Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 16, 3099-3126, 2016
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
09 Mar 2016
Atmospheric methane evolution the last 40 years
Stig B. Dalsøren1, Cathrine L. Myhre2, Gunnar Myhre1, Angel J. Gomez-Pelaez3, Ole A. Søvde1, Ivar S. A. Isaksen1,4, Ray F. Weiss5, and Christina M. Harth5 1CICERO – Center for International Climate and Environmental Research Oslo, Oslo, Norway
2NILU – Norwegian Institute for Air Research, Kjeller, Norway
3Izaña Atmospheric Research Center (IARC), Meteorological State Agency of Spain (AEMET), Izaña, Spain
4University of Oslo, Department of Geosciences, Oslo, Norway
5Scripps Institution of Oceanography University of California, San Diego La Jolla, California, USA
Abstract. Observations at surface sites show an increase in global mean surface methane (CH4) of about 180 parts per billion (ppb) (above 10 %) over the period 1984–2012. Over this period there are large fluctuations in the annual growth rate. In this work, we investigate the atmospheric CH4 evolution over the period 1970–2012 with the Oslo CTM3 global chemical transport model (CTM) in a bottom-up approach. We thoroughly assess data from surface measurement sites in international networks and select a subset suited for comparisons with the output from the CTM. We compare model results and observations to understand causes for both long-term trends and short-term variations. Employing Oslo CTM3 we are able to reproduce the seasonal and year-to-year variations and shifts between years with consecutive growth and stagnation, both at global and regional scales. The overall CH4 trend over the period is reproduced, but for some periods the model fails to reproduce the strength of the growth. The model overestimates the observed growth after 2006 in all regions. This seems to be explained by an overly strong increase in anthropogenic emissions in Asia, having global impact. Our findings confirm other studies questioning the timing or strength of the emission changes in Asia in the EDGAR v4.2 emission inventory over recent decades. The evolution of CH4 is not only controlled by changes in sources, but also by changes in the chemical loss in the atmosphere and soil uptake. The atmospheric CH4 lifetime is an indicator of the CH4 loss. In our simulations, the atmospheric CH4 lifetime decreases by more than 8 % from 1970 to 2012, a significant reduction of the residence time of this important greenhouse gas. Changes in CO and NOx emissions, specific humidity, and ozone column drive most of this, and we provide simple prognostic equations for the relations between those and the CH4 lifetime. The reduced lifetime results in substantial growth in the chemical CH4 loss (relative to its burden) and dampens the CH4 growth.

Citation: Dalsøren, S. B., Myhre, C. L., Myhre, G., Gomez-Pelaez, A. J., Søvde, O. A., Isaksen, I. S. A., Weiss, R. F., and Harth, C. M.: Atmospheric methane evolution the last 40 years, Atmos. Chem. Phys., 16, 3099-3126, doi:10.5194/acp-16-3099-2016, 2016.
Publications Copernicus
Short summary
Methane is a key greenhouse gas. Observations at surface sites show a more than 10 % increase over the period 1984–2012. Using an atmospheric model we calculate a growth in the atmospheric chemical methane loss the last decades. Without this, the rise in atmospheric methane would have been even higher. The model reproduces trends and short-term variations in observation data. However, some discrepancies in model performance question the accuracy in estimates of emission increases in Asia.
Methane is a key greenhouse gas. Observations at surface sites show a more than 10 % increase...