Extended Data Fig. 8: PETM temperature gradient proxy-model comparison.

a, Top, tropical SST compilation; proxy data are compiled as described in Methods (red symbols). The fitted LOESS model is plotted as a red line, with the 95% confidence interval as pink shading. Bottom, ice-free deep-ocean temperature compilation; δ¹⁸O-based proxy data are compiled as described in Methods. Generalized cross-validation (GCV)-optimized fitted LOESS model (as in Fig. 3) plotted as grey line, with the 95% confidence interval as grey shading. An alternative LOESS model with small bandwidth (0.25 times the GCV-optimized span) that tracks deep-ocean PETM temperature more closely is shown as the blue line, with the 95% confidence interval as blue shading. Data are plotted together with the PETM simulation of Kiehl and Shields²⁸ (black open squares, seasonal range error bars) with altered cloud parameters (CP_PETM). b, Calculated MTG based on LOESS fits of proxy data and of the model simulation CP_PETM. The grey line is obtained using a GCV-optimized fitted LOESS model and the blue line using a smaller-bandwidth deep-ocean LOESS model (propagated 95% confidence intervals are shown as shading). We note that the PETM MTG pattern is complex owing to its sensitivity to the specific records, age models and filtering used, and might have evolved over the course of the event. Nevertheless, peak PETM MTG matches the simulation CP_PETM poorly. The age is in Myr ago, following GTS2012.