Three dimensional coupled model approaches to terrestrial methane cycling during Paleogene greenhouse climates

Abstract

The biogeochemical cycling of methane presents a major challenge for paleoclimate scientists, as although this critical greenhouse gas (GHG) has the forcing potential to drive significant changes in the Earth System, there are no proxy methods for reconstructing it’s ancient atmospheric concentration. This is especially important as biogenic methane emissions are controlled by environmental conditions such as temperature and precipitation, and so methane has significant power as a positive or negative feedback on global climate change. Understanding how methane emissions and cycling acted in the high pCO2 greenhouse worlds of the Paleogene potentially bridges the gap between our understanding of other, better (although arguably still poorly-) constrained GHGs and global temperature. The recent application of advanced three dimensional global modelling strategies to the problem of Eocene trace GHG concentrations has begun to show how important these may be in high-CO2 worlds (Valdes et al, 2005; Singarayer et al, 2011; Beerling et al, 2011), suggesting that as much as 2.7 °C of global warming may be contributed. This approach couples the unified Hadley Centre climate model (HadCM3L) with Sheffield Dynamic Global Vegetation Model (SDGVM; Beerling et al, 1997) to simulate trace gas emissions, and the atmospheric chemistry model STOCHEM to simulate the concentration of methane and ozone in the troposphere Here we present early results of a project to extend and develop the results of these earlier works, with revised and improved biogeochemistry and vegetation models, and broader consideration of boundary and initial conditions. These new results will be compared to indirect proxy methods to determine Paleogene methane emissions using lipid biomarker carbon isotopes in Eocene wetland settings (preserved as lignites) to probe past methanogenic and methanotrophic populations and constrain ancient methane emissions in high-CO2 worlds.