Extended Data Fig. 6: Sensitivity of main results to late Eocene ice volume.

a, Top, tropical SST compilation; proxy data are compiled as described in Methods (red symbols). The fitted LOESS model is plotted as a black line and the 95% confidence interval as grey shading. Bottom, deep-ocean temperature compilation; δ¹⁸O-based proxy data are compiled as described in Methods. Ice-free deep-ocean temperatures and fitted LOESS model are shown as grey dots and line, respectively, and the deep-ocean temperature compilation and fitted LOESS model including late Eocene ice volume effect (Methods) as blue dots and line, correspondingly. 95% LOESS confidence intervals are shown as shading. b, Calculated MTG based on LOESS fits of proxy data (lines; propagated 95% confidence intervals are shown as silhouettes). The black line with grey silhouette shows results obtained using ice-free deep-ocean temperatures, and the blue line with blue silhouette includes the late Eocene ice volume effect on the deep-ocean temperature. c, Proxy (blue diamonds, tropical compilation; red diamonds, Site 959) deep-ocean temperature, including the late Eocene ice volume effect, against tropical SST. Lines represent Deming regression analysis through proxy data. The slope (polar amplification factor) is 2.07 ± 0.25 (±1 standard error) for the tropical compilation and 1.19 ± 0.06 for Site 959). Proxy data grouped into 1-Myr bins from 34–58 Myr ago, with error bars representing one standard deviation due to binning. d, Sensitivity of δ¹⁸O of Eocene seawater (‰ VSMOW) to the build-up of 0–10⁷ km³ of ice with varying isotopic composition.