Croll revisited: Why is the northern hemisphere warmer than the southern hemisphere?

The question of why, in the annual mean, the Northern Hemisphere is warmer than the Southern Hemisphere is addressed, revisiting an 1870 paper by James Croll. We first show that the ocean is warmer than the land in general which, acting alone, would make the Southern Hemisphere warmer because of its greater fraction of ocean. Croll recognized this and thought it was caused by greater humidity and greenhouse trapping over the ocean than over the land. However, for any given temperature, greenhouse trapping is actually greater over the land than the ocean. Instead, oceans are warmer than land because of smaller surface albedo. However, inter-hemispheric differences in total albedo are negligible because the impact of differences in land-sea fraction are offset by Southern Hemisphere ocean and land reflecting more than their Northern Hemisphere counterparts. In agreement with Croll, it is shown that northward cross-equatorial ocean heat transport is critical for the Northern Hemisphere to be warmer. This is examined in a simple box model based on the energy budget of each hemisphere. This inter-hemispheric difference forced by ocean heat transport is enhanced by the positive water vapor-greenhouse feedback, and is partly compensated by the southward atmospheric energy transport, which is accomplished by locating the Intertropical Convergence Zone in the Northern Hemisphere. However, to fully explain the temperature difference in this way requires a northward ocean heat transport at the extreme of observational estimates. A better fit to data is found when a larger basic state greenhouse trapping in the NH, conceived as imposed by continental geometry, is also imposed. Therefore, despite some modifications to his theory, analysis of modern data confirms Croll’s 140 year-old theory that the Northern Hemisphere is warmer than the Southern Hemisphere in part because of northward cross-equatorial ocean heat transport.