Detection of human influence on twentieth-century precipitation trends

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Human influence on climate has been detected in surface air temperature1,2,3,4,5, sea level pressure6, free atmospheric temperature7, tropopause height8 and ocean heat content9. Human-induced changes have not, however, previously been detected in precipitation at the global scale10,11,12, partly because changes in precipitation in different regions cancel each other out and thereby reduce the strength of the global average signal13,14,15,16,17,18,19. Models suggest that anthropogenic forcing should have caused a small increase in global mean precipitation and a latitudinal redistribution of precipitation, increasing precipitation at high latitudes, decreasing precipitation at sub-tropical latitudes15,18,19, and possibly changing the distribution of precipitation within the tropics by shifting the position of the Intertropical Convergence Zone20. Here we compare observed changes in land precipitation during the twentieth century averaged over latitudinal bands with changes simulated by fourteen climate models. We show that anthropogenic forcing has had a detectable influence on observed changes in average precipitation within latitudinal bands, and that these changes cannot be explained by internal climate variability or natural forcing. We estimate that anthropogenic forcing contributed significantly to observed increases in precipitation in the Northern Hemisphere mid-latitudes, drying in the Northern Hemisphere subtropics and tropics, and moistening in the Southern Hemisphere subtropics and deep tropics. The observed changes, which are larger than estimated from model simulations, may have already had significant effects on ecosystems, agriculture and human health in regions that are sensitive to changes in precipitation, such as the Sahel.

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Figure 1: Comparison between observed (solid black) and simulated zonal mean land precipitation trends for 1925–1999 (left) and 1950–1999 (right).
Figure 2: 1925–1999 changes in observed and simulated precipitation anomalies.
Figure 3: Results from detection and attribution analysis of zonal precipitation anomalies.


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We thank R. Vose (National Climatic Data Centre, NOAA) for the observed precipitation data, S.-K. Min for the provision of ECHO-G data, J. Wang (supported by the Canadian Foundation for Climate and Atmospheric Sciences) for processing model data and Y. Feng for computational assistance. We acknowledge the international modeling groups who contributed to the multi-model data archive at PCMDI (the Program for Climate Model Diagnostics and Intercomparison), which is supported by the Office of Science, US Department of Energy. Part of this work was supported by NOAA’s Office of Global Programs (G.C.H. and S.S.) and the DOE’s Office of Biological and Environmental Research (G.C.H.). G.C.H. was also supported by the NSF and by Duke University. N.P.G., F.H.L. and T.N. were supported by the Leverhulme Trust, the Comer Science and Education Foundation, and the MEXT (Ministry of Education, Culture, Sports, Science and Technology), respectively. P.A.S. was supported by the UK Department for Environment, Food and Rural Affairs.

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Correspondence to Francis W. Zwiers.

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This file contains Supplementary Notes, Supplementary Figures 1-7 with Legends, Supplementary Tables 1-3 and additional references. (PDF 292 kb)

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