The impact of future climate change on the sub-Saharan northern African growing season is investigated using regional climate model projections produced by the NCAR/NOAA WRF model. Two ensembles are generated. The first is a late 20th century control, while the second is a future projection representative of mid-21st century under the IPCC AR4 A1B emissions scenario. These ensembles are used to produce daily climate information needed to predict the growing season based on a precipitation/potential evapotranspiration model developed by the Food and Agriculture Organization of the United Nations.

Results from this analysis indicate a robust 10 - 25 day decrease in the number of growing season days over southern Ethiopia and southern Mali/eastern Guinea, and a 5 – 20 day increase over the central Sahel. The shortening of the growing season over southern Mali/eastern Guinea is associated with both a later onset and early cessation. Over the central Sahel the lengthening of the growing season is associated with an earlier onset and a later cessation as rainfall intensity increases 30 – 60 percent. Over southern Ethiopia both the boreal spring and fall growing seasons decrease in length. These changes are attributed to modifications in both precipitation and potential evapotranspiration.

The projected changes in the climate and growing season are related to changes in oceanic warming and land surface warming over the Sahara. While SSTAs are projected to warm in the future almost everywhere, the increase is not uniform. Late boreal spring mid-latitude North Atlantic SSTA warming is not as strong as mid-latitude South Atlantic SSTA warming, while the opposite occurs during the boreal summer. These changes in oceanic surface heating affect the inter-hemispheric temperature gradient and are found to be associated with a southward displacement in rainfall over the tropical Atlantic and West Africa in the late boreal spring, and a northward displacement in the rainfall and enhanced low-level westerly flow over the Sahel in the boreal summer. The circulation and rainfall response over the Sahel is further perturbed by increased future heating over the eastern Sahara.