Early Paleogene Climate: A Glimpse of Extreme Warming

As the world warms, the Earth system moves towards a climate state without societal precedent. This challenges predictions of the future, as climate models need to be tested and calibrated with real-world data from high carbon dioxide climates. Despite the many advances in climate modeling, predictions of precipitation have particularly high uncertainties. Earth history provides an opportunity to observe how the Earth system responded to high greenhouse gas emissions, enabling us to better predict how it may do so in the future. Here, we compile global terrestrial proxy data from the Early Paleogene (66-49 Ma), a period with a warm climate overprinted by multiple rapid global warming events and suggested as a possible analogue for future worst-case scenarios. We show surprising results in the timing and duration of dramatic shifts in the hydrologic cycle occurring well prior to maximum temperatures and persisting well beyond. We provide a glimpse of an extremely warm Earth with ever-wet or monsoonal conditions in the northern and southern polar regions, and sustained aridity interrupted by extreme rainfall events at mid-latitudes. Our results indicate inconsistencies between proxy data and state-of-the-art paleoclimate models that are commonly used to predict and understand future climate change. Our focus on precipitation intermittency and intensity provides new data on long-term precipitation trends in high greenhouse gas climates to help address large uncertainties in future precipitation trends. A high-resolution focus on mid-latitude proxy data produces trends where some locations become drier during the PETM, while others become wetter, indicating dynamically-driven changes that differ from the wet-gets-wetter, dry-gets-dryer" thermodynamic response on a regional scale.