Atmospheric science. Methane on the rise--again.

A factor of 2.5 increase in the global abundance of atmospheric methane (CH(4)) since 1750 contributes 0.5 Wm(-2) to total direct radiative forcing by long-lived greenhouse gases (2.77 Wm(-2) in 2009), while its role in atmospheric chemistry adds another approximately 0.2 Wm(-2) of indirect forcing. Since CH(4) has a relatively short lifetime and it is very close to a steady state, reductions in its emissions would quickly benefit climate. Sensible emission mitigation strategies require quantitative understanding of CH(4)'s budget of emissions and sinks. Atmospheric observations of CH(4) abundance and its rate of increase, combined with an estimate of the CH(4) lifetime, constrain total global CH(4) emissions to between 500 and 600 Tg CH(4) yr(-1). While total global emissions are constrained reasonably well, estimates of emissions by source sector vary by up to a factor of 2. Current observation networks are suitable to constrain emissions at large scales (e.g. global) but not at the regional to national scales necessary to verify emission reductions under emissions trading schemes. Improved constraints on the global CH(4) budget and its break down of emissions by source sector and country will come from an enhanced observation network for CH(4) abundance and its isotopic composition (δ(13)C, δD(D=(2)H) and δ(14)C). Isotopic measurements are a valuable tool in distinguishing among various sources that contribute emissions to an air parcel, once fractionation by loss processes is accounted for. Isotopic measurements are especially useful at regional scales where signals are larger. Reducing emissions from many anthropogenic source sectors is cost-effective, but these gains may be cancelled, in part, by increasing emissions related to economic development in many parts of the world. An observation network that can quantitatively assess these changing emissions, both positive and negative, is required, especially in the context of emissions trading schemes.

This study quantitatively estimates the spatial distribution of anthropogenic methane sources in the United States by combining comprehensive atmospheric methane observations, extensive spatial datasets, and a high-resolution atmospheric transport model. Results show that current inventories from the US Environmental Protection Agency (EPA) and the Emissions Database for Global Atmospheric Research underestimate methane emissions nationally by a factor of ∼1.5 and ∼1.7, respectively. Our study indicates that emissions due to ruminants and manure are up to twice the magnitude of existing inventories. In addition, the discrepancy in methane source estimates is particularly pronounced in the south-central United States, where we find total emissions are ∼2.7 times greater than in most inventories and account for 24 ± 3% of national emissions. The spatial patterns of our emission fluxes and observed methane-propane correlations indicate that fossil fuel extraction and refining are major contributors (45 ± 13%) in the south-central United States. This result suggests that regional methane emissions due to fossil fuel extraction and processing could be 4.9 ± 2.6 times larger than in EDGAR, the most comprehensive global methane inventory. These results cast doubt on the US EPA's recent decision to downscale its estimate of national natural gas emissions by 25-30%. Overall, we conclude that methane emissions associated with both the animal husbandry and fossil fuel industries have larger greenhouse gas impacts than indicated by existing inventories.