Synchronous tropical and polar temperature evolution in the Eocene

Abstract

Palaeoclimate reconstructions of periods with warm climates and high atmospheric CO concentrations are crucial for developing better projections of future climate change. Deep-ocean and high-latitude palaeotemperature proxies demonstrate that the Eocene epoch (56 to 34 million years ago) encompasses the warmest interval of the past 66 million years, followed by cooling towards the eventual establishment of ice caps on Antarctica. Eocene polar warmth is well established, so the main obstacle in quantifying the evolution of key climate parameters, such as global average temperature change and its polar amplification, is the lack of continuous high-quality tropical temperature reconstructions. Here we present a continuous Eocene equatorial sea surface temperature record, based on biomarker palaeothermometry applied on Atlantic Ocean sediments. We combine this record with the sparse existing data to construct a 26-million-year multi-proxy, multi-site stack of Eocene tropical climate evolution. We find that tropical and deep-ocean temperatures changed in parallel, under the influence of both long-term climate trends and short-lived events. This is consistent with the hypothesis that greenhouse gas forcing, rather than changes in ocean circulation, was the main driver of Eocene climate. Moreover, we observe a strong linear relationship between tropical and deep-ocean temperatures, which implies a constant polar amplification factor throughout the generally ice-free Eocene. Quantitative comparison with fully coupled climate model simulations indicates that global average temperatures were about 29, 26, 23 and 19 degrees Celsius in the early, early middle, late middle and late Eocene, respectively, compared to the preindustrial temperature of 14.4 degrees Celsius. Finally, combining proxy- and model-based temperature estimates with available CO reconstructions yields estimates of an Eocene Earth system sensitivity of 0.9 to 2.3 kelvin per watt per square metre at 68 per cent probability, consistent with the high end of previous estimates.

Climate Sensitivity on Geological Timescales Controlled by Nonlinear Feedbacks and Ocean Circulation

“…Indeed, the dependence of feedbacks on baseline temperature highlighted in this work provide a plausible explanation for why Anagnostou et al () found a higher climate sensitivity (∼4 °C per CO 2 doubling) in the relatively warm early Eocene climatic optimum than in the relatively cool late Eocene (∼3 °C per CO 2 doubling). Our findings are also consistent with Cramwinckel et al (, their Extended Data Figure 9), who, relative to the late Eocene, find a higher sensitivity for the warm EECO than for the cooler middle Eocene.…”

“…The change in global climate sensitivity over time includes a gradual increase in sensitivity from the earliest Cretaceous to the latest Cretaceous, a decrease to a minimum in the Paleocene, and then an increase followed by a decrease in the Eocene. Our modeled range of climate sensitivities in the Eocene (3.8 to 4.7 °C) is consistent with estimates of 3.5 to 8.9 °C (Cramwinckel et al, ) and 2.9 to 4.0 °C (Anagnostou et al, ), inferred from temperature and CO 2 proxies for this time period.…”

“…In addition, the Friedrich et al () benthic δ 18 O data prior to the Paleocene could be regarded as representing an upper bound on temperature, because much of the warmest underlying data comes from Demerara Rise, which is bathed by a warm, saline water mass forming on the South American shelf (Friedrich et al, ; MacLeod et al, ). The cooling trend through the Eocene in our < T > inferred is also seen in the recent estimates of global mean temperature from Cramwinckel et al (; blue line in Figure ). However, our trend is less pronounced, being cooler in the early Eocene (Ypresian) by about 6 °C.…”

“…The gray lines are global mean temperatures estimated from the Cramer et al (; light gray) and Friedrich et al (; dark gray) benthic δ 18 O data sets, using the method of Hansen et al (). The blue line shows global mean temperatures in the Eocene from Cramwinckel et al ().…”

“…We take T proxy from planktic δ 18 O estimates of Cretaceous, Paleocene, and Eocene SST compiled in O'Brien et al (; their Figure 9b) and Cramwinckel et al (; supporting information Figure S8a; the modern site locations and their paleorotations are shown in Figure S8b). Prior to calculating < T inferred >, we remove planktic δ 18 O records from one location (Blake Nose, sites 1050 and 1052) that appear exceptionally cold biased, which we suggest potentially results from diagenesis.…”