Rapid environmental change in African and South American tropics around 4000 years before present: a review

Palaeoecological data recording a pronounced environmental shift centred about 4000 years BP are presented from tropical Africa and South America. The environmental shift is particularly manifested as a change in the hydrological budget and vegetation within numerous swamp and lake catchments. The majority of sites in tropical Africa record a shift to drier environmental conditions that is in opposition to South America, where a shift to a wetter environment is generally recorded. The strength of between site signals varies from being relatively complacent to dramatic, in some cases reflective of whole-scale vegetation change. The magnitude of change is mainly dependent of the location of the site and the proxy under investigation. These ecosystem changes are likely to reflect changed precipitation regimes, increased evaporation, and or an extension/contraction of the dry season. This pronounced Holocene environmental shift is particularly interesting, as the marked changes within the tropics are either weakly recorded, or non-existent, at more extensively studied temperate latitudes and polar regions. The climate mechanisms responsible for this shift are reviewed and a model developed to explain such a strong signal from the tropical areas without associated changes at high latitudes, such as changes in polar ice-sheet extent. We propose changes in Pacific Ocean sea-surface temperature regime, and the establishment of El Niño conditions have imparted a direct influence on tropical Atlantic SST that could explain the rapid changes in terrestrial palaeoecological records. Other components of the terrestrial–ocean–atmosphere system are also likely to be important contributory factors in the environmental shift, in particular the large changes in land surface conditions could contribute to the climate shift. Given the scenario for tropical environmental change, to a degree independent from high latitudes, targeted areas of future research are indicated that incorporate development of climate and ecosystem modelling and palaeoenvironmental investigation with a tropical focus.