Abstract
This thesis aims at increasing the knowledge on past changes in extremes through the analysis of historical records of observations at meteorological stations. The key question addressed is:
How did the extremes of daily surface air temperature and precipitation change in Europe's climate of the 20th century, and what can
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we learn from this?
The contents is structured along the lines of four follow-up questions:
Are the available observational datasets adequate to analyse extremes?
Which trends are observed for the daily extremes of surface air temperature and precipitation?
Can the observed changes in temperature extremes in recent decades be regarded as a fingerprint of anthropogenic climate change?
Do the observed changes guide the development of temperature scenarios for our future climate?
Europe is one of the regions of the world that lacked a readily available and accessible dataset of high-resolution observational series with sufficient density and quality to study extremes. Such a dataset was developed for temperature and precipitation and used to detect statistically significant and non-trivial changes in extremes. The temperature trends indicate a coarsening of our climate and the precipitation trends indicate an increase of wet extremes. The calculated trends represent changes that can be due to natural internal processes within the climate system and/or external forcing, which can either be natural (solar irradiance, volcanic aerosols, ozone, etc.) or anthropogenic (greenhouse gases, etc.). Comparisons between the trend patterns of temperature extremes in the station records, the patterns associated with natural variability in the observations, and the patterns of future warming and natural variability as simulated by a climate model reveal fingerprints of anthropogenic warming over Europe. The last part of this thesis goes beyond the observations of the climate of the past and speculates on future changes in extremes. It presents a 'what- if scenario with seasonal details for a Gulf Stream induced cooling of the temperature climate in Western Europe as a result of a shutdown of the thermohaline ocean circulation.
Although our knowledge on past changes in extremes improved, the new daily dataset needs to be updated at a regular basis, as well as further quality controlled and elaborated with metadata information to keep its value for climate change research. The physical causes of the observed trends in extremes are unresolved even though the detected trends are statistically significant. Formal climate change detection / attribution studies require fingerprints that are well represented by climate models. Finally, the Gulf Stream scenario describes one of many possible futures. We still don't know how extremes will change.
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