On the mechanisms of warming the mid-Pliocene and the inference of a hierarchy of climate sensitivities with relevance to the understanding of climate futures

Abstract

Abstract. We present results from our investigation into the physical mechanisms through which the mid-Pliocene, with a pCO2 of only ∼ 400 ppmv, could have supported the same magnitude of global warmth as has been projected for the climate at the end of the 21st century when pCO2 is expected to be 3 times higher. These mechanisms allow us to understand the warming in terms of changes to the radiative properties of the surface, the clouds, greenhouse gases, and changes to the meridional heat transport. We find that two-thirds of the warming pervasive during the mid-Pliocene, compared to the preindustrial, could be attributed to the reduction in the planetary emissivity owing to the higher concentrations of the greenhouse gases CO2 and water vapor, and the remaining one-third to the reduction in planetary albedo. We also find that changes to the orography and the pCO2 are the leading causes of the warming with each contributing in roughly equal parts to a total of 87 % of the warming and changes to the polar ice sheets responsible for the remaining warming. Furthermore, we provide a mid-Pliocene perspective on ongoing efforts to understand the climate system's sensitivity at various timescales and using multiple lines of evidence. The similarities in the boundary conditions between the mid-Pliocene and the present day, together with the globally elevated temperatures, make the mid-Pliocene an ideal paleo time period from which to derive inferences of climate sensitivity and assess the impacts of various timescale-dependent feedback processes. We assess a hierarchy of climate sensitivities of increasing complexity in order to explore the response of the climate over a very large range of timescales. The picture that emerges is as follows: on the short timescale, owing to the influence of fast feedback processes, the climate sensitivity is 3.25 °C per doubling of CO2; sensitivity increases to 4.16 °C per doubling of CO2 on an intermediate timescale as the ice–albedo feedback becomes active, and then sensitivity further increases to 7.0 °C per doubling of CO2 on long timescales due to the feedback from the glacial isostatic adjustment of the Earth's surface in response to the melting of the polar ice sheets. Finally, once the slow feedbacks have stabilized, the sensitivity of the system drops to 3.35 °C per doubling of CO2. Our inference of the intermediate-timescale climate sensitivity suggests that the projected warming by 2300 CE, inferred using Earth system models of intermediate complexity on the basis of an extension to the RCP4.5 emission scenario in which atmospheric pCO2 stabilizes at roughly twice the PI level in year 2150 CE, could be underestimated by ∼ 1 °C due to the absence of ice-sheet-based feedbacks in those models.

Survival of the Qaidam mega-lake system under mid-Pliocene climates and its restoration under future climates

“…3.3-3.0 Ma BP) and before. Although spatial details of the mega-lake system are not known, Chen and Bowler (1986) reported that the total lake surface was about 59 000 km 2 during the early Pleistocene. The onset of the Pleistocene glaciations at ca.…”

“…This approach follows the general idea underlying studies that have linked climates of the past with those projected for the near future using the concept of climate sensitivity (e.g. Chandan and Peltier, 2018).…”

“…Since the mid-Pliocene is often regarded as a past analogue for modern climate changes (Zubakov and Borzenkova, 1988;Haywood et al, 2016;Chandan and Peltier, 2018), this study also intends to provide a rational basis for assessing environmental changes that might be caused by climate changes as projected in this region for the future (Burke et al, 2018;Gu et al, 2018;Hui et al, 2018).…”

An energy balance model for paleoclimate transitions

“…Even for changes as recent as the mid-Holocene, there is a debate, for example, over whether the abrupt desertification of the Sahara is due to a bifurcation, as suggested in Claussen et al (1999) using an Earth model of intermediate complexity (EMIC), or is a transient response of the AMOC to a sudden termination of freshwater discharge to the North Atlantic, as proposed in Liu et al (2009), using a coupled atmosphere-ocean GCM (AOGCM). Similarly, for the glaciation of Greenland at the Pliocene-Pleistocene transition, recent work in Tan et al (2018), using a coupled GCM-ice-sheet model, shows good agreement with proxy records without the need for bifurcations.…”

Multiproxy Reduced‐Dimension Reconstruction of Pliocene Equatorial Pacific Sea Surface Temperatures

“…For these reasons, the Pliocene is considered a potential analog for climate conditions predicted for the latter part of this century (Crowley, 1996;Dowsett et al, 1996;Dowsett & Robinson, 2009). Not surprisingly, Pliocene climate model simulations with concentrations of CO 2 greater than those of pre-industrial time (Burls et al, 2017;Chandan & Peltier, 2018, 2017Haywood et al, 2010Haywood et al, , 2013Lunt et al, 2008;Salzmann et al, 2013) and/or with assumed or simulated reduced northern hemisphere ice cover (e.g., Feng et al, 2017;Haywood & Valdes, 2004;Howell et al, 2016) lead to higher temperatures at mid and high latitudes. Other model runs, however, that exploit perturbations to equatorial Pacific SSTs can also lead to warmer midlatitude and high-latitude (e.g., Barreiro et al, 2005;Brierley & Fedorov, 2010;Burls & Fedorov, 2014a, 2014bGoldner et al, 2011;Shukla et al, 2009Shukla et al, , 2011Vizcaíno et al, 2010).…”