Global Warming just so happens that most of the posts on this site have tried to counteract arguments from those who would sow fake "uncertainty" in the climate debate. But lest our readers feel that we are unjustifiably certain about our knowledge, let us look at a recent example of the opposite tendency: too much certainty.
A recent BBC Horizon documentary (transcript) raised the issue of 'global dimming' and argued that this 'killer' phenomena's newly-recognised existence would lead to huge re-assessments of future global warming. As part of the hyperbole, the process of global dimming was linked very clearly to the famines in Ethiopia in the 1980s and the implication was left that worse was to come. Media reports with headlines like "Fossil Fuel Curbs May Speed Global Warming" swiftly followed. So what's the real story?
Global dimming is indicated by measurements over land areas in many regions in the world and may therefore be a real phenomena. Though there are serious issues with the quality of some of the data (birds drinking out of uncovered evaporation pans, drift and inhomogeneities in the solar radiation measuring instruments), in the most global assessment, Beate Liepert estimated that there was globally a reduction of about 4% in solar radiation reaching the ground between 1961 and 1990. While more recent indications are that the trend may have reversed in the last decade, it could still be significant. Assuming for the sake of argument that these data are valid, what could have caused this? A change of that magnitude in the incoming solar radiation itself is not possible since satellite observations would have seen it. Thus, it must be something that is happening in the atmosphere to intercept solar radiation. There are only a few possibilities: clouds, water vapour or aerosols.
First of all it is important to note that even pure greenhouse gas forcing will lead to a slight decrease in surface solar radiation (due to the concurrent increased humidity) and potential cloud feedbacks. Cloud cover and thickness are both like to vary as a function of climate change.
