Our data illustrate how the Earth system responds to rapidly increasing, high carbon dioxide concentrations, and underline fundamental differences in global climate under such conditions. We also show how Earth system responses vary temporally and latitudinally.
While the lack of polar ice caps and the equitable nature of the extremely warm Early Eocene climates have been previously documented26, 27, we show that CEW climates already existed at high northern latitudes during the Early Paleocene when carbon dioxide concentrations were only 400 ppm2, indicating significant shifts in climate already at relatively low carbon dioxide concentrations (Fig. 1B). Monsoonal conditions (CM) were observed on the Antarctic Peninsula during all time periods where proxy data was available. Despite dramatic increases and decreases in atmospheric carbon dioxide concentrations and global temperature through the Paleocene and Early Eocene, polar climate was relatively stable, particularly in the Arctic (Fig. 1).
Another striking change in climate was the drying and increased precipitation intermittency indicated by expansion of arid to semi-arid conditions (WA/CA and WS/CS) at mid-latitudes beginning in the Late Paleocene and persisting until after the EECO (Fig. 2). There are a few data points indicating aridity and intermittent precipitation at about 30° latitude in the Early Paleocene (Fig. 2), but by the Late Paleocene, the most common climate type at mid-latitudes was WS/CS (Fig. 2), indicating a significant increase in aridity and precipitation intermittency at increasingly high latitudes in response to greenhouse gas emissions. A global, mid-latitude increase in precipitation intermittency for the PETM has been noted previously12, 15, but here we show that these conditions persisted throughout the Early Paleogene. Basin-specific studies have noted dramatic changes in climate prior to and persisting after the PETM28–30, and this compilation suggests a global significance of these trends. This highlights the scale of potential future changes in precipitation, and the importance of understanding the drivers and timing of shifts in climate variability31.
As temperatures and atmospheric carbon dioxide concentrations were highest during the PETM, the largest changes in precipitation would also be expected at that time. Much of the prior research has focused on the PETM due to the expectation of dramatic Earth system changes at that time8, 9, 12. However, our data are indicative of a significant intensification of the hydrologic cycle that already occurred in the Late Paleocene. Furthermore, precipitation did not significantly change in the post-PETM when mean temperature and atmospheric carbon dioxide concentrations dropped globally (Fig. 1E and Fig. 2)2. Based on this global data compilation, the hydrologic cycle did not return to “normal” conditions, similar to the modern day, until the post-EECO cooling period.
Due to the global prevalence of study sites with increased precipitation intermittency and drying persisting after the PETM, our data suggest that spatial shifts in precipitation persist longer than initial carbon dioxide sequestration, especially by the geologically rapid sources such as ocean uptake. Furthermore, based on shifts that occurred specifically during and after the PETM, regions on the border of climate “zones,” such as ~ 45° latitude in the western United States and Eurasia, were the most affected by the geologically rapid changes in carbon dioxide emissions, as arid (WA/CA) proxy locations expanded by ~ 5° poleward (Fig. 1 and S2). These findings support interpretations of regionally complex changes in precipitation in high carbon dioxide climates15.