Global Dimming?

The reference is to the 1980s famine in Ethiopia, partly caused by the failure of the Sahel monsoon (but clearly exacerbated by the poor governance of the Menghistu regime then in power). The link is based on a single modelling sensitivity study (Rotstayn and Lohmann, 2002) which looked at only the changes in the indirect effect from the pre-industrial (ca. 1850) to the present day. In this study, there was a shift southward of the rainfall belts in a similar way to that observed over the whole century (i.e. not necessarily just the 1980s). While this is indeed very interesting and does suggest that aerosol indirect effects can have important climatic consequences, it is merely the first step to attributing any particular climatic effect (failure of Sahel monsoon) to a particular cause (aerosol indirect effects). The obvious open questions relate to the importance of other forcings, in particular, greenhouse gases (which were not changed in this experiment), and the robustness of any transient response (i.e. does a simulated drought occur in the Sahel in the 1980s more often than at any other time). Absent this further study (which we expect is ongoing as part of the assessments related to the IPCC 4th Assessment report), it is horribly premature to declare ‘global dimming’ the cause of this event. Note that while this study looked at the aerosol effects, it makes no claim to actually match the ‘global dimming’ results. Additionally, other model experiments (Giannini et al, 2003) point to warmer Indian Ocean temperatures to explain the 1980s droughts.

Aerosols are however much more clearly responsible for serious respiratory problems in big cities (London prior to 1950s, Beijing today), and their health impacts are well known. This was one of the big pushes behind initiatives like the Clean Air acts in many countries which reduced aerosol emissions from power stations. While in the developed world (US, Europe, the ex-USSR) emissions have been falling, the global burden is increasing because of development in India and China. Since, on average, aerosols have a cooling effect (although some absorbing aerosols like black carbon (soot) are actually adding to global warming), reducing current aerosol levels (particularly sulphates) is equivalent to an extra warming effect.

An important point to note is that while cooling from aerosols and warming from greenhouse gases may have a slight cancelling effect in the global mean, this is not true regionally. Ideas that we should increase aerosol emissions to counteract global warming have been described as a “Faustian bargain” because that would imply an ever increasing amount of emissions in order to match the accumulated GHG in the atmosphere, with ever increasing monetary and health costs.

Does this all have either an implication for the global climate sensitivity (how much warming would result from a doubling of CO2) or the scenarios used by IPCC to project climate changes out to 2100? This is where I have to disagree most strongly with the commentary in the program. First, if we were trying to estimate climate sensitivity purely from the response over the 20th century, we would need to know a number of things quite exactly: chiefly the magnitude of all the relevant forcings. However, the uncertainties in the different aerosol effects in particular, preclude an accurate determination from the instrumental period alone. While it is true that, holding everything else equal, an increase in how much cooling was associated with aerosols would lead to an increase in the estimate of climate sensitivity, the error bars are too large for this to be much of a constraint. The estimate of 3+/-1 deg C (for doubled CO2) based on paleo-data and model studies is therefore still valid, even after this program.

Secondly, would a re-evaluation of the aerosol effect imply that projections to 2100 must be worse than previously suggested? If the climate sensitivity lies within the bounds considered in IPCC TAR (which I would argue is still the case), the answer is no. The most extreme scenario postulated in TAR (A1F1) already has a big reduction in sulphate aerosol forcing, and so the temperature changes by 2100 are almost purely a function of the GHG forcing. They are therefore unaffected by a re-evaluation of the aerosol indirect effect.

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