1. Climate Research Division, Environment Canada, Toronto, Ontario M3H 5T4, Canada
2. Nicholas School for the Environment and Earth Sciences, Box 90227, Duke University, Durham, North Carolina 27708, USA
3. Department of Geography, 507 McCone Hall, University of California, Berkeley, California 94720, USA
4. Climatic Research Unit, School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
5. NOAA Earth System Research Laboratory, 325 Broadway, Boulder, Colorado 80305, USA
6. Met Office Hadley Centre (Reading Unit), Meteorology Building, University of Reading, Reading RG6 6BB, UK
7. National Institute for Environmental Studies, Tsukuba, Ibaraki 305-8506, Japan

Correspondence to: Francis W. Zwiers¹ Correspondence and requests for materials should be addressed to F.W.Z. (Email: francis.zwiers@ec.gc.ca).

Abstract

Human influence on climate has been detected in surface air temperature^{1, 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 scale^{10, 11, 12}, partly because changes in precipitation in different regions cancel each other out and thereby reduce the strength of the global average signal^{13, 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 latitudes^{15, 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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