More intermittent mid-latitude precipitation accompanied extreme early Paleogene warmth

Warming is pushing the Earth system toward unfamiliar climate conditions, complicating predictions. Geological archives of past greenhouse climates provide essential tests for models under extreme forcing. We investigate how precipitation responded to extreme warmth during early Paleogene global warming events (66–47.8 million years ago) – a period considered a possible analogue for worst-case future scenarios. Here we compile global paleoclimate data and develop a multi-proxy approach that integrates sedimentary proxies - such as plant fossils, ancient soils and river deposits - providing constraints on global precipitation intermittency (seasonal and inter-annual variability) and intensity (rainfall rate). The data reveal wet or monsoonal polar regions, and aridity punctuated by intense rainfall at mid- and low-latitude continental interiors. This hydroclimate shift occurred 3 million years before and persisted 7 million years after the Paleocene-Eocene Thermal Maximum – the warmest period of the Cenozoic Era, suggesting that extreme warmth induces non-linearities in the hydrological cycle’s sensitivity to temperature increase. Polar humidity and mid-latitude aridity further indicate a departure from the expected wet-gets-wetter and dry-gets-drier response. Shifts towards aridity were decoupled from mean annual precipitation and driven by seasonal and interannual precipitation distribution, such as shorter wet season length and longer interannual rainfall recurrence interval. This highlights the importance of considering precipitation intermittency and intensity, as similar shifts may occur under future warming despite differences in boundary conditions.

Methods
This dataset was collected by compiling published literature on terrestrial temperature and precipitation during the Early Paleogene (66-47.8 Ma). Mean annual precipitation (MAP), mean annual temperature (MAT), and qualitative precipitation intermittency  and intensity values were used to assign a climate type with a distinct range of MAP and MAT values. Where quantitative information was not available, qualitative proxies were assigned a range of potential MAT and MAP values based on comparison to modern environments. For a more detailed explanation of the methods, please see the published article's supplementary information.

气候变暖正将地球系统推向前所未有的气候状态，加剧了气候预测的不确定性。地质历史中记录的古温室气候档案，可为极端强迫条件下的气候模型提供关键验证依据。本研究聚焦早古近纪（Early Paleogene，6600万至4780万年前）全球变暖事件期间降水对极端暖期的响应——这一时期被视为未来极端气候情景的潜在类比参照。我们整合全球古气候数据，构建了多代用指标研究方法，涵盖植物化石、古土壤、河流沉积等沉积代用指标（sedimentary proxies），以此约束全球降水的间歇性（季节与年际变率）与强度（降雨速率）。数据显示，极地地区呈现湿润或季风气候特征，中低纬度大陆内部则以干旱为主，间有强降雨事件。此次水文气候转变发生在古新世-始新世极热事件（Paleocene-Eocene Thermal Maximum，新生代（Cenozoic Era）以来最温暖的气候事件）之前300万年，并在该事件发生后持续了700万年，表明极端暖化会引发水文循环对温度升高的非线性响应。极地湿度升高与中纬度地区干旱加剧，进一步偏离了“湿区更湿、干区更干”的预期响应模式。干旱化趋势与年平均降水量解耦，而是由降水的季节与年际分布格局驱动，例如湿季长度缩短、年际降雨重现周期变长。这凸显了关注降水间歇性与强度的重要性：尽管未来暖化的边界条件存在差异，但类似的水文气候转变仍可能发生。

方法
本数据集通过汇编早古近纪（Early Paleogene，66–47.8 Ma）陆地气温与降水的已发表文献构建。我们采用年平均降水量（mean annual precipitation, MAP）、年平均气温（mean annual temperature, MAT）以及定性的降水间歇性与强度数值，结合年平均降水量与年平均气温的特定值域范围划定气候类型。在缺乏定量数据的情况下，我们通过与现代环境类比，为定性代用指标赋予潜在的年平均气温与年平均降水量值域范围。关于研究方法的详细说明，请参阅已发表论文的补充材料。