Global Dimming II

Guest commentary on BBC documentary on “Global Dimming” aired on January 13th 2005 by Beate Liepert, LDEO, Columbia University

I haven’t yet seen the documentary. I have only read the transcript and hence was spared the pictures of the potential apocalypse and the invocation of biblical-scale famines. However, as one of the lead scientists on the topic [and who was interviewed by the BBC for the Horizon documentary (transcript, previous post)], I feel I should explain a few things about it without using religious analogies and stoking unnecessary fear.

First though, this is a nice example of the power of words: Gerry Stanhill coined the observed reduction in solar energy reaching the ground “global dimming”. He called it “global” dimming because the technical term for the radiative energy is called “global solar radiation” and it contrasts nicely with the more common “global warming”.

Secondly, there are three published studies out on long term changes in solar radiation (or “global dimming” if preferred). All use the same data sources. Solar radiation has been measured at weather stations worldwide since about 1956-57. As with many other measurements most of the data are from the Northern Hemisphere and all are taken on land. A reduction in downward solar radiation of about 4% or about 7W/m2 from 1961 to 1990 was found at stations worldwide by Gilgen et al., (1998). Gilgen et al. did a quick analysis and used all the available data with increasingly shorter records for their trend statistics. Stanhill and Cohen (2001) calculated a stronger reduction of about 8% per decade. The reason for the discrepancy might be that only 30 records were used in the latter study and it seems only the ones with the declining trend. My own analysis was based on 110 continuously recording stations worldwide from 1961 to 1990 (Liepert 2002). I confirmed Gilgen et al.’s estimate of a reduction of about 4% in three decades. Since the late 1980s a recovery seems to be occurring but the studies demonstrating this are not yet published.

Why is solar radiation changing? From observations we can separate cloud-free skies and cloudy conditions. We can hence infer clouds or atmospheric transparency as possible causes for the dimming. In my study of the US data I identified clouds as the main reason for the dimming of sunlight. Only about a fifth of the dimming could be observed during cloud-free conditions.

Why should the atmospheric transparency change at all in cloud-free conditions? V. Ramanathan explained it in the BBC documentary. Sunlight is reflected by air pollution or absorbed in the atmosphere before it reaches the ground. Field campaigns like INDOEX show this clearly (well, not “clearly” in the literal sense!). Advanced climate models include this “direct” aerosol effect and base their inputs on experiments like INDOEX.

Why should clouds change? Global warming for example. Surprised? Most climate simulations predict some “global dimming” due to the water vapor and cloud feedback of greenhouse gas forced global warming. Global warming, however, affects the entire atmosphere whereas global dimming is only a surface and near-surface phenomena. Hence global warming and global dimming are not exclusive or contradictory. (Incidentally, the decline of solar energy at the surface inferred in my study is about 60% of the increasing longwave radiation in a typical global warming climate simulation (Feichter et al. 2004)). With global warming, atmospheric moisture increases and this makes the atmosphere slightly less transparent to sunlight. Furthermore once clouds are formed, they tend to hold more water and therefore look a little darker.

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