world. Proceedings of the National Academy of Sciences (early edition), [doi:10.1073/pnas.1101766108]

The article is available upon request at: pnasnews@nas.edu

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