Variations in global methane sources and sinks during 1910–2010

Apurna Ghosh ,T Nakazawa,P J Fraser,D M Etheridge,C M Trudinger,P B Krummel,J B Miller,L P Steele,R L Langenfelds,Kentaro Ishijima ,Edward J Dlugokencky ,Bruce H Vaughn ,Satoshi Sugawara ,Kenji Kawamura ,Tazu Saeki ,James W C White ,Prabir K Patra ,Shuji Aoki ,Taku Umezawa ,Akihiko Ito

doi:10.5194/acp-15-2595-2015

Abstract

Abstract. Atmospheric methane (CH4) increased from ~900 ppb (parts per billion, or nanomoles per mole of dry air) in 1900 to ~1800 ppb in 2010 at a rate unprecedented in any observational records. However, the contributions of the various methane sources and sinks to the CH4 increase are poorly understood. Here we use initial emissions from bottom-up inventories for anthropogenic sources, emissions from wetlands and rice paddies simulated by a~terrestrial biogeochemical model, and an atmospheric general circulation model (AGCM)-based chemistry-transport model (i.e. ACTM) to simulate atmospheric CH4 concentrations for 1910–2010. The ACTM simulations are compared with the CH4 concentration records reconstructed from Antarctic and Arctic ice cores and firn air samples, and from direct measurements since the 1980s at multiple sites around the globe. The differences between ACTM simulations and observed CH4 concentrations are minimized to optimize the global total emissions using a mass balance calculation. During 1910–2010, the global total CH4 emission doubled from ~290 to ~580 Tg yr−1. Compared to optimized emission, the bottom-up emission data set underestimates the rate of change of global total CH4 emissions by ~30% during the high growth period of 1940–1990, while it overestimates by ~380% during the low growth period of 1990–2010. Further, using the CH4 stable carbon isotopic data (δ13C), we attribute the emission increase during 1940–1990 primarily to enhancement of biomass burning. The total lifetime of CH4 shortened from 9.4 yr during 1910–1919 to 9 yr during 2000–2009 by the combined effect of the increasing abundance of atomic chlorine radicals (Cl) and increases in average air temperature. We show that changes of CH4 loss rate due to increased tropospheric air temperature and CH4 loss due to Cl in the stratosphere are important sources of uncertainty to more accurately estimate the global CH4 budget from δ13C observations.

Highlights

Methane (CH4), the second most important anthropogenic greenhouse gas, plays an important role in the chemical and radiative balances in the Earth’s atmosphere
We use combined emissions from bottom-up inventories, emissions from wetlands and rice paddies simulated by a terrestrial biogeochemical model, and a 3-dimensional chemistry-transport model to simulate atmospheric CH4 for the past 100 years (1910–2010)
The initial ACTM simulation using Eini (Fig. 2) underestimates the growth rate by 0.6 ppb yr−1 (4.8 ppb yr−1) for the period of slow growth during 1910–1950 (1950–1990), and it fails to capture the slowdown of the observed CH4 growth rate during the 1990s

Summary

Introduction

Methane (CH4), the second most important anthropogenic greenhouse gas, plays an important role in the chemical and radiative balances in the Earth’s atmosphere. Due to its main removal by reaction with hydroxyl (OH) radical, which is a major atmospheric oxidant, CH4 actively participates in tropospheric air-pollution chemistry. A. Ghosh et al.: Variations in global methane sources and sinks during 1910–2010. 1984) and is the primary sink for chlorine radicals (Cicerone and Oremland, 1988). The global warming potential (GWP) of CH4 is 28 over a time horizon of 100 years (Myhre et al, 2013). Methane is released into the atmosphere from both anthropogenic and natural sources (Patra et al., 2011; Kirschke et al, 2013; and references therein).

Methods

Results

Conclusion