"Enhanced chemistry-climate feedbacks in past greenhouse worlds," by David J. Beerling et al., PNAS, 108 (24) (2011); doi: 10.1073/pnas.1102409108

Proceedings of the National Academy of Sciences, Vol. 108, No. 24, pp. 9770-9775 (June 14, 2011), published online before print May 31, 2011; doi: 10.1073/pnas.1102409108

Enhanced chemistry-climate feedbacks in past greenhouse worlds

1. David J. Beerling^a,*, 
2. Andrew Fox^a,², 
3. David S. Stevenson^b,
4. and
5.  
6. Paul J. Valdes^c

1. ^aDepartment of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, United Kingdom;
2. ^bSchool of GeoSciences, University of Edinburgh, Edinburgh EH9 3JN, United Kingdom; and
3. ^cDepartment of Geographical Sciences, University of Bristol, Bristol BS8 1SS, United Kingdom

1. Edited by Ralph J. Cicerone, National Academy of Sciences, Washington, DC, and approved April 26, 2011 (received for review February 11, 2011).

Abstract

Trace greenhouse gases are a fundamentally important component of Earth’s global climate system sensitive to global change. However, their concentration in the pre-Pleistocene atmosphere during past warm greenhouse climates is highly uncertain because we lack suitable geochemical or biological proxies. This long-standing issue hinders assessment of their contribution to past global warmth and the equilibrium climate sensitivity of the Earth system (E_ss) to CO₂. Here we report results from a series of three-dimensional Earth system modeling simulations indicating that the greenhouse worlds of the early Eocene (55 Ma) and late Cretaceous (90 Ma) maintained high concentrations of methane, tropospheric ozone, and nitrous oxide. Modeled methane concentrations were four- to fivefold higher than the preindustrial value typically adopted in modeling investigations of these intervals, even after accounting for the possible high CO₂-suppression of biogenic isoprene emissions on hydroxyl radical abundance. Higher concentrations of trace greenhouse gases exerted marked planetary heating (> 2 K), amplified in the high latitudes (> 6 K) by lower surface albedo feedbacks, and increased E_ss in the Eocene by 1 K. Our analyses indicate the requirement for including non-CO₂ greenhouse gases in model-based E_ss estimates for comparison with empirical paleoclimate assessments, and point to chemistry-climate feedbacks as possible amplifiers of climate sensitivity in the Anthropocene.

• biogenic aerosols
 
• coals
 
• stratosphere-troposphere exchange

*Correspondence e-mail :d.j.beerling@sheffield.ac.uk

http://www.pnas.org/content/108/24/9770.abstract