MODELLING THE POTENTIAL IMPACTS OF FORESTATION ON EXTREME CLIMATE EVENTS OVER WEST AFRICA

Abstract

Previous studies on climate change projections over West Africa did not include the influence of on-going forestation activities on future climate extremes over the region. The present study aimed to examine the potential impacts of a large scale forestation activity on the future characteristics of extreme climate events (extreme rainfall and heatwaves) over West Africa using Regional Climate Models (RCMs). The specific objectives of the study were to: (i) examine RCMs ability to simulate extreme climate events over West Africa; (ii) investigate the potential impacts of climate and forestation on extreme rainfall events over West Africa; and (iii) examine the potential impacts of climate and forestation on heatwaves characteristics over West Africa. The study applied two RCMs (RegCM and WRF) to simulate the present day (PRS, 1970-2004) and the future (2030-2064) climates, with and without forestation (GHG and FRS, respectively). The simulations account for the potential impacts of forestation over the Savannah zone (8°N - 12°N) in West Africa. In this study, an extreme rainfall event is said to occur when the daily rainfall amount exceeds a threshold value (i.e. 95th percentile of the daily rainfall) and a Widespread Extreme Rainfall Event (WERE) is defined as the simultaneous occurrence of extreme rainfall that covers at least 50% of a given area. Heatwaves are identified using two metrics: the excess heat factor (EHF) and a percentile based index (TXI: 90th percentile of daily maximum temperature). The RCMs ability to simulate the characteristics of extreme events for PRS is assessed against observed datasets: the Global Precipitation Climatology Project (GPCP) and Tropical Rainfall Measurement Mission (TRMM) for extreme rainfall events analysis; and the Princeton University Global Meteorological Forcing Dataset (PGFD) for heatwaves analysis. The results show that both RCMs reproduce well the extreme rainfall threshold values (95th percentile) over West Africa and WEREs over Savannah in comparison with the observation datasets (GPCP and TRMM), though with some notable discrepancies. The RCMs generally overestimate the threshold of extreme rainfall over coastal areas and highlands, and simulated WEREs earlier than observed. For heatwaves, the two methods (EHF and TXI) generally produce similar patterns of heatwave characteristics over West Africa, except that heatwave number and days are substantially greater with TXI than with EHF. Also, the models give a realistic simulation of extreme temperature thresholds and heatwave characteristics over West Africa, although with some apparent biases. The results agree with previous studies that the Representative Concentration Pathway (RCP4.5) emission scenario would increase the frequency and intensity of extreme climate events over West Africa in future. In fact climate change would increase the frequency of extreme rainfall events over parts of the Guinea coast (and lower it over the Sahel zone), and heatwave number, days and duration over the whole region in future. The results further indicate that forestation would enhance the characteristics of extreme events over West Africa in future. Forestation generally increases the frequency and intensity of extreme rainfall events over the forested zone and decreases it elsewhere. Also, both models suggest that forestation would increase WERE frequency in parts of the Savannah zone. Similarly, forestation would also increase heatwave number and days over the forested area as the forestation would decrease surface albedo which during the dry season would increase the net solar radiation making more energy available at the surface. The outcomes of the present study suggest that the use of forestation to mitigate the impacts of climate change over West Africa might induce undesirable climatic impacts (increase in extreme rainfall and heatwave events) over some locations of the subcontinent, thereby increasing the climatic risk on human health and security. Therefore, the results of this study may guide decision makers in improving the resilience of West African countries to the consequences of climate and weather extremes and also in choosing appropriate climate change mitigation and adaptation options.