| 英文摘要: | Sahelian summer rainfall, controlled by the West African monsoon, exhibited large-amplitude multidecadal variability during the twentieth century. Particularly important was the severe drought of the 1970s and 1980s, which had widespread impacts1, 2, 3, 4, 5, 6. Research into the causes of this drought has identified anthropogenic aerosol forcing3, 4, 7 and changes in sea surface temperatures (SSTs; refs 1, 2, 6, 8, 9, 10, 11) as the most important drivers. Since the 1980s, there has been some recovery of Sahel rainfall amounts2, 3, 4, 5, 6, 11, 12, 13, 14, although not to the pre-drought levels of the 1940s and 1950s. Here we report on experiments with the atmospheric component of a state-of-the-art global climate model to identify the causes of this recovery. Our results suggest that the direct influence of higher levels of greenhouse gases in the atmosphere was the main cause, with an additional role for changes in anthropogenic aerosol precursor emissions. We find that recent changes in SSTs, although substantial, did not have a significant impact on the recovery. The simulated response to anthropogenic greenhouse-gas and aerosol forcing is consistent with a multivariate fingerprint of the observed recovery, raising confidence in our findings. Although robust predictions are not yet possible, our results suggest that the recent recovery in Sahel rainfall amounts is most likely to be sustained or amplified in the near term. The Sahel drought of the 1970s and 1980s had devastating impacts on local populations and has been widely studied as one of the most important examples in instrumental records of changes in the hydrological climate of any region1, 2, 3, 4, 5, 6. Between the 1950s and 1980s, Sahelian summer rainfall declined by around 40%, associated with a spatially coherent pattern of rainfall change across most of North Africa1, 2, 3, 4. Early studies on the causes of the drought focused on the role of the land surface15 and changes in sea surface temperatures (SSTs) in the Atlantic1, 2, 6, 8, 9, 11 and Indian 10, 11 ocean basins. The relevant changes in SST may have arisen from natural internal variability or in response to changing forcings. More recent studies have explored the role of changing anthropogenic forcings, with several studies concluding that increases in anthropogenic aerosol precursor emissions from North America and Europe were a particularly important factor3, 4, 7. One likely consequence of these aerosol precursor emissions was to cool SST in the North Atlantic relative to the South Atlantic, making a link with the previous research on the influence of SST changes. Since the 1980s, the amounts of Sahel summer (July–August–September) rainfall have increased2, 3, 4, 5, 6, 11, 12, 13, 14 (Fig. 1a–c and Supplementary Fig. 1). The increase between the recent period 1996–2010/2011 and the drought period 1964–1993 was 0.26–0.31 mm day−1 (hereafter mm d−1, estimated from three data sets), corresponding to around one-third of the decrease that occurred between the 1950s and the drought period. The largest increases have occurred in August, which is climatologically the wettest month (Fig. 1b). In view of the devastating impacts of the drought, understanding the reasons for this recent recovery, and whether it is likely to be sustained, is a key challenge.
| http://www.nature.com/nclimate/journal/v5/n8/full/nclimate2664.html
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