Interannual variability and trends in atmospheric methane over the western Pacific from 1994 to 2010

Abstract

We present an analysis of interannual variability (IAV) and trends in atmospheric methane (CH 4) mixing ratios over the western Pacific between 55°N and 35°S from 1994 to 2010. Observations were made by the Center for Global Environmental Research (CGER) of the National Institute for Environmental Studies (NIES), using voluntary observation ships sailing between Japan and Australia/New Zealand and between Japan and North America, sampling background maritime air quasi-monthly (∼10 times per year) with high latitudinal resolution. In addition, simulations of CH 4 were performed using NIES atmospheric transport model. A large CH 4 increase was observed in the tropics (10°N-5°S) during 1997 (between 15 ± 3 and 19 ± 3 ppb yr -1) and during 1998 for other regions (40°N-50°N: 10 ± 2-16 ± 1 ppb yr -1; 10°S-25°S: 12 ± 2-22 ± 4 ppb yr -1). The CH 4 increase leveled off from 1999 to 2006 at all latitudes. The CH 4 growth rate was enhanced in 2007 (25°N-50°N: 10 ± 1-12 ± 3 ppb yr -1; 15°S-35°S: 7 ± 1-8 ± 1 ppb yr -1) but diminished thereafter; however, a large CH 4 growth (10 ± 1-17 ± 1 ppb yr -1) was observed in 2009 over the northern tropics (0-15°N). These observations, combined with the simulation results, suggest that to explain the CH 4 increase in 2007 would require an increase in surface emissions of ∼20 ± 3 Tg-CH 4 yr -1 globally and an increase in the Northern Hemisphere (NH) of 4-7 ± 3 Tg-CH 4 yr -1 more than that in the Southern Hemisphere (SH), assuming no change in OH concentrations; alternatively, a decrease in OH concentrations of 4.5 ± 0.6%-5.5 ± 0.5% yr -1 globally would be required if we assume no change in surface emissions. Over the western Pacific, the IAV in CH 4 within the northern tropics was characterized by a high growth rate in mid-1997 and a reduced growth in 2007. The present data indicate that these events were strongly influenced by the IAV in atmospheric circulation associated with El Niño and La Niña events. Our observations captured the CH 4 anomaly in 1997 associated with forest fires in Indonesia. The IAV and trends in CH 4 as seen by our data sets capture the global features of background CH 4 levels in the northern midlatitudes and the SH, and regional features of CH 4 variations in the western tropical Pacific. Copyright 2011 by the American Geophysical Union.