The latitudinal temperature gradient and its climate dependence as inferred from foraminiferal delta O-18 over the past 95 million years

The latitudinal temperature gradient is a fundamental state parameter of the climate sys-tem tied to the dynamics of heat transport and radiative transfer. Thus, it is a primary target for temperature proxy reconstructions and global climate models. However, recon-structing the latitudinal temperature gradient in past climates remains challenging due to the scarcity of appropriate proxy records and large proxy-model disagreements. Here, we develop methods leveraging an extensive compilation of planktonic foraminifera delta O-18 to reconstruct a continuous record of the latitudinal sea-surface temperature (SST) gradient over the last 95 million years (My). We find that latitudinal SST gradients ranged from 26.5 to 15.3 degrees C over a mean global SST range of 15.3 to 32.5 degrees C, with the highest gradients during the coldest intervals of time. From this relationship, we calcu-late a polar amplification factor (PAF; the ratio of change in >60 degrees S SST to change in global mean SST) of 1.44 6 0.15. Our results are closer to model predictions than previ-ous proxy-based estimates, primarily because delta O-18-based high-latitude SST estimates more closely track benthic temperatures, yielding higher gradients. The consistent covariance of delta O-18 values in low-and high-latitude planktonic foraminifera and in ben-thic foraminifera, across numerous climate states, suggests a fundamental constraint on multiple aspects of the climate system, linking deep-sea temperatures, the latitudinal SST gradient, and global mean SSTs across large changes in atmospheric CO2, continen-tal configuration, oceanic gateways, and the extent of continental ice sheets. This implies an important underlying, internally driven predictability of the climate system in vastly different background states.