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20th-Century Drying of the Sahel and Southern Africa
Reference
Hoerling, M., Hurrell, J., Eischeid, J. and Phillips, A. 2006. Detection and attribution of twentieth-century northern and southern African rainfall change. Journal of Climate 19: 3989-4008.

Background
The authors note that "a trend toward increased aridity since 1950 has emerged over southern Africa (Hulme, 1996)," which they say was driven by "a 20% reduction in February-March-April rainfall." In addition, they report there was a 35% reduction in rainfall over the same period throughout the Sahel.

What was done
In a study that "explores the nature and causes for the regional downward trajectories of African rainfall," Hoerling et al. compared real-world measurements with the rainfall simulations of 18 climate models employed in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change.

What was learned
In the words of the four researchers, "the ensemble of greenhouse-gas-forced experiments, conducted as part of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, fails to simulate the pattern or amplitude of the twentieth-century African drying, indicating that the drought conditions were likely of natural origin." In fact, they say that for both of the regions studied, "the observed trend amplitude exceeded that of the greenhouse gas signal by an order of magnitude," and they state once again that they "therefore concluded that greenhouse gas forcing played little or no role in the 1950-99 observed African drying trends." What is more, they say there is "considerable spread" among the 18 model projections, making their mean trend so small that they suggest that "natural variability will continue to be the primary driver of [Africa's] low-frequency rainfall variations during the next century."

What it means
In spite of the climate-alarmist contention that the earth is now warmer than it has been at any time throughout the prior two millennia, and that it is close to the all-time level of warmth experienced over the past million years (thanks, supposedly, to anthropogenic CO2 emissions), there is absolutely no evidence that the 20th-century drying of much of Africa was in any way related to CO2-induced global warming, nor is there any model-based reason for supposing it will be so related over the next century. If anything, the models suggest it should actually get a little wetter there ... and for decades to come.

Reference
Hulme, M. 1996. Recent climatic change in the world's drylands. Geophysical Research Letters 23: 61-64.

Reviewed 14 February 2007