Global methane emissions have risen and fallen several times since the 1980s, Turner said, but they’ve been rising continuously since 2007. “The causes for this renewed growth are currently unknown,” he said,
In the US, the government tally of the country’s annual human-caused methane emissions between 2002 and 2012 shows that emissions have been about 29 million metric tons annually, without any significant trends up or down. Research by Turner’s team, however, showed that emissions ranged from about 39 million tons to about 52 million tons during that period. The team based its findings on satellite data.
The Harvard study, published last week in the journal Geophysical Research Letters, adds to mounting research that shows that the government’sdata on methane emissions is not consistent with observations made by universities and other institutions.
The official tally, taking what is known as a “bottom-up” approach, calculates methane emissions based on expected leak rates at oil and gas well sites, not actual measurements. The Harvard researchers used a “top-down” method, calculating emissions based on actual satellite measurements, showing that US methane emissions are far greater than those estimated by the government.
Turner said he and other researchers are working with the EPA to reconcile those differences.
“Both the bottom-up and top-down communities are trying to understand the sources of methane, but the different methods have different strengths, and we are working on making them more compatible,” he said.
EPA spokeswoman Enesta Jones said the Harvard study adds to a wide body of new research about how to best quantify methane emissions. The EPA is reviewing those studies, she said.
With the US responsible for as much as 60% of global methane emissions growth, it’s critical that the country reduce natural gas use as quickly as possible, said Robert Howarth, a Cornell University ecologist and methane researcher who is unaffiliated with the Harvard study. Failing to do so could jeopardise commitments made as part of the Paris climate agreement’s goal of keeping global warming to below 2C.
“There is simply no way to do that by reducing carbon dioxide emissions alone because of lags in the climate system,” he said. “Even with major carbon dioxide emission reductions starting now, the planet would reach 1.5C in 12 years and 2C in 35 years. But the planet responds much more rapidly to methane, so a reduction in methane emissions now would slow the rate of global warming immediately.”
Howarth said the Turner team’s study is consistent with his own research.
“I believe the US probably is responsible for this much of an increase in global methane emissions,” Howarth said. “And, the increase almost certainly must be coming from the fracking and from the increase in use of natural gas.”
Excerpt from The Guardian