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Hydroclimate lessons from the early Paleogene indicate decoupling of mean annual precipitation from precipitation intensity and intermittency in an extremely warm world
Slawson, Jacob S.
Slawson, Jacob S.
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2025
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Abstract
The Earth is moving towards a climate state without societal precedent. To assess the effects of greenhouse gas driven warming on hydroclimate, we turn to Earth’s past where changes in climate are recorded in the form of proxies. The early Paleogene (66-47.8 Ma) offers a unique opportunity to evaluate this relationship due to its high global temperatures and superimposed extreme warm periods (hyperthermals). This period includes the Paleocene-Eocene Thermal Maximum (PETM) and Early Eocene Climatic Optimum (EECO). We compiled global proxy data across this time period and classified proxy locations using a novel climate type system focused on precipitation intensity and intermittency. By comparing changes in climate before, during, and after these extreme warming events, we gained insight into the hydroclimatic response to extreme warming and find (1) a decoupling of mean annual precipitation from precipitation intensity and intermittency, (2) non-linearities in the hydrological cycle’s sensitivity to temperature increase, and (3) departures from the wet-gets-wetter and dry-gets-drier response.
These conclusions were treated as hypotheses and further tested with the construction of a high-resolution paleoclimate record from the same time period in the Uinta Basin, Utah. We used paleosol-based geochemical proxies to quantify changes in precipitation during the PETM and subsequent hyperthermals. We find no significant change in mean annual precipitation over nearly a 10 million year period. However, paleosol mass-balance results and concurrent fluvial stratigraphy indicate that the hyperthermals were associated with an increase in the intensity and intermittency of precipitation. These changes persisted for 105-106 years after the PETM, highlighting non-linearities in the hydrologic response to warming. Our results and precipitation in the larger Rocky Mountain West may have been driven by long-term changes in the transport direction, seasonality, and moisture transport capability of the North American monsoon and a poleward shift in the storm tracks. Significantly, these results were compared to a state-of-the-art climate ensemble and indicate that the model does a poor job of simulating precipitation during the early Paleogene in the region. These results support our interpretations in the global literature analysis and imply the need for high-resolution regional model in the Rocky Mountain West to resolve if these discrepancies are a function of coarse model resolution, missing processes, or incorrect boundary conditions. To accurately predict future changes in hydroclimate, climate models need to be able to simulate the effects of extreme warmth.
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