Future methane, hydroxyl, and their uncertainties: key climate and emission parameters for future predictions 1Department of Earth System Science, University of California, Irvine, CA 92697-3100, USA
11 Jan 2013
2Center for International Climate and Environmental Research (CICERO), Oslo, Norway
Received: 01 August 2012 – Published in Atmos. Chem. Phys. Discuss.: 20 August 2012 Abstract. Accurate prediction of future methane abundances following a climate scenario
requires understanding the lifetime changes driven by anthropogenic
emissions, meteorological factors, and chemistry-climate feedbacks.
Uncertainty in any of these influences or the underlying processes implies
uncertainty in future abundance and radiative forcing. We simulate methane
lifetime in three chemical transport models (CTMs) – UCI CTM, GEOS-Chem, and
Oslo CTM3 – over the period 1997–2009 and compare the models' year-to-year
variability against constraints from global methyl chloroform observations.
Using sensitivity tests, we find that temperature, water vapor, stratospheric
ozone column, biomass burning and lightning NOx are the dominant
sources of interannual changes in methane lifetime in all three models. We
also evaluate each model's response to forcings that have impacts on decadal
time scales, such as methane feedback, and anthropogenic emissions. In
general, these different CTMs show similar sensitivities to the driving
variables. We construct a parametric model that reproduces most of the
interannual variability of each CTM and use it to predict methane lifetime
from 1980 through 2100 following a specified emissions and climate scenario
(RCP 8.5). The parametric model propagates uncertainties through all steps
and provides a foundation for predicting methane abundances in any climate
scenario. Our sensitivity tests also enable a new estimate of the methane
global warming potential (GWP), accounting for stratospheric ozone effects,
including those mediated by water vapor. We estimate the 100-yr GWP to be 32,
which is 25% larger than past assessments.
Revised: 31 December 2012 – Accepted: 02 January 2013 – Published: 11 January 2013
Citation: Holmes, C. D., Prather, M. J., Søvde, O. A., and Myhre, G.: Future methane, hydroxyl, and their uncertainties: key climate and emission parameters for future predictions, Atmos. Chem. Phys., 13, 285-302, doi:10.5194/acp-13-285-2013, 2013.