A just-released PNAS paper by Robert Kaufmann and others claims to account for observed global surface temperatures since 1998 by including in its model the ‘global dimming’ effect of sulfate aerosols emitted with the combustion of coal, oil and natural gas, and the smelting of copper, zinc, lead and nickel.
It has attracted some attention from the mainstream news media (which welcomed it either as a chance to tut about Chinese pollution or spout hogwash); so too have recent warnings of drought and near-famine conditions in the Horn of Africa.
This brings to mind something that has received very little attention outside the world of environmental science, a 2002 paper co-written by Leon Rotstayn from CSIRO Marine and Atmospheric Research. ‘Tropical Rainfall Trends and the Indirect Aerosol Effect’ modelled the effects on rainfall of sulfate emissions and found that they helped account for decades of strikingly decreased rainfall observed in northeast Africa from the 1960s onwards, which led ultimately to the calamitous Sahel famine of the 1980s. Simulations revealed that precipitation in the low-latitude band between Senegal and Eritrea is very sensitive to changes to oceanic temperature gradients in the nearby Atlantic and Indian Ocean basins. Such marine temperature changes, and the human misery that resulted, may have been induced by the activity of coal-fired power stations, diesel vehicles and oil refineries in the northern hemisphere, which caused the latter to cool relative to the south. For now that is an open question.
Much interesting work has been done on the climate and water-cycle effects of atmospheric brown clouds over the Indian Ocean and East Asia formed by black-carbon and sulfate aerosols resulting from nearby and upwind industrial activity and household burning. These may include the recently observed moderate cooling in China and India, disruption of the summer monsoon, floods in southern China and drought in the north. Uncertainties, apparently, abound when estimating the precise effect on cloud formation, precipitation and radiative forcing of atmospheric aerosols. But, just like in Ethiopia and Sudan during the 1980s, what was previously attributed to overfarming and desertification can be simulated by other means, and may thus have another explanation.
The Ethopian-Sudanese famine of the 1980s was one of the great televised and photographed horrors of the late 20th century. The reality of these events and their use as a business opportunity has remained obscure, as Alex de Waal, formerly of Human Rights Watch, argued in his book on the ‘disaster relief industry’, Famine Crimes. De Waal’s detailed retelling of the famine, focused on its social and political underpinnings, ought to be supplemented with knowledge of research into its possible physical basis.
The alternative is the grotesque line of BBC foreign correspondents about ‘biblical famine’, the video below having become the template for all subsequent treatments of the subject.