Global "dimming" and "brightening" are not well understood

Retrieved August 11, 2009, from http://www.sciencedaily.com­ /releases/2009/07/090719195200.htm

ScienceDaily (Aug. 11, 2009) — A special volume of the Journal of Geophysical Research reviews the growing research field of “global dimming” and “global brightening” in over 20 articles. These phenomena, supposedly human-induced, control solar radiation incident at the Earth’s surface and thus influence climate......
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The articles provide the first indication of the magnitude of these effects, how they vary in terms of time and space and what the possible consequences might be for climate change. They also discuss in detail the underlying causes and mechanisms, which are still under debate.

It is particularly unclear as to whether it is the clouds or the aerosols that trigger global dimming/brightening, or even interactions between clouds and aerosols, as aerosols can influence the “brightness” and lifetime of the clouds. The investigation of these relations is complicated by the fact that insufficient – if any – observational data are available on how clouds and aerosol loadings have been changing over the past decades. The recently launched satellite measurement programs should help to close this gap for the future from space, however.

“There is still an enormous amount of research to be done as many questions are still open”, explains Wild. This includes the magnitude of the dimming and brightening effects on a global level and how greatly the effects differ between urban and rural areas, where fewer aerosols are released into the atmosphere. Another unresolved question is what happens over the oceans, as barely any measurement data are available from these areas.

A further challenge for the researchers is to incorporate the effects of global dimming/brightening more effectively in climate models, to understand their impact on climate change better. After all, studies indicate that global dimming masked the actual temperature rise – and therefore climate change – until well into the 1980s. Moreover, the studies published also show that the models used in the Intergovernmental Panel on Climate Change’s (IPCC) fourth Assessment Report do not reproduce global dimming/brightening adequately: neither the dimming nor the subsequent brightening is simulated realistically by the models. According to the scientists, this is probably due to the fact that the processes causing global dimming/brightening were not taken into account adequately and that the historical anthropogenic emissions used as model input are afflicted with considerable uncertainties.

“This is why at ETH Zurich we are working with a research version of a global climate model, which contains much more detailed aerosol and cloud microphysics and can reproduce global dimming/brightening more effectively”, says Wild. For him, the studies so far constitute “initial” estimates that need to be followed up with further research.

Global Dimming and Brightening, Special Issue of Journal of Geophysical Research, vol. 114, no. , 2009.




Adapted from materials provided by ETH Zurich.