The Eocene (56–34 million years ago) is characterised by declining sea surface temperatures (SSTs) in the low latitudes (~4°C) and high southern latitudes (~8-11°C), in accord with decreasing CO2 estimates. However, in the mid-to-high northern latitudes there is no evidence for surface water cooling, suggesting thermal decoupling between northern and southern hemispheres and additional non-CO2 controls. To explore this further, we present a multi-proxy (Mg/Ca, δ18O, TEX86) SST record from the western North Atlantic (~36°N paleolatitude). Our data confirm a long-term decline in SSTs of ~5°C between the early (~53 Ma) and the middle (~42 Ma) Eocene, supporting declining atmospheric CO2 as the primary mechanism of Eocene cooling. However, from the middle Eocene onwards, North Atlantic zonal temperature gradients are decoupled, which we attribute to the incursion of warmer waters into the eastern North Atlantic and the inception of Northern Component Water across the early-middle Eocene transition.

Reconstructing global mean surface temperature (GMST) is one of the key contributions that paleoclimate science can make in addressing societally relevant questions and is required to determine equilibrium climate sensitivity (ECS). GMST has been derived from the temperature of the deep ocean (Td), with previous work suggesting a simple Td-GMST scaling factor of 1 prior to the Pliocene. However, this factor lacks a robust mechanistic basis, and indeed, is intuitively difficult to envisage given that polar amplification is a ubiquitous feature of past warm climate states and deep water overwhelmingly forms at high latitudes. Here, we interrogate whether and crucially, why, this relationship exists using a suite of curated data compilations and two sets of paleoclimate model simulations. We show that models and data are in full agreement that a 1:1 relationship is a good approximation. Taken together, the two sets of climate models suggest that i) a lower sensitivity of SST in the season of deep water formation than high latitude mean annual SST in response to climate forcing, and moreover ii) a greater degree of land versus ocean surface warming are the two processes that act to counterbalance a possible polar amplification-derived bias on Td-derived GMST. Using this knowledge, we provide a new Cenozoic record of GMST. Our estimates are substantially warmer than similar previous efforts for much of the Paleogene and are thus consistent with a substantially higher-than-modern ECS during deep-time high CO2 climate states.