How much and why precipitation changes as the climate warms is uncertain, even for the global mean. In the 21st Century of the IPCC AR4 A1b forcing scenario, global-mean precipitation increase per degree warming varies among models by over a factor of three. Clear-sky atmospheric shortwave absorption change explains over half of the intermodel spread (r2= 0.61) in precipitation increase. Removing the part of clear-sky atmospheric shortwave absorption change due to water vapor increase reveals that shortwave absorption forcing decreases in NCAR CCSM 3.0 but increases in GFDL CM 2.1, which we attribute to differences in black carbon forcing reported by these modeling groups. The range of applied forcing causes a range in atmospheric shortwave absorption increase, which leads to spread in precipitation increase. In contrast, in the CO2-doubling forcing scenario, clear-sky atmospheric shortwave absorption explains an insignificant amount of spread (r2 = 0.04).