Past terrestrial hydroclimate sensitivity controlled by Earth system feedbacks

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.

Ran Feng, Tripti Bhattacharya, Bette L. Otto-Bliesner, Esther C. Brady, Alan M. Haywood, Julia C. Tindall, Stephen J. Hunter, Ayako Abe-Ouchi, Wing-Le Chan, Masa Kageyama, Camille Contoux, Chuncheng Guo, Xiangyu Li, Gerrit Lohmann, Christian Stepanek, Ning Tan, Qiong Zhang, Zhongshi Zhang, Zixuan Han, Charles J. R. Williams, Daniel J. Lunt, Harry J. Dowsett, Deepak Chandan, W. Richard Peltier