Natural gas operations are leaking more of a potent greenhouse gas than previously assumed, but even so, the fossil fuel is better than coal for the climate, according to a comprehensive new study.
The study found that U.S. EPA's greenhouse gas inventory is underestimating the amount of methane emitted in the United States by about 50 percent. Some of that excess is from the natural gas sector.
The study was published yesterday in the journal Science and was funded by the Cynthia and George Mitchell Foundation, a philanthropic organization founded by the pioneer of hydraulic fracturing, the late George Mitchell.
Methane leakage from natural gas systems has caused significant concern as utilities are increasingly switching from coal to gas. The worry is that the leaks could well negate the benefits of burning a fuel that is much cleaner than coal.
The Science study finds this is not the case. Even when the scientists assumed that all the emissions unaccounted for in the EPA inventory came from the natural gas sector, the fuel still was better for the climate than coal.
Better does not mean using natural gas will solve the climate problem confronting the world.
Avoiding the worst impacts of climate change would require significantly deeper cuts in emissions than are afforded by a simple switch from coal to gas, said Doug Arent of the Joint Institute for Strategic Energy Analysis, who was a co-author of the study. Carbon capture and sequestration and the development of renewables would be key.
"Over the long term, one has to realize [natural] gas is still a greenhouse gas source," Arent said.
EPA underestimates emissions
The study analyzed more than 200 technical publications that looked into methane leakage from oil and gas operations. Leakages from all parts of the natural gas sector, including production, processing and distribution, were considered.
The scientists looked at studies that made direct measurements at individual pieces of equipment and well pads, as well as broader atmospheric studies that tracked methane across entire basins or regions. The studies were compared with EPA's greenhouse gas inventory. The inventory is released yearly and is based on estimates of leakage and counts of activity provided by companies. The latest inventory suggests that 1.5 percent of the natural gas system is leaking.
The Science study found that this is an underestimate, and methane emissions are about 50 percent higher. The results are similar to a study out of Harvard University last year (EnergyWire, Nov. 26, 2013).
The scientists could not say exactly what the leakage rate is. That's because some of the observed emissions in the studies were from oil wells (which also produce natural gas), abandoned oil and gas wells and geologic seeps, which are technically not part of the natural gas sector.
The Science paper also found that the studies with the highest methane leakage estimates are likely overstating the problem.
Part of the reason the EPA inventory is off is that it does not include "super-emitters," the study found. These are facilities that leak much more gas than is the norm and account for a major portion of the emissions recorded in atmospheric studies.
"There is a high likelihood that a large proportion of emissions are coming from a relatively small number of sites," said Steve Hambug, chief scientist at the Environmental Defense Fund. "This strongly indicates that there is a relatively straightforward capacity to reduce those emissions."
Coal vs. gas
To analyze the benefits of coal versus natural gas, the scientists reviewed an earlier EDF study (EnergyWire, April 10, 2012) and found that gas is more beneficial than coal when the leakage rate from the well head to the power plant is less than 3.2 percent.
Whether this is worth the switch is a matter of judgment, said Adam Brandt, an assistant professor in the Department of Energy Resources Engineering at Stanford University and lead author of the study. And in his opinion, even if coal and gas look roughly equal over the next 20 years, that does not give us license to burn coal, he said. That would simply shift the climate problem into the future.
"What that means is, we are talking equal impacts in the present time period, but we are shifting a much larger burden to our grandchildren," he said.