Late Quaternary climatic variability in intertropical Africa
Tropical Africa is at the geographical heart of the PEP III transect and forms part of the heat engine which drives the meridional circulation of the atmosphere. The tropics are therefore central to studies of climate change not only in the equatorial belt but also in sub-tropical regions (Yin and Battisti 2001), and may even lead some high latitude climate changes (Henderson and Slowey 2000). Despite this pivotal role, the tropics have historically been the poor relation of temperate regions in palaeoenvironmental research. Here we synthesise newpalaeoclimate information derived primarily from forest and savanna regions of tropical Africa. These recent studies have helped to close some empirical gaps in data coverage within the region and at the same time have led to a new conceptual understanding of past tropical climates. The latter has been achieved through the rigorous application of classical methods as well as the development of several new proxies of environmental change.
We focus our discussion on several key palaeoclimate issues that fall within the scope of PAGES time stream 2 (cf. Gasse and Battarbee (this volume)). Although this is defined as spanning the last two glacial cycles, we will place greatest emphasis on the events since the penultimate glacial (marine isotope stage 6, hereafter MIS 6) and especially on the last 30,000 calendar years BP (abbreviated below to kyr) (broadly MIS 1-3). At these long time-scales, orbital-forcing factors, modified by earth-surface feedback mechanisms, ought to be dominant and be recognisable in the biotic and hydrological systems. We will then examine the evidence for abrupt events at century to millennial scales, which are prominent features in many climate reconstructions since the Last Glacial Maximum (LGM). Climate forcing at these time-scales is tightly coupled to oceanic changes and so we will extend our discussion to a number of offshore sites. Numerous terrestrial archives are now being explored including some excellent high-resolution speleothem records (Holmgren et al. 2001), but to limit our study we will confine our discussion to lakes and the major river systems (Fig. 1).
Few proxies are directly related to the fundamental climate variables of precipitation and temperature. Even in the tropics where shifts from wet to dry conditions commonly outweigh temperature changes, it is difficult to tease these variables apart. Moreover, the part played by atmospheric gases such as CO2 in terrestrial environmental change is now also recognised (Street-Perrott 1994; Jolly and Haxeltine 1997; Street-Perrott et al. 1997). Identifying the specific contribution of the different variables is of primary concern to tropical palaeoenvironmental studies and has promoted the development of several novel approaches to environmental reconstruction.
KeywordsOxygen Isotope Lake Level Last Glacial Maximum Marine Isotope Stage African Climate
Unable to display preview. Download preview PDF.