Human influence on climate has been detected in surface air temperature on global and regional scales, in sea level pressure, in tropopause height, and in ocean heat content. However, large scale precipitation changes have not been formally attributed to human influence, perhaps because compensating increases and decreases in different regions of the globe reduce the global average signal. Here we compare in-situ precipitation observations over the 20th century averaged not globally, but in latitude bands, to precipitation simulated by fifteen climate models. We show that the anthropogenic contribution to zonal precipitation changes can be separated from climate variability and the effect of natural forcing, thus detecting human influence on this key impact-relevant climate variable. We estimate that external, primarily anthropogenic, forcings likely contributed significantly to observed increases in precipitation in Northern Hemispheric mid-latitudes, a finding that is consistent with observed changes in runoff and salinity. Human forcings likely also contributed substantially to the drying trend in the northern subtropics and tropics, and to moistening in the southern subtropics and tropics. These changes have significant implications for ecosystems, agriculture, and human health.