WHEN the worst drought in 60 years hit America’s corn belt this summer, many people wondered if it was caused by climate change. It is too early to say much about such a recent episode but various studies have attributed earlier individual heatwaves or drought to global warming, notably those in Europe in 2003, Russia in 2010 and the sweltering summer of 2011 in Texas. The most recent (2007) assessment by the Intergovernmental Panel on Climate Change (IPCC) said bluntly: “higher temperatures and decreased precipitation have contributed to changes in drought.” This week a study for the World Bank by the Potsdam Institute for Climate Impact Research argued that “extreme summer temperatures can now largely be attributed to climatic warming since the 1960s.”
Global warming might cause drought because warm air holds on to water vapour, making rain even less likely in places that are already dry. But a study published recently in Nature casts doubt about whether that is actually happening.
Drought is an imprecise term. Arid conditions in one area might count as plentiful rainfall in another. To make matters more exact, scientists employ an index, called the Palmer index, to monitor changes in agricultural conditions. This index uses precipitation and temperature data to calculate evaporation rates and hence moisture levels in the soil. Because temperature plays such a big role, the index is sensitive to global warming.
The trouble is, argue the authors of the study, Justin Sheffield and Eric Wood of Princeton University, and Michael Roderick of the Australian National University, in Canberra, “evaporation is a function of more than just temperature.” Wind-speed, vapour pressure, clouds and many other factors matter. Until recently, there have not been enough data on these to feed into the index. Now the data are better.
To test the effect of these other factors, the authors calculated the Palmer index using two different methods: the traditional temperature-based one, and another that includes more varied meteorological information. For 1980-2008, the temperature-based Palmer index rises in 98% of the world’s land area. Using the more varied data, the index rises in only 58% of the land area and falls in a significant minority of places (South and Central America, the eastern United States, East Africa, western Russia, South and South-East Asia and Australia). Droughts in Europe, most of Africa, the American West and East Asia do seem to have become more severe. So it may be that the perception of a global increase in the severity of droughts is driven by local increases. That does not, of course, reduce the human impact of drought in those areas that suffer. But achieving a better understanding of what is really going on is important. Without it, anticipating the effects of climate change, rather than just reacting to them, will be difficult.