While rising carbon dioxide emissions are a primary concern of those worried about climate change, emissions of methane, another potent greenhouse gas, have also risen in recent years.
Curbing these emissions would help reduce warming, but scientists have found it difficult to determine exactly how much methane comes from human sources. It's also challenging to trace the methane back to the source and find out whether it is coming from cows, coal mines or gas patches.
Now, a study published yesterday in the journal Proceedings of the National Academy of Sciences reports that U.S. methane emissions are significantly higher than U.S. EPA estimates.
The analysis, led by Scot Miller, a doctoral student in atmospheric science at Harvard University, points to underestimates in emissions from livestock and fossil fuel activities as the primary reason for the disparity.
The study also focuses attention on Texas and nearby states as a source of nearly a quarter of the country's human-related methane emissions. "We've learned that methane emissions from the south-central United States are probably a lot higher than existing estimates," Miller explained.
Miller's research finds that, in 2007 and 2008, U.S. emissions of methane from human-related sources were 33.4 teragrams of carbon equivalent per year. That number is significantly higher than EPA's methane budget, which puts U.S. emissions for 2008 at 22.1 teragrams of carbon equivalent per year.
"The results show that the emissions ... are about 1½ times the EPA estimate," said Steven Wofsy, a professor of atmospheric and environmental chemistry at Harvard and a co-author of the study.
Bottom-up versus top-down
"EPA has not yet had the opportunity to fully review the PNAS study on methane emissions; however we are encouraged that more methane emissions measurement data are now available to the public," the agency said in a statement.
The agency has also said it is interested in studies such as this one that result in different estimates of methane emissions.
The study's numbers for methane emissions are also higher than the U.S. methane emissions reported by a European-based inventory, the Emissions Database for Global Atmospheric Research (EDGAR).
Both the EDGAR and the EPA methane emissions numbers come from what are called bottom-up estimates and are calculated very differently from Miller's study. Such bottom-up estimates rely on emissions amounts that are associated with given practices or activities.
For example, a cow might be associated with a given amount of methane emissions per year. Multiply the number of cows by that quantity, and that will essentially determine the bottom-up estimate of how much methane is emitted from cows in a year.
The error in such estimates can come from uncertainties associated with the number of cows or variations in how much methane each cow emits. The same can be the case with the oil and gas industry, where differences in equipment or practices might lead to very different methane emissions.
In contrast, the method used by Miller and his colleagues, called a top-down method, uses measurements of methane in the atmosphere, taken from a national network of greenhouse gas monitoring stations and aircraft measurements conducted by the National Oceanic and Atmospheric Administration and the Department of Energy.
The researchers took those observations from 2007 and 2008, nearly 12,700 of them, and essentially ran a weather model in reverse to trace those measurements back in time and space. This allowed them to learn where the methane is coming from and what quantity is being emitted.
The strength of such a method, according to Miller, is that it provides a good measurement of how much methane is being emitted in total. However, because the measurements calculate the total amount of methane at a regional level, it is hard to pinpoint the specific source of the methane.
"The bottom up is great because it can tell us where all the cows are that are emitting methane, or where are all of the landfills that we should be concerned about," Miller said.
Harvard's Wofsy said the research team plans to conduct this same analysis for other years as well. "We have plans to essentially run it backward in time to 2004 and then forwards in time to yesterday," he said.
Texas, Okla. and Kan. in spotlight
Another surprising finding of the study was that it pointed out high levels of methane emissions in Texas, Oklahoma and Kansas, and linked those emissions to fossil fuel extraction.
"We did not expect to see methane emissions that high over Texas and Oklahoma," Miller said.
Based on emissions of hydrocarbons associated with oil and gas production that they also measured along with the methane, the researchers estimated that about 45 percent of the methane emissions in the south-central part of the country come from fossil fuel activities, he added.
Robert Harriss, a methane researcher at the Environmental Defense Fund who did not participate in the study, said the disparity between top-down and bottom-up estimates often has to do with the way in which methane leaks are measured and counted.
"The very limited current data on fugitive methane leakage also suggests that a relatively small number of oil and gas operations may be responsible for a large fraction of the methane leakage," he said.
EDF has been active in trying to quantify the amount of methane leaking from natural gas operations. If the fuel is to be used as a bridge between coal and renewable sources of energy, the amount of fugitive methane leaking from the natural gas system needs to be minimal.
David Allen, a University of Texas researcher who conducted measurements from natural gas fields to measure methane emissions, called the study an "important contribution."
Allen's study found some sources of methane from natural gas activities were larger than EPA estimates, while others were lower (ClimateWire, Sept. 17).
"Both regional measurements and analyses, as reported by Miller et al., and source specific studies, are needed to identify opportunities for emission reductions."
Robert Howarth, a researcher at Cornell University who has been critical of natural gas because of fears about fugitive emissions, released a statement saying the study showed that natural gas could not bridge the gap between coal-fired power and renewables.
"Using this new information as well as other independent studies on methane emissions published since 2011, and the latest information on the climate influence of methane compared to carbon dioxide from the latest synthesis report from the Intergovernmental Panel on Climate Change released in September of this year, it is clear that natural gas is no bridge fuel," Howarth wrote.
Wofsy, however, said that although the research contributed to the body of knowledge on methane emissions, it was not meant to address the issue raised by Howarth.
"I think we contribute a lot to the discussion, but we do not really have anything to say about whether or not natural gas is or is not a good bridge fuel. It shouldn't be presented that way," he said.
Monitoring network lacks funding
NOAA scientists whose observations were used in the study said the research showed the importance of the set of towers and aircraft measurements that monitor greenhouse gas emissions.
"It really does show what we can do with these larger networks," said Colm Sweeney, a research scientist at the University of Colorado, Boulder, who runs NOAA's carbon cycle aircraft program at its Earth System Research Laboratory. Sweeney was also a co-author on the paper.
Funding cuts in the NOAA budget, however, have led to decreases in the number of greenhouse gas sampling flights Sweeney's group has been able to do in recent years, he said.
As a result, the team has turned to other projects funded by outside groups, such as measuring methane emissions from natural gas fields in Utah (ClimateWire, Aug. 7).
And currently there are about 10 tall towers that monitor greenhouse gases, whereas the original plan for the measurement network called for around 35, said Arlyn Andrews, a NOAA researcher at the Earth System Research Laboratory who is responsible for the tall tower network and was a co-author on the paper.
With more towers and observations, researchers would be better able to pinpoint emissions sources.
"If we want to start being area- and source-specific, then we need to dramatically increase our network," Sweeney said. "This study is a great example of how we can use a larger network to get at individual sources."