Global warming: record years no coincidence

For the first time, the frequency of above-average warm years between 1880 and 2006 has been statistically analyzed

Head or figure © GKSS
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Scientists have for the first time statistically examined the global frequency of above average warm years between 1880 and 2006. Result: The observed increase of warm years after 1990 is no statistical coincidence. The researchers report on their results in the journal "Geophysical Research Letters".

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Between 1880 and 2006, the average global annual average temperature was 15 degrees Celsius. In the years after 1990, years accumulated in which this average was exceeded. The GKSS researchers therefore asked themselves the question: is it pure coincidence that the warmest 13 years have been measured after 1990, or is this accumulation influenced?

Probability was calculated

Using the so-called mathematical "Monte Carlo simulation", the coastal researchers Eduardo Zorita and Professor Hans von Storch of the GKSS Research Center Geesthacht together with Professor Thomas Stocker from the University of Bern have calculated that the frequency of warm record years is extremely unlikely after 1990 is a coincidence, but that it is influenced by certain external drives.

The fact that the 13 warmest years since 1880 after 1990 had taken place by chance would then correspond to a probability of not more than 1 in 10, 000. This can be illustrated by the game of chance "Head or Number": If the warm record years pure coincidence, would fall 14 times in a row head or only number in the game. display

Climate more complicated than game

"But to understand and statistically analyze the climate system and its interactions between ocean, land atmosphere and human activity, comparison with gambling is no longer enough. The natural sequence of warm and cold years does not work on the simple principle of zero or one, "Zorita explains the challenges of his calculations. Because the climate system reacts relatively slowly.

Results consistent with IPCC results

For example, after a warm year, mild years usually follow, as the oceans store heat. This natural inertia had to be included in the calculations.

"Our study is purely statistical in nature and can not attribute the increase in warm years to individual factors, but is fully consistent with the IPCC's findings that increased greenhouse gas emissions are largely responsible for recent global warming", Zorita summarizes the new results.

(idw - GKSS Research Center Geesthacht, 09.01.2009 - DLO)