Polar amplification of Pliocene climate by elevated trace gas radiative forcing

Peter Hopcroft, Gilles Ramstein, Thomas Pugh, Stephen Hunter, Fabiola Murguia-Flores, Aurelien Quiquet, Yong Sun, Ning Tan, Paul J. Valdes

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Warm periods in Earth's history offer opportunities to understand the dynamics of the Earth system under conditions that are similar to those expected in the near future. The Middle Pliocene warm period (MPWP), from 3.3 to 3.0 My B.P, is the most recent time when atmospheric CO 2 levels were as high as today. However, climate model simulations of the Pliocene underestimate high-latitude warming that has been reconstructed from fossil pollen samples and other geological archives. One possible reason for this is that enhanced non-CO 2 trace gas radiative forcing during the Pliocene, including from methane (CH 4), has not been included in modeling. We use a suite of terrestrial biogeochemistry models forced with MPWP climate model simulations from four different climate models to produce a comprehensive reconstruction of the MPWP CH 4 cycle, including uncertainty. We simulate an atmospheric CH 4 mixing ratio of 1,000 to 1,200 ppbv, which in combination with estimates of radiative forcing from N 2O and O 3, contributes a non-CO 2 radiative forcing of 0.9 [Formula: see text] (range 0.6 to 1.1), which is 43% (range 36 to 56%) of the CO 2 radiative forcing used in MPWP climate simulations. This additional forcing would cause a global surface temperature increase of 0.6 to 1.0 °C, with amplified changes at high latitudes, improving agreement with geological evidence of Middle Pliocene climate. We conclude that natural trace gas feedbacks are critical for interpreting climate warmth during the Pliocene and potentially many other warm phases of the Cenezoic. These results also imply that using Pliocene CO 2 and temperature reconstructions alone may lead to overestimates of the fast or Charney climate sensitivity.

Original languageEnglish
Pages (from-to)23401-23407
Number of pages7
JournalNational Academy of Sciences. Proceedings
Issue number38
Early online date4 Sept 2020
Publication statusPublished - 22 Sept 2020

Bibliographical note

Funding Information:
P.O.H. is supported by a University of Birmingham Fellowship. G.R. is supported by the French project Les Enveloppes Fluides et l'Environnement "ComPreNdrE" (A2016-992936), French State Program Investissements d'Avenir (managed by Agence Nationale de la Recherche [ANR]), ANR Human Ancestors Dispersal: the role of Climate project, Grant ANR-17-CE31-0010 of the French National Research Agency. T.A.M.P. acknowledges support from the European Research Council under the European Union Horizon 2020 program (Grant 758873, TreeMort). This work was carried out using the computational facilities of the Birmingham Environment for Academic Research (http://www.bear.bham.ac.uk).We thank Linda Sohl and Mark Chandler from NASA GISS for providing model outputs and Nan Rosenbloom (National Center for Atmospheric Research) who performed the original CCSM4 simulations. We thank Sophie Szopa for informative discussions on these topics. This is paper number 48 of the Birmingham Institute of Forest Research.

Publisher Copyright:
© 2020 National Academy of Sciences. All rights reserved.

Copyright 2020 Elsevier B.V., All rights reserved.


  • Biogeochemistry
  • GCM
  • Methane
  • Pliocene
  • Trace gas
  • Wetland

ASJC Scopus subject areas

  • General


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