The new findings based on satellite data from 2004 provide the clearest picture yet of methane emissions over the entire U.S. They show human activities released more of the gas into the atmosphere than previously thought and the sources of these emissions could be much different than government estimates.
The Environmental Protection Agency estimates that, in 2004, 28.3 megaton's (31 million tons) of methane were released into the atmosphere by human activities in the United States. The EPA estimates that 8.8 megatons (9.7 million tons) of methane came from livestock while 9.0 megatons (9.9 million tons) came from the oil and gas industries.
These EPA estimates are based on a “bottom-up” approach. The EPA collects data about greenhouse gases through national energy data, information on agricultural activities, and a data reporting program that requires industries to submit their annual emissions of methane and other gases. The EPA estimates the amount of methane in the atmosphere from this data, which includes information about the amount of methane each cow would produce or how much natural gas is sold, said Wecht.
The satellite tool used in the new study directly measured the amount of gas released into the atmosphere, which is more accurate than estimating methane emissions using the “bottom-up” approach, according to the paper.
Satellite observations of atmospheric methane provide a resource for constraining emissions, as first demonstrated by Bergamaschi et al. . Satellites deliver dense spatial coverage unachievable by surface networks or aircraft campaigns, albeit with lower precision. Methane has been retrieved from nadir satellite measurements of solar backscatter in the short-wave infrared (SWIR) and terrestrial radiation in the thermal infrared (TIR). SWIR retrievals are available from Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) for 2003–2012 [Frankenberg et al., 2011] and Greenhouse Gases Observing Satellite (GOSAT) for 2009 to present [Parker et al., 2011; Schepers et al., 2012]. TIR retrievals are available from Atmospheric Infrared Sounder for 2002 to present [Xiong et al., 2008], Tropospheric Emission Spectrometer for 2004–2011 [Worden et al., 2012], and Infrared Atmospheric Sounding Interferometer for 2007 to present [Xiong et al., 2013; Crevoisier et al., 2013]. SWIR retrievals provide total atmospheric columns. TIR retrievals provide vertical profiles but with low sensitivity to the lower troposphere due to lack of thermal contrast, and this limits their value for detecting regional sources [Wecht et al., 2012]. SCIAMACHY had full global coverage with a 6 day return time. Current coverage by GOSAT is much sparser. Instrument degradation limited the value of the SCIAMACHY data after 2005 [Frankenberg et al., 2011]. The Topospheric Monitoring Instrument to be launched in 2015 will provide SWIR methane data with global daily coverage and 7 × 7 km2 nadir resolution [Veefkind et al., 2012].
Not all scientists agree that methane emissions from livestock are easily measured or should be curtailed as a way to combat climate change. Agricultural methane emissions are difficult to measure and highly uncertain because livestock operations are so widespread and their emissions are often intermittent, said Raymond Pierrehumbert, a geophysical sciences professor at the University of Chicago. Cutting methane emissions regardless of their source is pointless, Pierrehumbert said.
“As for methane control strategy, the short lifetime of methane in the atmosphere means that the harm it does to the climate is reversible, so we can defer action until later after we’ve gotten CO2 under control,” he said. “CO2 action can’t be deferred without causing irreversible harm.”
However, Wecht said that methane emissions have accounted for roughly half the warming as CO2 emissions since the industrial revolution, and slashing methane emissions now could buy humanity time to figure out how to more effectively cut CO2 emissions.
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