Earth’s climate sensitivity, defined as the temperature increase for a doubling of partial pressure of carbon dioxide (\(p_{\mathrm{CO}_2}\)), and the mechanisms responsible for amplification of high-latitude warming remain controversial. The latest Palaeocene/earliest Eocene (LPEE; 57–55 million years ago) is a time when atmospheric CO₂ concentrations peaked between 1,400 and 4,000 ppm, which allows us to evaluate the climatic response to high \(p_{\mathrm{CO}_2}\). Here we present a reconstruction of continental temperatures and oxygen isotope compositions of precipitation (reflective of specific humidity) based on clumped and oxygen isotope analysis of pedogenic siderites. We show that continental mean annual temperatures reached 41 °C in the equatorial tropics, and summer temperatures reached 23 °C in the Arctic. The oxygen isotope compositions of precipitation reveal that compared with the present day the hot LPEE climate was characterized by an increase in specific humidity and the average residence time of atmospheric moisture and by a decrease in the subtropical-to-polar specific humidity gradient. The global increase in specific humidity reflects the fact that atmospheric vapour content is more sensitive to changes in \(p_{\mathrm{CO}_2}\) than evaporation and precipitation, resulting in an increase in the residence time of moisture in the atmosphere. Pedogenic siderite data from other super-greenhouse periods support the evidence that the spatial patterns of specific humidity and warmth are related, providing a new means to evaluate Earth’s climate sensitivity.

地球对气候的敏感性定义为二氧化碳分压（\（p _ {\ mathrm {CO} _2} \））加倍时的温度升高，以及引起高纬度变暖放大的机制仍然存在争议。最新的古新世/最早的始新世（LPEE； 57-5500万年前）是大气中CO ₂浓度达到1400 ppm至4,000 ppm峰值的时期，这使我们能够评估对高\（p _ {\ mathrm {CO} _2} \）。在这里，我们基于成岩侧铁矿的成团和氧同位素分析，提出了大陆温度和降水的氧同位素组成（反映特定湿度）的重建。我们显示，赤道热带大陆的年平均温度达到41°C，而北极的夏季温度达到23°C。降水的氧同位素组成表明，与今天相比，炎热的LPEE气候的特征在于比湿度增加和大气水分的平均停留时间增加，并且副热带与极性的相对湿度梯度减小。全球特定湿度的增加反映了这样一个事实，即大气蒸气含量对\（p _ {\ mathrm {CO} _2} \）的变化更敏感与蒸发和沉淀相比，增加了水分在大气中的停留时间。来自其他超级温室时期的成岩性菱铁矿数据证明了特定湿度和温暖的空间格局是相关的，这为评估地球的气候敏感性提供了一种新的手段。