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Spatial pattern of super-greenhouse warmth controlled by elevated specific humidity


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.

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Fig. 1: Temperature reconstructions during the LPEE.
Fig. 2: Siderite-based reconstructions of the oxygen isotope composition of mean annual precipitation (δ18Op) during the LPEE.
Fig. 3: Spatial pattern in elevated specific humidity during the LPEE.

Data availability

All data discussed in the main text and plotted in the main figures and extended data figures are included in Supplementary Excel files stored in a Pangaea database ( and available along with the Supplementary Information. Siderite clumped and oxygen isotope data can be found in Supplementary Data 3. Raw clumped and oxygen isotope data is stored in an offline Easotope database that can be provided upon request by S.M.B. A compilation of siderite-based and previous early Palaeogene temperatures, both on land and in the ocean, can be found in Supplementary Data 2 together with the respective references that are also included in the Supplementary Information. A compilation of siderite-based reconstructions of δ18Op and previous reconstructions of δDp can be found in Supplementary Data 1. Microprobe data on siderite trace elemental concentrations can be found in Supplementary Data 4. Calculated LPEE saturation vapour pressures can be found in Supplementary Data 5.


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We thank M. Jaggi and S. Bishop for assistance in the laboratory. We thank PAREX resources and G. Tellez for allowing sampling, and G. Bayona for identifying the siderite in the sample from Cuervos, Colombia. We thank G. Suan, J. Schnyder and F. Baudin for providing the sample from Arctic Siberia. We thank M. Dechesne, E. D. Currano, R. Dunn and L. E. Schmidt for providing the sample from the Hanna Basin. We thank R. Peters for assistance with the WorldClim-2 and Digital Elevation Model data. We thank D. J. J. van Hinsbergen for assistance with the palaeolatitudes. We thank C. Jaramillo, K. Snell, J. Kelson, E. Middlemas, D. Colwyn, T. Kukla and R. Wills for internal discussions. We acknowledge financial support through ETH project ETH-33 14-1 and Swiss SNF project 200021_169849.

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T.W. and S.M.B. designed the study. J.v.D. wrote the manuscript. J.v.D., A.F. and S.M.B. developed the method for siderite analysis and J.v.D. and A.F. performed the measurements. S.R.P. provided the Faroese sample. T.W. provided most of the other samples. All authors contributed to discussions and editing of the final manuscript.

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Correspondence to Joep van Dijk.

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Extended data

Extended Data Fig. 1 Paleo-geographic map of the Paleocene/Eocene Thermal Maximum (56 Ma) with study sites.

Siderite bearing paleosols (red stars) and previous latest Paleocene/ earliest Eocene study sites (yellow circles; Supplementary Data 2). 1) Cerrejon and Cuervos, Colombia, 2) Elgin and McQueeney, Texas, 3) Pruden mine, Arkansas, 4) Alberhill, California, 5) Calhan, Colorado, 6) Hanna basin, Wyoming, 7) Black diamond, Washington State, 8) Prince of Wales Island, Alaska, 9) Paris basin, France, 10) Faroe Islands, 11) Faddeevsky Island, Russia. Light-grey gridlines represent latitudes and longitudes, with 30° spacing. The map was generated with GPlates® using the rotation frame and tectonic reconstruction of ref. 73.

Extended Data Fig. 2 Schematic on the formation of pedogenic siderite spherules and their proxy information.

Pedogenic siderite forms in permanently waterlogged soils with high organic matter content. From top left to bottom right: 1) Oxygen isotopes in precipitation percolate down into the soil and get incorporated into the dissolved inorganic carbon pool. 2) Bacterial dissimilatory iron reduction in the anoxic soil produces aqueous CO2 and ferrous iron (Fe2+). 3, 4) Pedogenic siderite precipitates in the soil pore spaces under the supersaturation of dissolved inorganic carbon and Fe2+ recording the soil groundwater temperature, the oxygen isotope composition of precipitation and the carbon isotope composition of the soil groundwater (reflective of the dominant soil redox pathway). 5) Clumped isotope-based soil groundwater temperature can be related to the surface air temperature. 6) Oxygen isotope composition of siderite can be related to the oxygen isotope composition of precipitation.

