Klimawandel-Faustregel entpuppt sich als falsch: Trockene Gebiete werden nicht immer trockener

Stets hatte man uns erklärt, der Klimawandel würde alle Extreme immer schlimmer werden lassen. Zwar würde eine wärmere Atmosphäre mehr Wasserdampf enthalten, jedoch würde der Regen vor allem in den bereits jetzt gut mit Regen versorgten Gebieten niedergehen, während die Trockengebiete der Erde leer ausgingen. Eine böse Todesspirale.

Leider alles falsch, wie die Klimaforscher nun angesäuert einräumen mussten. Auch die trockenen Regionen der Erde würden von den gesteigerten Niederschlägen profitieren. Bereits im September 2014 war dies bekannt, als die schweizerische Zeitung 20 Minuten über die neuen Ergebnisse berichtete:

Klimawandel-Faustregel: Trockene Gebiete werden nicht immer trockener
Trockene Regionen werden trockener, feuchte feuchter – so lautet eine Faustregel zum Klimawandel. Nun haben ETH-Forscher herausgefunden, dass das nicht immer stimmt.

Anderthalb Jahre später legten australische Forscher nach. In einer Pressemitteilung der University of New South Wales vom 7. März 2016 heißt es:

Global warming increases rain in world’s driest areas

Global warming will increase rainfall in some of the world’s driest areas over land, with not only the wet getting wetter over land but the dry getting wetter as well. New research published today in Nature Climate Change has revealed that in the Earth’s dry regions, global warming will bring an overall increase in rainfall and in extreme precipitation events that could lead to flash flooding becoming a more regular event. “We found a strong relationship between global warming and an increase in rainfall, particularly in areas outside of the tropics,” said lead author Dr Markus Donat from the ARC Centre of Excellence for Climate System Science. “Within the tropics we saw an increase in rainfall responding to global warming but the actual rate of this increase was less clear.”

Unfortunately for societies, businesses and agricultural activities, that exist in arid regions, the expected increase in rainfall over dry areas does not necessarily mean that more water will become available according to the researchers. The additional heat caused by global warming will likely lead to increased evaporation. This means that while there may be more extreme flooding events it may have little impact on overall water storage rates. “The concern with an increased frequency and in particular intensity of extreme precipitation events in areas that are normally dry is that there may not be infrastructure in place to cope with extreme flooding events,” said Dr Donat. “Importantly, this research suggests we will see these extreme rainfall events increase at regional levels in dry areas, not just as an average across the globe.”

The researchers were able to reach this conclusion because they looked at regions with similar characteristics rather than trying to compare complex climate variations found when comparing one country or continent with another. This meant that dry regions in Australia were compared with similarly dry regions in Asia, Africa and many other countries. At the same time, wetter regions across different countries were also compared. This allowed the researchers to directly compare like with like. Importantly, the findings remained consistent across observations and models.

“With precipitation models and observations don’t always tell the same story regarding regional changes, but we were very surprised to find that our results turned out to be highly robust across models and observations,” said Dr Donat. “It appears the uncertainties in climate models were greatest where the observational uncertainties were greatest. This suggests that improved observations will be vital if we are to reasonably determine how in the future precipitation will change in every corner of the world with global warming.”

Man muss den australischen Wissenschaftlern nachsehen, dass sie die Vorteile der vermehrten Niederschläge in den Trockengebieten kleinreden. Ansonsten wäre es sicher zu peinlich geworden. Wer wirft schon gerne sein liebgewonnenes Katastrophenmodell über den Haufen? Im Folgenden die Kurzfassung des dazugehörigen Papers von Donat et al. (2016) in Nature Climate Change:

More extreme precipitation in the world’s dry and wet regions
Intensification of the hydrological cycle is expected to accompany a warming climate1, 2. It has been suggested that changes in the spatial distribution of precipitation will amplify differences between dry and wet regions3, 4, but this has been disputed for changes over land5, 6, 7, 8. Furthermore, precipitation changes may differ not only between regions but also between different aspects of precipitation, such as totals and extremes. Here we investigate changes in these two aspects in the world’s dry and wet regions using observations and global climate models. Despite uncertainties in total precipitation changes, extreme daily precipitation averaged over both dry and wet regimes shows robust increases in both observations and climate models over the past six decades. Climate projections for the rest of the century show continued intensification of daily precipitation extremes. Increases in total and extreme precipitation in dry regions are linearly related to the model-specific global temperature change, so that the spread in projected global warming partly explains the spread in precipitation intensification in these regions by the late twenty-first century. This intensification has implications for the risk of flooding as the climate warms, particularly for the world’s dry regions.