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1. Response of Coastal California Hydroclimate to the Paleocene-Eocene Thermal Maximum

   https://doi.org/10.5194/cp-2023-89  

   Zhang, Xiaodong; Tipple, Brett J; Zhu, Jiang; Rush, William D; Shields, Christian A; Novak, Joseph B; Zachos, James C (November 2023, European Geosciences Union)

   Abstract. The effects of anthropogenic warming on the hydroclimate of California are becoming more pronounced, with increased frequency of multi-year droughts and flooding. As a past analog for the future, the Paleocene-Eocene Thermal Maximum (PETM) is a unique natural experiment for assessing global and regional hydroclimate sensitivity to greenhouse gas warming. Globally, extensive evidence (i.e., observations, climate models with high pCO2) demonstrates hydrological intensification with significant variability from region to region (i.e., dryer or wetter, or greater frequency and/or intensity of extreme events). Central California (paleolatitude ~42° N), roughly at the boundary between dry subtropical highs and mid-latitude low pressure systems, would have been particularly susceptible to shifts in atmospheric circulation and precipitation patterns/intensity. Here, we present new observations and climate model output on regional/local hydroclimate responses in central California during PETM. Our findings based on multi-proxy evidence within the context of model output suggest a transition to an overall drier climate punctuated by increased precipitation during summer months along the central coastal California during the PETM. 

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2. Paleosol‐Based Reconstruction Indicates Decoupling of Mean Annual Precipitation and Precipitation Intensity During the Paleocene‐Eocene Thermal Maximum in the Uinta Basin, Utah

   https://doi.org/10.1029/2024PA004966  

   Slawson, J; Plink‐Bjorklund, B P; Beverly, E J; Bachtadse, V (April 2025, Paleoceanography and Paleoclimatology)

   Abstract The Earth is transitioning to a state unprecedented in human history. This transition poses a challenge for predicting the future, as climate models require testing and calibration with real‐world data from high greenhouse gas climates. Despite significant progress in climate modeling, changes in the precipitation remain highly uncertain. The Paleocene‐Eocene Thermal Maximum (PETM) was the warmest period of the Cenozoic Era, and thus serves as an analog for a hydrological cycle altered by extreme greenhouse gas warming. Here, we use paleosol‐based geochemical proxies to quantify changes in mean annual precipitation (MAP) during the PETM in the Uinta Basin, Utah. We find no change in MAP during this warming event. However, paleosol mass balance results track increased translocation of carbonates, increased clay illuviation, and increased accumulation of redox‐sensitive elements. These results, along with shifts in fluvial stratigraphy, provide evidence for increased intensity and intermittency of extreme precipitation events that may be related to changes in the transport direction, seasonality, and moisture transport capability of the North American Monsoon. Surprisingly, changes in fluvial stratigraphy, clay illuviuation, and redoximorphy continued for 10⁵–10⁶ years after the PETM, suggesting persistent changes in precipitation intensity despite a decrease in global temperature. These findings provide further support for an intensification of the hydrological cycle during and after the PETM, provide evidence for a decoupling between mean and extreme precipitation, and indicate the importance of multi‐proxy, regional studies for understanding the complexities of climate change. 

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3. Incoherency in Central American Hydroclimate Proxy Records Spanning the Last Millennium

   https://doi.org/10.1029/2022PA004445  

   Obrist‐Farner, Jonathan; Steinman, Byron A.; Stansell, Nathan D.; Maurer, Jeremy (March 2023, Paleoceanography and Paleoclimatology)

   Abstract Continued global warming is expected to result in reduced precipitation and a drier climate in Central America. Projections of future changes are highly uncertain, however, due to the spatial resolution limitations of models and insufficient observational data coverage across space and time. Paleoclimate proxy data are therefore critical for understanding regional climate responses during times of global climate reorganization. Here we present two lake‐sediment based records of precipitation variability in Guatemala along with a synthesis of Central American hydroclimate records spanning the last millennium (800–2000 CE). The synthesis reveals that regional climate changes have been strikingly heterogeneous, even over relatively short distances. Our analysis further suggests that shifts in the mean position of the Intertropical Convergence Zone, which have been invoked by numerous studies to explain variability in Central American and circum‐Caribbean proxy records, cannot alone explain the observed pattern of hydroclimate variability. Instead, interactions between several ocean‐atmosphere processes and their disparate influences across variable topography appear to have resulted in complex precipitation responses. These complexities highlight the difficulty of reconstructing past precipitation changes across Central America and point to the need for additional paleo‐record development and analysis before the relationships between external forcing and hydroclimate change can be robustly determined. Such efforts should help anchor model‐based predictions of future responses to continued global warming. 

