Impact of climate change on mid-twenty-first century growing seasons in Africa

Kerry H Cook,Edward K Vizy

doi:10.1007/s00382-012-1324-1

Abstract

Changes in growing seasons for 2041–2060 across Africa are projected using a regional climate model at 90-km resolution, and confidence in the predictions is evaluated. The response is highly regional over West Africa, with decreases in growing season days up to 20% in the western Guinean coast and some regions to the east experiencing 5–10% increases. A longer growing season up to 30% in the central and eastern Sahel is predicted, with shorter seasons in parts of the western Sahel. In East Africa, the short rains (boreal fall) growing season is extended as the Indian Ocean warms, but anomalous mid-tropospheric moisture divergence and a northward shift of Sahel rainfall severely curtails the long rains (boreal spring) season. Enhanced rainfall in January and February increases the growing season in the Congo basin by 5–15% in association with enhanced southwesterly moisture transport from the tropical Atlantic. In Angola and the southern Congo basin, 40–80% reductions in austral spring growing season days are associated with reduced precipitation and increased evapotranspiration. Large simulated reductions in growing season over southeastern Africa are judged to be inaccurate because they occur due to a reduction in rainfall in winter which is over-produced in the model. Only small decreases in the actual growing season are simulated when evapotranspiration increases in the warmer climate. The continent-wide changes in growing season are primarily the result of increased evapotranspiration over the warmed land, changes in the intensity and seasonal cycle of the thermal low, and warming of the Indian Ocean.

Highlights

A reliance on rain-fed agricultural across Africa increases the population’s vulnerability to climate variability and change
The response is highly regional over West Africa, with decreases in growing season days up to 20% in the western Guinean coast and some regions to the east experiencing 5–10% increases
In Angola and the southern Congo basin, 40–80% reductions in austral spring growing season days are associated with reduced precipitation and increased evapotranspiration

Summary

Introduction

A reliance on rain-fed agricultural across Africa increases the population’s vulnerability to climate variability and change. Precipitation-only models define the growing season(s) using precipitation threshold criteria that can vary regionally, and are useful because rainfall observations are generally available (Stern and Coe 1982). Sivakumar (1988) defines onset over Burkina Faso and Niger as the date after May 1st when the 3-day accumulated rainfall exceeds 20 mm and no dry spells within the month exceed 7 days. Odekunle (2004) defines onset (cessation) over Nigeria as the date when an accumulated 7–8% (90%) of the annual rainfall total is obtained for a given station. Segele and Lamb (2005) find that regionally-specific onset and cessation definitions are needed over Ethiopia due to complex orography and high variability. Onset is defined as the date when the 3 day accumulated rainfall is greater than 20 mm provided there are no dry spells within the month longer than 8 days. The cessation date over wet (dry) regions is taken as the first day of a dry period that lasts for at least 20 (15) days

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