Scientists at the GKSS Research Centre of Geesthacht and the University of Bern have investigated the frequency of warmer than average years between 1880 and 2006 for the first time. The result: the observed increase of warm years after 1990 is not a statistical accident.
Between 1880 and 2006 the average global annual temperature was about 15°C. However, in the years after 1990 the frequency of years when this average value was exceeded increased.
The GKSS Research Centre asks: is it an accident that the warmest 13 years were observed after 1990, or does this increased frequency indicate an external influence?
Calculating the likelihood
With the help of the so called "Monte-Carlo-Simulation“ the coastal researchers Dr. Eduardo Zorita and Professor Hans von Storch at the GKSS-Research Centre together with Professor Thomas Stocker from the University of Bern estimated that it is extremely unlikely that the frequency of warm record years after 1990 could be an accident and concluded that it is rather influenced by a external driver.
The fact that the 13 warmest years since 1880 could have occurred by accident after 1990 corresponds to a likelihood of no more than 1:10,000.
These likelihood can be illustrated by using the game of chance "heads or tails": the likelihood is the same as flipping a coin and getting 14 heads in a row.
Climate is more complicated than a game
"In order to understand and statistically analyse the climate system and its interaction between the ocean, land, atmosphere and human activity, the comparison with a game of chance is no longer sufficient.
The natural sequence of warm and cold years no longer functions according to the simple principle of 'zero or one,'" explains the GKSS scientist Dr. Eduardo Zorita about the challenges of his calculations, because the climate system possesses some inertia.
An example: After a warm year milder years tend to follow, since the oceans have stored some heat. This natural inertia must also be included in the calculations.
"Our study is pure statistical nature and can not attribute the increase of warm years to individual factors, but is in full agreement with the results of the IPCC that the increased emission of greenhouse gases is mainly responsible for the most recent global warming“, says Zorita in summary.
- Zorita et al. How unusual is the recent series of warm years? Geophysical Research Letters, 2008; 35 (24): L24706 DOI: 10.1029/2008GL036228
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