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4. Zonal Indian Ocean Variability Drives Millennial‐Scale Precipitation Changes in Northern Madagascar

   https://doi.org/10.1029/2023PA004626  

   Tiger, Benjamin H.; Burns, Stephen; Dawson, Robin R.; Scroxton, Nick; Godfrey, Laurie R.; Ranivoharimanana, Lovasoa; Faina, Peterson; McGee, David (November 2023, Paleoceanography and Paleoclimatology)

   Abstract The low latitude Indian Ocean is warming faster than other tropical basins, and its interannual climate variability is projected to become more extreme under future emissions scenarios with substantial impacts on developing Indian Ocean rim countries. Therefore, it has become increasingly important to understand the drivers of regional precipitation in a changing climate. Here we present a new speleothem record from Anjohibe, a cave in northwest (NW) Madagascar well situated to record past changes in the Intertropical Convergence Zone (ITCZ). U‐Th ages date speleothem growth from 27 to 14 ka. δ¹⁸O, δ¹³C, and trace metal proxies reconstruct drier conditions during Heinrich Stadials 1 and 2, and wetter conditions during the Last Glacial Maximum and Bølling–Allerød. This is surprising considering hypotheses arguing for southward (northward) ITCZ shifts during North Atlantic cooling (warming) events, which would be expected to result in wetter (drier) conditions at Anjohibe in the Southern Hemisphere tropics. The reconstructed Indian Ocean zonal (west‐east) sea surface temperature (SST) gradient is in close agreement with hydroclimate proxies in NW Madagascar, with periods of increased precipitation correlating with relatively warmer conditions in the western Indian Ocean and cooler conditions in the eastern Indian Ocean. Such gradients could drive long‐term shifts in the strength of the Walker circulation with widespread effects on hydroclimate across East Africa. These results suggest that during abrupt millennial‐scale climate changes, it is not meridional ITCZ shifts, but the tropical Indian Ocean SST gradient and Walker circulation driving East African hydroclimate variability. 

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5. Changes in tree drought sensitivity provided early warning signals to the California drought and forest mortality event

   https://doi.org/10.1111/gcb.15973  

   Keen, Rachel_M; Voelker, Steven_L; Wang, S‐Y_Simon; Bentz, Barbara_J; Goulden, Michael_L; Dangerfield, Cody_R; Reed, Charlotte_C; Hood, Sharon_M; Csank, Adam_Z; Dawson, Todd_E; et al (November 2021, Global Change Biology)

   Abstract Climate warming in recent decades has negatively impacted forest health in the western United States. Here, we report on potential early warning signals (EWS) for drought‐related mortality derived from measurements of tree‐ring growth (ring width index; RWI) and carbon isotope discrimination (∆¹³C), primarily focused on ponderosa pine (Pinus ponderosa). Sampling was conducted in the southern Sierra Nevada Mountains, near the epicenter of drought severity and mortality associated with the 2012–2015 California drought and concurrent outbreak of western pine beetle (Dendroctonus brevicomis). At this site, we found that widespread mortality was presaged by five decades of increasing sensitivity (i.e., increased explained variation) of both tree growth and ∆¹³C to Palmer Drought Severity Index (PDSI). We hypothesized that increasing sensitivity of tree growth and ∆¹³C to hydroclimate constitute EWS that indicate an increased likelihood of widespread forest mortality caused by direct and indirect effects of drought. We then tested these EWS in additional ponderosa pine‐dominated forests that experienced varying mortality rates associated with the same California drought event. In general, drier sites showed increasing sensitivity of RWI to PDSI over the last century, as well as higher mortality following the California drought event compared to wetter sites. Two sites displayed evidence that thinning or fire events that reduced stand basal area effectively reversed the trend of increasing hydroclimate sensitivity. These comparisons indicate that reducing competition for soil water and/or decreasing bark beetle host tree density via forest management—particularly in drier regions—may buffer these forests against drought stress and associated mortality risk. EWS such as these could provide land managers more time to mitigate the extent or severity of forest mortality in advance of droughts. Substantial efforts at deploying additional dendrochronological research in concert with remote sensing and forest modeling will aid in forecasting of forest responses to continued climate warming. 

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