The Low-Down On Aerosols

  • Date: 24/01/13
  • Andrew Montford, Bishop Hill

The leak of the draft of the IPCC’s Fifth Assessment Report before Christmas has had perhaps rather less of an impact than might have been expected, but the rock that Alec Rawls’ chucked in the IPCC’s pond was a large one and not all of the ripples have reached the shore yet. Here’s a slightly unexpected one.

One of the immediate responses to the leak came from New Scientist, a bastion of climatological rectitude and stern upholder of the IPCC consensus. The article, by Michael Marshall and Fred Pearce, discussed what the IPCC report “really said”, much of it being a lengthy attempt to rebut Rawls’ case that the IPCC was acknowledging a much greater role for solar activity. However, the close of the article shifted to other aspects of the report:

Other conclusions are also more sobering. The IPCC is predicting greater sea level rise than it did in 2007, as it now includes models of ice sheet movements. And we now have a gloomier picture of the extent to which smogs and other human-made aerosols in the atmosphere shade us from the worst of global warming. This is still a big uncertainty in temperature forecasting. The draft says their cooling effect is 40 per cent less than thought in 2007, suggesting this positive side effect of air pollution has been overstated.

As BH readers no doubt know, the relatively slow pace of warming that the earth has exhibited since the start of the industrial revolution could have one of two explanations. The higher climate sensitivity argument is that the Earth tends to warm quickly in response to greenhouse gas emissions but that this warming has been partially countered by so-called aerosols – soot and chemical droplets released into the atmosphere by industrial activity. It is said that as industry cleans its act up, levels of aerosols in the atmosphere will fall and their damping down of the greenhouse warming will become decline too, leading to rapid temperature rises. The alternative explanation – the lower sensitivity case – is that a relatively modest warming in response to greenhouse gases is countered by only a modest aerosol effect. The effect of a cleaner atmosphere in this latter case would be only a relatively small warming.

Perceptive readers might now be able to see that Pearce and Marshall have got their arguments completely back to front, as Nic Lewis tried to explain in a letter to the editor at the start of the year:


Michael Marshall and Fred Pearce (22/29 December issue, page 8, “Sceptics misuse leaked IPCC report”) make several fair points, but they have got completely the wrong end of the stick in saying that we now have a gloomier picture of the extent to which human-made aerosols reduce global warming, being 40% less than thought in 2007. This is in fact excellent news.

Smoothed global temperatures have been virtually unchanged since 2007, while greenhouse gas warming net of the latest, well constrained, estimates of the offsetting effects of aerosols and ocean heat uptake is over 60% higher that it was thought to be in 2007. This implies that the climate system must be much less sensitive to greenhouse gas warming than previously thought: a doubling of carbon dioxide concentration can be expected to cause the mean global temperature ultimately to rise by 1.5-2°C (a detailed calculation is available That is much less than the central estimate of climate sensitivity per the 2007 IPCC report of 3°C.

While lower climate sensitivity doesn’t mean a continuing unchecked rise in carbon dioxide emissions is safe, it is certainly good news.”

In other words, the finding that aerosols have a smaller-than-expected shading effect makes the higher-sensitivity/higher-aerosol explanation for the slow warming much less plausible than the lower-sensitivity/lower-aerosol shading case. With the continuing rise in greenhouse gases producing no apparent rise in temperatures in recent years, the case for low climate sensitivity only looks stronger. This is unequivocally good news.

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