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Abstract
Despite tectonic conditions and atmospheric CO2 levels (pCO2) similar to those of present-day, geological reconstructions from the mid-Pliocene (3.3-3.0 Ma) document high lake levels in the Sahel and mesic conditions in subtropical Eurasia, suggesting drastic reorganizations of subtropical terrestrial hydroclimate during this interval. Here, using a compilation of proxy data and multi-model paleoclimate simulations, we show that the mid-Pliocene hydroclimate state is not driven by direct CO2 radiative forcing but by a loss of northern high-latitude ice sheets and continental greening. These ice sheet and vegetation changes are long-term Earth system feedbacks to elevated pCO2. Further, the moist conditions in the Sahel and subtropical Eurasia during the mid-Pliocene are a product of enhanced tropospheric humidity and a stationary wave response to the surface warming pattern, which varies strongly with land cover changes. These findings highlight the potential for amplified terrestrial hydroclimate responses over long timescales to a sustained CO2 forcing.
Highlights
Despite tectonic conditions and atmospheric CO2 levels similar to those of presentday, geological reconstructions from the mid-Pliocene (3.3-3.0 Ma) document high lake levels in the Sahel and mesic conditions in subtropical Eurasia, suggesting drastic reorganizations of subtropical terrestrial hydroclimate during this interval
Using atmosphere-ocean coupled global climate model (GCM) simulations from the most recent Pliocene Model Intercomparison Project Phase II (PlioMIP2)[33,37,38], we demonstrate that most current generation GCMs can reproduce the pattern of mid-Pliocene hydroclimate of the subtropical Sahel and East Asia suggested by proxies without any paleoclimate-specific changes to model parameterizations
We further develop several new simulations using the Community Earth System Model version 235,39 to explore the extent to which simulated mid-Pliocene hydroclimate changes can be generated by changes in CO2, tectonics, or vegetation and ice sheets
Summary
Mechanisms causing the positive δ(P–E) in the subtropical Sahel and East Asia are different from previous studies. A positive vegetation-precipitation feedback results in multiple equilibrium states of North African vegetation over the late Quaternary[62] Different from those mechanisms, in our simulations, atmospheric circulation responses to Fvegice facilitate moisture transport towards the subtropical Sahel and East Asia via a stationary wave response. Changes in regional circulation in the form of stationary wave responses lead to strengthened low-level winds that import moisture into the subtropical Sahel and East Asia from the tropical Atlantic and Indian Oceans, respectively This inland moisture influx is further amplified by enhanced tropospheric humidity. This inference is supported by the strong relationship between a model’s ability to capture the mid-Pliocene hydroclimate state and its simulated global mean warming (Fig. 5a) The latter reflects model diversity in Earth System Sensitivity (ESS), which incorporates surface temperature responses to longterm biome range shift and ice sheet changes in addition to CO2 changes. Changes in vegetation and ice sheet distributions should be carefully considered when simulating past and future climates
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