Past terrestrial hydroclimate sensitivity controlled by Earth system feedbacks

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.

Patterns and Mechanisms of Northeast Pacific Temperature Response to Pliocene Boundary Conditions

“…In addition, there is an anomalous anticyclonic circulation stretching from southern California south to Baja California, suggesting higher pressure in this region. Similar stationary wave pattern has been noted in other Pliocene modeling simulations, as well as in simulations of future warming (Feng et al., 2022b; Menemenlis et al., 2021; Wills et al., 2019). While the cause of this pattern remains unclear, it may be linked to changes in the Pacific Walker circulation, since this pattern is evident in both CESM1.2 and CESM2 simulations, both of which weaken the Walker circulation, and is absent in the CCSM4 simulation (Feng et al., 2020).…”

“…Recent work suggests that non‐CO 2 boundary conditions may play an important role in regional Pliocene climate patterns (Feng et al., 2022b; Fu et al., 2021; Menemenlis et al., 2021). To further diagnose the causes of temperature change along the California coast in the mid‐Pliocene, we decompose the influence of CO 2 , topography, and vegetation changes in the MPWP on various climate parameters using a series of single forcing simulations of CESM2 (Figure 9).…”

“…These simulations are described in Feng et al. (2022b). Briefly, our E400 simulation features 400 ppm of CO 2 and PI boundary conditions.…”

“…To further decompose the changes to SST and other variables in the CESM2 mid-Pliocene simulation along the California Margin, we performed a series of single forcing simulations to isolate the influence of greenhouse gases, biome distributions and ice sheets, and paleogeography and topography. These simulations are described in Feng et al (2022b). Briefly, our E400 simulation features 400 ppm of CO 2 and PI boundary conditions.…”

“…Liu et al, 2012Liu et al, , 2016. All simulations and model versions used in this study are further described by Feng et al (2020Feng et al ( , 2022b.…”