Abstract
Abstract. The Pliocene is regarded as a potential analogue for future climate with conditions generally warmer-than-today and higher-than-preindustrial atmospheric CO2 levels. Here we present the first orbitally resolved records of continental hydrology and vegetation changes from West Africa for two Pliocene time intervals (5.0–4.6 Ma, 3.6–3.0 Ma), which we compare with records from the last glacial cycle (Kuechler et al., 2013). Our results indicate that changes in local insolation alone are insufficient to explain the full degree of hydrologic variations. Generally two modes of interacting insolation forcings are observed: during eccentricity maxima, when precession was strong, the West African monsoon was driven by summer insolation; during eccentricity minima, when precession-driven variations in local insolation were minimal, obliquity-driven changes in the summer latitudinal insolation gradient became dominant. This hybrid monsoonal forcing concept explains orbitally controlled tropical climate changes, incorporating the forcing mechanism of latitudinal gradients for the Pliocene, which probably increased in importance during subsequent Northern Hemisphere glaciations.
Highlights
The hydrologic cycle is of vital importance for the global climate system, owing to its function in regulating the heat and moisture balance (Lindzen, 1990, 1994)
Two modes of interacting insolation forcings are observed: during eccentricity maxima, when precession was strong, the West African monsoon was driven by summer insolation; during eccentricity minima, when precession-driven variations in local insolation were minimal, obliquity-driven changes in the summer latitudinal insolation gradient became dominant
Pliocene δ13C31 values display a narrow range around the average of −25.4 ‰, clearly below the enriched values of up to −23 ‰ as observed for the last glacial cycle (Fig. 4; Kuechler et al, 2013)
Summary
The hydrologic cycle is of vital importance for the global climate system, owing to its function in regulating the heat and moisture balance (Lindzen, 1990, 1994) This is mainly achieved through the atmospheric and ocean circulation, which tend to equalize the differences in solar heating between low and high latitudes. The importance of this heat redistribution is illustrated by the fact that around 50 % of the annual energy budget of high latitudes originates from lower latitudes (Peixoto and Oort, 1992), peaking in winter, when polar night conditions prevail at high latitudes (Davis and Brewer, 2011). To assess the dynamics of a changing climate, palaeoclimate studies provide means to investigate long-term developments of the monsoon (Mohtadi et al, 2016)
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