Cenozoic Proxy Constraints on Earth System Sensitivity to Greenhouse Gases

Abstract

Climate sensitivity is defined as the global surface temperature response (ΔGST) to a given radiative forcing (ΔF) and, as such, forms an important factor in predicting the evolution of the climate system during the Anthropocene. The surface temperature at which the planet achieves radiative equilibrium is set by the interaction of numerous components of the Earth system, such that the magnitude and behavior of climate sensitivity under various levels of forcing remain difficult to constrain (Knutti et al., 2017). The target quantity of interest in most studies using climate models and instrumental observations is called the equilibrium climate sensitivity (ECS). ECS only considers the behavior of climate feedbacks with a centennial response timescale, including those that amplify (e.g., water vapor, sea ice, and clouds) or attenuate temperature changes (e.g., lapse rate adjustment), and is therefore considered the best measure for the expected global-scale atmospheric response to human perturbation within the near future (e.g., Sherwood et al., 2020).The geologic record is recognized as a powerful tool to elucidate the sensitivity of the climate system, as it contains numerous examples of a planet at steady state with elevated climate forcing created by changes in greenhouse gas concentrations, solar luminosity, and ocean area (Farnsworth et al., 2019;Rohling et al., 2012).

Toward a Cenozoic history of atmospheric CO 2

“…Our reconstructed Cenozoic CO 2 trends are broadly coherent with those for global temperature as inferred, for instance, from the oxygen isotopic composition (δ 18 O) of fossil benthic foraminifera shells ( 43 , 44 ) and compilations of global surface temperature ( 45 ) (Fig. 2B).…”

“…( B ) Global mean surface temperatures estimated from benthic δ 18 O data following Westerhold et al . ( 43 ) (solid line, individual proxy estimates as symbols, and statistically reconstructed 500-kyr mean values shown as the continuous curve, with 50 and 95% credible intervals) and from surface temperature proxies (gray boxes) ( 45 ). ( C ) Sea level after ( 66 ) with gray dots displaying raw data; the solid black line reflects median sea level in a 1-Myr running window.…”

“…In addition, we pair a set of multiproxy global surface temperature estimates for eight Cenozoic time intervals (Fig. 2B) ( 45 ) with posterior CO 2 estimates from time bins corresponding to each interval. The two temperature reconstructions are broadly similar, although the benthic record suggests relatively higher temperatures during the hothouse climate of the Paleocene and Eocene, whereas the multiproxy reconstruction is elevated relative to the benthic record during the Oligocene and Neogene.…”

“…The slopes between two points in time reflect the average ESS [CO2] . Circles reflect 500-kyr binned Category 1 CO 2 estimates paired with corresponding GMST means from ( 43 ); squares pair CO 2 and GMST means from compilations of sea surface temperature ( 45 ) in seven coarsely resolved time intervals. Note that this figure omits the Pliocene temperature estimate of ( 45 ) because it samples too short a time interval (compare with Fig.…”

Increased Biogenic Calcification and Burial Under Elevated pCO2 During the Miocene: A Model‐Data Comparison

“…One possible argument is that the ice albedo that comes into play in the last 3 Myr tends to enhance effective climate sensitivity at low p CO 2 . As a result, a small increase in p CO 2 from 280 p.p.m.v might have a large effect on equilibrium climate if the Earth becomes ice‐free (Ring et al., 2022). However, available Miocene simulations with less than 600 p.p.m.v cannot produce the magnitude of mid‐Miocene warming (Burls et al., 2021).…”