Extended Data Fig. 3 Physical soil temperature depth model in a typical siderite-bearing soil.

We consider a one-meter thick lignite74 overlaying a waterlogged clay horizon and a seasonality of 10 °C at the soil surface. Soil physical properties are taken from ref. 75. Based on the model, the maximum siderite formation temperature at a depth of one meter, just below the A-horizon, can only be slightly biased towards the temperature of the warmest months even if the siderite forms in the summer due to high rates of microbial organic matter degradation. The uncertainty of the clumped isotope-based formation temperature overlaps the mean annual temperature at a soil depth of one meter.

Extended Data Fig. 4 Latest Paleocene/ earliest Eocene siderite-bearing paleosols in California.

Ione Fm. at Ione, California (a) and siderite-bearing paleosol at Alberhill, California (bf). Siderite-bearing horizon at Alberhill with patches of siderite spherules (e) and fossilized roots (d) is located roughly one meter below overlying A-horizon (c, b, f).

Extended Data Fig. 5 Latest Paleocene/ earliest Eocene (LPEE) siderite continental temperatures in comparison to the present-day.

Reconstructions are compared to present-day temperatures at the respective sites or the longitudinal range of present-day temperatures considering the 2σ uncertainty in clumped isotope-based temperatures. We compared the Colombian, southern Alaskan and Siberian reconstructions to the entire present-day longitudinal range of temperatures (circles and dashes lines). Other reconstructions are compared to the respective temperature at the site in the present-day (stars), with the exception of the sites in Wyoming and Colorado that are currently located at high altitude. Black represents comparisons to present-day mean annual temperature and red represents comparisons to present-day temperature of the warmest months, considering the summer-bias in high latitude temperatures.

Extended Data Fig. 6 Compilation of all late Paleocene/ early to middle Eocene continental temperatures without elevation filter.

Yellow stars: siderite Δ47 (this study), green circles: fossil flora, orange triangles: paleosol weathering climofunctions, blue diamonds: MBT’-CBT, purple plus signs: tooth-enamel δ18O, red triangle: goethite δ18O, blue cross: cellulose δ18O, blue hexagons: calcite Δ47 (see Pangaea database). Faint symbols represent the late Paleocene or early/ middle Eocene. Open symbols are from elevations above 200 meters above sea level. Shaded grey represents present-day mean annual temperature and cross-hatched grey represents the present-day temperature of the warmest months.

Extended Data Fig. 7 Super-greenhouse siderite oxygen isotope (δ18O) records.

Latest Paleocene/ earliest Eocene, late Permian/ early Triassic and Albian/ Cenomanian siderite δ18O records76,77,78,79,80,81,82,83,84 show a similar decrease from the tropics to the polar latitudes. Black dashed line is symmetrical with the center at 0 ° and drawn to depict the main features of the datasets.

Supplementary information

Supplementary Information

Supplementary discussion, Figs. 1–11 and Table 1.

Supplementary Data 1

Oxygen and deuterium isotope records presented in Figs. 2 and 3a.

Supplementary Data 2

Temperature records presented in Fig. 1.

Supplementary Data 3

Siderite-based temperature reconstructions presented in Fig. 1.

Supplementary Data 4

Siderite microprobe measurements discussed in the Supplementary Information.

Supplementary Data 5

LPEE saturation vapour pressures presented in Fig. 3b.

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van Dijk, J., Fernandez, A., Bernasconi, S.M. et al. Spatial pattern of super-greenhouse warmth controlled by elevated specific humidity. Nat. Geosci. 13, 739–744 (2020).

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