Hot Air, Hot Lakes, or both? Exploring Holocene African Temperatures using Proxy System Modeling

Abstract:

Current-generation climate models predict Africa will warm by up to 5 ℃ in the coming century, severely stressing African societies. To help constrain these model predictions, this study compares ten reconstructions of East African lake and air temperatures to those predicted by Paleoclimate Model Intercomparison Project (PMIP3) and transient TraCE simulations, focusing on the Mid-Holocene. Reconstructions of tropical African temperature derived from lake sedimentary archives indicate 1-3℃ of warming during the Middle Holocene relative to the 20th century. Importantly, most climate models do not replicate the warming observed in these paleoclimate data. We investigate this discrepancy using a new lake proxy system model and climate model simulations, with special attention to the variable relationship between lake surface temperature and air temperature. Focusing on geochemical reconstructions of temperature from ten equatorial African lakes, we compare these reconstructions to coupled general circulation model simulations spanning the Mid-Holocene and the present. Ongoing analysis of two large lakes finds amplified lake surface temperature changes compared to air temperature during the Mid-Holocene due to heightened seasonality and precessional forcing; however, the lake heating bias is insufficient to reconcile the large data-model discrepancy 6k.a. This analysis also finds further evidence of changes in mixing depth over time, potentially driven by changes in cloud cover and shortwave radiative fluxes penetrating the lake surface. This may confound proxy interpretation for glycerol dialkyl glycerol tetraether compounds which exist in the mixed layer, and suggests a need for independent constraints on mixed layer depth. This presentation will expand this analysis to ten lakes to provide a new interpretive framework for these invaluable paleoclimate records of past temperature changes over the African continent.