Earth's climate sensitivity – defined as the temperature increase for a doubling of pCO2 – and the mechanisms responsible for amplification of high latitude warming remain controversial. The latest Paleocene/earliest Eocene (LPEE; 57-55 million years ago) is a time when pCO2 peaked between 1400 and 4000 ppm, which allows us to evaluate the climatic response to high pCO2. 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 temperature 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 to 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 vapor content is more sensitive to changes in pCO2 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 new means to evaluate Earth's climate sensitivity.

Data availability statementAll data in the manuscript 'Spatial pattern of super-greenhouse warmth controlled by elevated specific humidity' is accessible in the attached files. Pedogenic siderite clumped and oxygen isotope data can be found in 'LPEE siderite clumped and oxygen isotope data'. Raw clumped and oxygen isotope data is stored in an offline Easotope® database that can be provided upon request by S. Bernasconi. A compilation of siderite-based and previous early Paleogene temperatures – both on land and in the ocean – can be found in 'A compilation of siderite-based and previous early Paleogene temperature – both on land and in the ocean'. A compilation of siderite-based reconstructions of δ18Oprecipitation and previous reconstructions of δDprecipitation can be found in 'LPEE reconstructions of oxygen isotope and hydrogen isotope compositions of precipitation based on the analysis of pedogenic siderites and published records of plant wax n-alkanes'. Microprobe data on cation substitutions in the pedogenic siderites can be found in 'Cation substitutions in LPEE pedogenic siderites'. Calculated LPEE saturation vapor pressures can be found in 'LPEE saturation vapor pressure calculations using siderite clumped isotope-based reconstructions of atmospheric temperature'.