Attribution of climate change to causes

The recent years saw a multitude of attempts to attribute observed climate parameter changes to either natural or anthropogenic forcing. One of the earliest attributions was: Cooling of the lower stratosphere (around 20 km height) is due to the depletion of the stratospheric ozone layer by decay products of the chlorofluorocarbons (CFCs) and not so much due to the enhanced greenhouse effect, which also leads to cooling of the stratosphere. By the research group of one of the authors (H. Grassl) several regional attribution attempts have been published recently, showing firstly that the decrease of air pollution over Europe and the increase over China have led to cloud property changes.

While the reflectivity of clouds was reduced over large parts of Europe from the 1980s to the 1990s, because of lower aerosol particle concentrations as a consequence of both emission reduction by new environmental laws in western European countries and collapse of parts of industry in former East Block countries, it was reduced over China also, but because of strong pollution growth with a large soot proportion making clouds reflecting less (Krüger and Grassl, 2002; Krüger and Grassl, 2004). In contrast to it, clouds over major European harbour areas did not show reduced but enhanced reflectivity because the emissions are not regulated and ship traffic was growing fast (Dev-asthale et al., 2006).

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