Abstract
Atmospheric CH4 is arguably the most interesting of the anthropogenically influenced, long-lived greenhouse gases. It has a diverse suite of sources, each presenting its own challenges in quantifying emissions, and while its main sink, atmospheric oxidation initiated by reaction with hydroxyl radical (OH), is well-known, determining the magnitude and trend in this and other smaller sinks remains challenging. Here, we provide an overview of the state of knowledge of the dynamic atmospheric CH4 budget of sources and sinks determined from measurements of CH4 and δ13CCH4 in air samples collected predominantly at background air sampling sites. While nearly four decades of direct measurements provide a strong foundation of understanding, large uncertainties in some aspects of the global CH4 budget still remain. More complete understanding of the global CH4 budget requires significantly more observations, not just of CH4 itself, but other parameters to better constrain key, but still uncertain, processes like wetlands and sinks.This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 1)'.
Highlights
Atmospheric CH4 absorbs terrestrial infrared radiation in a band at approximately 7.6 μm, so it affects Earth’s radiation balance and is a primary driver of the human impact on climate
Changes to the global CH4 budget are recorded in the atmosphere in CH4 abundance and δ13CCH4 spatial and temporal gradients
High-quality, calibrated observations of atmospheric CH4 and its stable isotopic composition are crucial for decreasing uncertainty in quantifying the evolving global CH4 budget, yet the world is still greatly under-sampled
Summary
Atmospheric CH4 absorbs terrestrial infrared radiation in a band at approximately 7.6 μm, so it affects Earth’s radiation balance and is a primary driver of the human impact on climate. From measurements of air extracted from ice cores, we know that δ13CCH4 was increasing for approximately 200 yr [30] before the recent decline began This suggests a rather significant change in the global budget of CH4 sources and sinks that will be explored below, the timing of the change in emissions is difficult to match directly with the δ13CCH4 observations because of the slow response of δ13CCH4 to changes in CH4 emissions or average source signatures [31]. The scenarios most consistent with both CH4 and δ13CCH4 observations and most plausible in regard to other evidence involve increased emissions from microbial sources (agriculture and wetlands) in the tropics with a small contribution from fossil sources [39,42,43,44], with a small decrease in biomass burning to maintain isotope mass balance
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