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Global dimming: does it affect the UKCIP02 climate change scenarios?
Geoff Jenkins and Jim Haywood, Hadley Centre, Met Office, Exeter
Last updated: 21 April 2005
Download a pdf of this article here.
A recent BBC2 Horizon programme has led to an upsurge in interest in global dimming, a reduction in solar radiation at the ground over the past few decades, and some UKCIP stakeholders have asked what the implications are for the UKCIP02 scenarios. The programme featured Gerald Stanhill of the Israeli Institute of Soil, Water and Environmental Sciences, who was one of the first to highlight the phenomenon, and Pete Cox of the Met Office's Hadley Centre, who explained that one possible implication of global dimming is that climate change in the future may have been underestimated.
A very readable review of the topic, by Stanhill, can be found in the January 2005 edition of Weather*. A network of pyranometers measures "global irradiance", the amount of solar radiation, both that direct from the clear sun and that transmitted or reflected by clouds. Over the past 3 or 4 decades global irradiance appears to have shown a global-average decrease of about 2-3% per decade, with a peak in latitudes of 20-50N. Some stations have shown larger decreases and some have shown increases (for example Rothamsted in Kent has an average trend of +4% per decade from 1958-2000).
It seems likely that reductions may be associated with changes in aerosols (small particles in the atmosphere) or clouds. As explained in UKCIP02, sulphate aerosols from industrial emissions of SO2 give a haze in the lower atmosphere that will reflect back solar radiation and hence cool the earth. They also act as nuclei on which cloud particles form, and hence in industrial areas cloud tends to be composed of more, but smaller, water droplets; this (paradoxically) makes them brighter as seen from outer space and they reflect back more solar radiation; this is the "indirect" cooling effect of sulphate aerosols. (There is a further indirect cooling effect because clouds with smaller droplets will tend to give less rain and hence last longer, but this is poorly quantified). The other main aerosol species are organic carbon and black carbon or soot, emitted from transport and industry, and also from biomass burning (for example, forest fires). While organic carbon has effects similar to sulphate aerosol (i.e. a cooling via direct and indirect effects), soot absorbs solar radiation and so has a warming effect. It is important to realise however that all aerosol types, whether scattering or absorbing, will contribute to global dimming at the surface.
The Hadley Centre climate model used for the UKCIP02 scenarios (HadCM3) does show a global dimming over the past few decades, but only a small fraction of that observed. What does this imply for its predictions of global warming? The model includes the direct and indirect (cloud brightness) effects of sulphate aerosols, but does not include soot. If the discrepancy between model estimates and observations is due to soot, then the addition of soot to the model may give better agreement with dimming observations but because it will have had a warming effect to date, probably not change the predictions markedly. If the discrepancy is because the cooling effects of sulphate aerosols are underestimated, then this could also mean that the warming effects of greenhouse gases are underestimated too, if between them they are required to explain the observed warming over the past few decades. This could also lead to predictions of greater climate change in the future, as the aerosol burden (and its cooling effect) is expected to decrease once greater controls are placed on emitters such as power stations to reduce local pollution effects such as acid rain.
Recent modelling studies with the new Hadley Centre model (HadGEM1), which includes the effects of sulphate, soot, and biomass burning aerosols (mainly soot and organic carbon), show a more significant global dimming than those performed with HadCM3.
We hope to investigate the implications of global dimming on predictions of climate change further; the observations place a useful new constraint on climate models. It is worth pointing out, though, that this is but one of the uncertainties in climate change predictions, some of which (for example, cloud feedback) are much larger. Appendix 1 of UKCIP02 pointed out the need to take these modelling uncertainties into account, and a Hadley Centre Note (HCTN 44) describes some of the ways that can be used to manage uncertainties.
*Royal Meteorological Society, 104 Oxford Rd, READING, RG1 7LL or contact weather@rmets.org
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