Past extreme warming events linked to massive carbon release from thawing permafrost

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Between about 55.5 and 52 million years ago, Earth experienced a series of sudden and extreme global warming events (hyperthermals) superimposed on a long-term warming trend1. The first and largest of these events, the Palaeocene–Eocene Thermal Maximum (PETM), is characterized by a massive input of carbon, ocean acidification2 and an increase in global temperature of about 5°C within a few thousand years3. Although various explanations for the PETM have been proposed4, 5, 6, a satisfactory model that accounts for the source, magnitude and timing of carbon release at the PETM and successive hyperthermals remains elusive. Here we use a new astronomically calibrated cyclostratigraphic record from central Italy7 to show that the Early Eocene hyperthermals occurred during orbits with a combination of high eccentricity and high obliquity. Corresponding climate–ecosystem–soil simulations accounting for rising concentrations of background greenhouse gases8 and orbital forcing show that the magnitude and timing of the PETM and subsequent hyperthermals can be explained by the orbitally triggered decomposition of soil organic carbon in circum-Arctic and Antarctic terrestrial permafrost. This massive carbon reservoir had the potential to repeatedly release thousands of petagrams (1015 grams) of carbon to the atmosphere–ocean system, once a long-term warming threshold had been reached just before the PETM. Replenishment of permafrost soil carbon stocks following peak warming probably contributed to the rapid recovery from each event9, while providing a sensitive carbon reservoir for the next hyperthermal10. As background temperatures continued to rise following the PETM, the areal extent of permafrost steadily declined, resulting in an incrementally smaller available carbon pool and smaller hyperthermals at each successive orbital forcing maximum. A mechanism linking Earth’s orbital properties with release of soil carbon from permafrost provides a unifying model accounting for the salient features of the hyperthermals.

At a glance


  1. Orbital phasing of Early Eocene hyperthermals.
    Figure 1: Orbital phasing of Early Eocene hyperthermals.

    a, b, The Cenozoic record of benthic oxygen (a) and carbon (b) isotope values31 shows the late Palaeocene–early Eocene gradual warming trend (shaded blue to red) that culminates at the Early Eocene Climatic Optimum. Positions of hyperthermals PETM (ETM1), ETM2 and ETM3 are shown. Panels above show the shaded area in more detail. c, Major early Eocene hyperthermals recorded in the bulk carbon isotope record from Contessa Road (blue line) relative to the benthic carbon isotope stack in ref. 31 (dots and pink line). d, CaCO3 record from the Contessa Road section (grey line) and MTM reconstruction of individual components exceeding the 99% confidence level (green line). e, Envelope (Hilbert transform) of the obliquity parameter from the La04 astronomical solution18. f, The long-term (405kyr) modulation component of eccentricity from La0418. g, The very long-term (~2.4Myr) modulation component of eccentricity from La0418. The astrochronological age of PETM (ETM1), ETM2 and ETM3 (grey bands) is based on Contessa Road. The inferred positions of other minor dissolution events and carbon isotope excursions (CIE; dashed lines), including F, G, H2, I1, I2 and the two unnamed events between 54.6 and 54.8Myr ago16, 17, 19, are also shown. All major events (ETMs) occur at maximum values of obliquity and minimal CaCO3 values, and mimic the long-term modulation of obliquity. h, Evolutive spectrum of combined mean summer insolation (CMSI) at high latitudes shows intervals of maximum power in the higher frequency components (precession and obliquity) across hyperthermals. i, The CMSI series obtained by summing mean summer insolation at 65°N (21 June to 21 September) and 75°S (21 December to 21 March). The 65°N insolation values are doubled to account for the larger land area of the Northern Hemisphere (see Supplementary Information).

  2. Climate-biome and permafrost simulations in response to increasing background GHG levels and orbital forcing.
    Figure 2: Climate-biome and permafrost simulations in response to increasing background GHG levels and orbital forcing.

    Left, simulated biomes (corresponding vegetation types are shown below); right, permafrost (blue). CO2 concentrations in equivalent parts per million by volume (e.p.p.m.v.) are shown at the top left of each panel and total permafrost area is shown at top right. Panels ac represent a scenario of gradually increasing background GHG levels, leading to experiment 2 (Table 1) at 900e.p.p.m.v. CO2 (a), initial permafrost thaw and carbon release triggered by high-obliquity and high-eccentricity orbital forcing (b; corresponds to experiment 5 in Table 1), and enough carbon mobilized to increase GHG concentrations above 2,680e.p.p.m.v. CO2, causing 6°C warming (c; corresponds to experiment 7 in Table 1).


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Author information


  1. Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01002, USA

    • Robert M. DeConto &
    • David Tracy
  2. Earth, Life, and Environmental Sciences Department, University of Urbino, 61029 Urbino, Italy

    • Simone Galeotti
  3. Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA

    • Mark Pagani
  4. National Snow and Ice Data Center, Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Boulder, Colorado 80309, USA

    • Kevin Schaefer &
    • Tingjun Zhang
  5. Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA

    • David Pollard
  6. Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK

    • David J. Beerling
  7. Ministry of Education Key Laboratory of West China's Environmental System, Lanzhou University, 222 Tianshuinanlu, Lanzhou, Gansu 730000, China

    • Tingjun Zhang


R.M.D. conceived the permafrost–hyperthermal model with input from S.G., M.P., D.T., D.P. and D.J.B. S.G. developed the cyclostratigraphic framework and performed MTM and SSA analyses. R.M.D., D.T. and D.P. designed the numerical modelling scheme and D.T. analysed the GCM results. D.J.B. and R.M.D. developed the changing GHG concentration scenarios for the model simulations. K.S. and T.Z. refined the carbon calculations. R.M.D. was the primary author and all co-authors contributed to the writing and response to reviewers.

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Supplementary information

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  1. Supplementary Information (1.9M)

    This file contains Supplementary Methods, Supplementary Figures 1-13, Supplementary Table 1 and additional references. This file was replaced on 22 August 2012, as Supplementary Table 1 contained incorrect data – see the corrigendum 11424 linked to the this paper for details.

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