Atmos. Chem. Phys. Discuss., 12, 22945-23005, 2012
© Author(s) 2012. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Analysis of present day and future OH and methane lifetime in the ACCMIP simulations
A. Voulgarakis1,2, V. Naik3, J.-F. Lamarque4, D. T. Shindell1, P. J. Young5,6, M. J. Prather7, O. Wild8, R. D. Field1,9, D. Bergmann10, P. Cameron-Smith10, I. Cionni11, W. J. Collins12, S. B. Dalsøren13, R. M. Doherty14, V. Eyring15, G. Faluvegi1, G. A. Folberth12, L. W. Horowitz16, B. Josse17, I. A. McKenzie14, T. Nagashima18, D. A. Plummer19, M. Righi15, S. T. Rumbold12, D. S. Stevenson14, S. A. Strode20, K. Sudo18, S. Szopa21, and G. Zeng22
1NASA Goddard Institute for Space Studies and Columbia Earth Institute, New York, NY, USA
2Department of Physics, Imperial College, London, UK
3UCAR/NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA
4National Center for Atmospheric Research (NCAR), Boulder, CO USA
5Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
6NOAA Earth System Research Laboratory, Boulder, CO, USA
7University of California at Irvine, CA, USA
8Lancaster University, Lancaster, UK
9Department of Applied Physics and Applied Mathematics, Columbia University, USA
10Lawrence Livermore National Laboratory, CA, USA
11Agenzia nazionale per le nuove tecnologie, l'energia e lo sviluppo economico sostenibile (ENEA), Bologna, Italy
12Met Office Hadley Centre, Exeter,~UK
13CICERO, Center for International Climate and Environmental Research Oslo, Oslo, Norway
14University of Edinburgh, Edinburgh, UK
15Deutsches Zentrum für Luft- und Raumfahrt (DLR), Germany
16NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA
17GAME/CNRM, Météo-France, CNRS – Centre National de Recherches Météorologiques, Toulouse, France
18National Institute for Environmental Studies, Tsukuba-shi, Ibaraki, Japan
19Environment Canada, Victoria, BC, Canada
20NASA Goddard Space Flight Center and Universities Space Research Association, Greenbelt, MD, USA
21Laboratoire des Sciences du Climat et de l'Environnement, LSCE/CEA/CNRS/UVSQ/IPSL, France
22National Institute of Water and Atmospheric Research, Lauder, New Zealand

Abstract. Results from simulations performed for the Atmospheric Chemistry and Climate Modeling Intercomparison Project (ACCMIP) are analysed to examine how OH and methane lifetime may change from present-day to the future, under different climate and emissions scenarios. Present-day (2000) mean tropospheric chemical lifetime derived from the ACCMIP multi-model mean is 9.8 ± 1.6 yr, lower than a recent observationally-based estimate, but with a similar range to previous multi-model estimates. Future model projections are based on the four Representative Concentration Pathways (RCPs), and the results also exhibit a~large range. Decreases in global methane lifetime of 4.5 ± 9.1% are simulated for the scenario with lowest radiative forcing by 2100 (RCP 2.6), while increases of 8.5 ± 10.4% are simulated for the scenario with highest radiative forcing (RCP 8.5). In this scenario, the key driver of the evolution of OH and methane lifetime is methane itself, since its concentration more than doubles by 2100, and it consumes much of the OH that exists in the troposphere. Stratospheric ozone recovery, which drives tropospheric OH decreases through photolysis modifications, also plays a~partial role. In the other scenarios, where methane changes are less drastic, the interplay between various competing drivers leads to smaller and more diverse OH and methane lifetime responses, which are difficult to attribute. For all scenarios, regional OH changes are even more variable, with the most robust feature being the large decreases over the remote oceans in RCP 8.5. Through a~regression analysis, we suggest that differences in emissions of non-methane volatile organic compounds and in the simulation of photolysis rates may be the main factors causing the differences in simulated present-day OH and methane lifetime. Diversity in predicted changes between present-day and future was found to be associated more strongly with differences in modelled climate changes, specifically global temperature and humidity. Finally, through perturbation experiments we calculated an OH feedback factor (F) of 1.29 from present-day conditions (1.65 from 2100 RCP 8.5 conditions) and a~climate feedback on methane lifetime of 0.33 ± 0.13 yr K−1, on average.

Citation: Voulgarakis, A., Naik, V., Lamarque, J.-F., Shindell, D. T., Young, P. J., Prather, M. J., Wild, O., Field, R. D., Bergmann, D., Cameron-Smith, P., Cionni, I., Collins, W. J., Dalsøren, S. B., Doherty, R. M., Eyring, V., Faluvegi, G., Folberth, G. A., Horowitz, L. W., Josse, B., McKenzie, I. A., Nagashima, T., Plummer, D. A., Righi, M., Rumbold, S. T., Stevenson, D. S., Strode, S. A., Sudo, K., Szopa, S., and Zeng, G.: Analysis of present day and future OH and methane lifetime in the ACCMIP simulations, Atmos. Chem. Phys. Discuss., 12, 22945-23005, doi:10.5194/acpd-12-22945-2012, 2012.
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