Evaluating Quaternary climatic change in West Africa using the NOAA AVHRR 1-km Land Dataset

Abstract

Previous estimates of the extent and magnitude of climatic change in West Africa and other desert marginal areas, based on observations of fossil fixed dunes, have assumed a maximum shift in climatic zones during the Quaternary period of 500-700 km, with rainfall totals some 600-700 mm higher today than at the height of desert expansion. Such estimates have been able to utilise air photo and satellite images readily available in desert marginal areas with little cloud cover, but have not previously extended into more humid regions. The daily coverage of the NOAA AVHRR sensor gives a higher probability of cloud free images in humid regions, permitting a synoptic view across whole continents and climatic zones, with similar date and illumination conditions. The current availability, on a USGS website of the NOAA 1-km Land Dataset has enabled observation of desert landforms well into the rainforest zone of Nigeria and Cameroon, approximating the 2000 mm isohyet and a former desert margin at 1200 km south of the present desert. Although the desert landforms are now highly degraded by elevation and weathering, the sands of the former dunes are highly fertile fine sandy clay loams, ideal for cereal and groundnut cultivation, though the former interdune areas, with heavier clay soils, often remain uncultivated. Surprisingly, traditional soil surveys in West Africa have been unaware of these highly contrasting soil differences over short distances, though inexplicable disparities have often been noted on soil survey sheets. The study is therefore relevant not only in terms of estimating the magnitude of climatic change in tropical regions but also provide a different perspective for soil surveys and agricultural development in the tropics.