In recent years, U.S. greenhouse gas (GHG) emissions appear to have fallen modestly, a development tied to rising reliance on natural gas. Yet there is new evidence that U.S. methane emissions are higher than was previously understood, per a new report from RealClimate:
Miller et al (2013) combine measurements of methane concentrations in various locations through time with model reconstructions of wind fields, and “invert” the information to estimate how much methane was released to the air as it blew over the land. This is a well-established methodology, pushed to constrain US anthropogenic emissions by including measurements from aircraft and communications towers in addition to the ever-invaluable NOAA flask sample network, and incorporating socioeconomic and industrial data. The US appears to be emitting 50-70% more methane than the EPA thought we were, based on “bottom up” accounting (adding up all the known sources).
Is this bad news for global warming?
Not really, because the one real hard fact that we know about atmospheric methane is that its concentration isn’t rising very quickly. Methane is a short-lived gas in the atmosphere, so to make it rise, the emission flux has to continually increase. This is in contrast to CO2, which accumulates in the atmosphere / ocean system, meaning that steady (non-rising) emissions still lead to a rising atmospheric concentration. There is enough uncertainty in the methane budget that tweaks of a few percent here and there don’t upset the apple cart. Since the methane concentration wasn’t rising all that much, its sources, uncertain as they are, have been mostly balanced by sinks, also uncertain. If anything, the paper is good news for people concerned about global warming, because it gives us something to fix.
RealClimate has more on methane emissions from other regions, including on the notion that there is an “Arctic methane bomb” waiting to go off as the Siberian Arctic melts.