Tropical African climate after LGM

Johan Weijers and colleagues (2007) found that rainfall likely increased in tropical Africa at the end of the last glaciation and during the Holocene, with a brief interruption that approximately corresponds to the Younger Dryas.

We analyzed the distribution of branched tetraether membrane lipids derived from soil bacteria in a marine sediment record that was recovered close to the Congo River outflow, and the results enabled us to reconstruct large-scale continental temperature changes in tropical Africa that span the past 25,000 years. Tropical African temperatures gradually increased from 21° to 25°C over the last deglaciation, which is a larger warming than estimated for the tropical Atlantic Ocean. A direct comparison with sea-surface temperature estimates from the same core revealed that the land-sea temperature difference was, through the thermal pressure gradient, an important control on central African precipitation patterns.

Their reconstruction indicates a greater temperature rise over land than over the Atlantic with Pleistocene and Holocene global warming, which is expected to draw moisture onto the land. Tropical rainforest area was reduced during the glaciations, which seems to have been known from climate modeling:

This difference between continental and oceanic deglacial warming in the tropics is in agreement with climate model studies, which suggest that the average continental deglacial warming in the tropics was about 1.5 times stronger than the deglacial warming of the tropical oceans (23, 24). This amplified continental warming may have been because the continents cool more during glacial times. At high and mid-latitudes, the presence of a changed and often reduced vegetation cover during glacials results, through increased albedo, in enhanced cooling of the land surface. However, in tropical areas, this effect is counteracted by a negative feedback from reduced evaporation, which results from decreased tropical rainforest area [again, during glacials]. The reduced evaporation leads to a decreased loss of latent heat and thus relatively warmer surface temperatures. As a result, vegetation changes in tropical Africa are thought to result eventually in a negligible temperature effect (25). A more likely explanation for the enhanced glacial cooling of the continental tropics may thus be an increased pole-to-equator temperature gradient, resulting in a strengthened and enlarged Hadley Cell circulation (26) and, consequently, an increase in relatively cool air that flows from higher toward lower latitudes.

The paper also includes a good review of other data on African climate during and after the last glacial maximum. They claim that tropical Africa reflects global climate changes because it receives airflow from both Northern and Southern hemispheres, and the data connect to similar patterns elsewhere.

References:

Weijers JWH, Schefuss E, Schouten S, Damsté JSS. 2007. Coupled thermal and hydrological evolution of tropical Africa over the last deglaciation. Science 315:1701-1704. doi:10.1126/science.1138131