“Agricultural and wetland emissions” from the planet’s tropical areas, not oil and gas activities in the United States, are more than likely responsible for a post-2007 global increase in methane levels, according to the National Oceanic and Atmospheric Administration’s Climate.gov.
But regulating or mitigating those methane sources could be difficult or impossible.
“Both of the likely contenders for the recent increase in emissions could be tricky to mitigate,” wrote Climate.gov’s Rebecca Lindsey and Michon Scott.
“In developing countries with burgeoning populations, methane control could wind up pitted against the need to expand food production.
If natural wetlands are the main source of the increase, control may not even be possible,” the authors wrote.
According to Climate.gov, following a 1999 to 2006 global methane plateau scientists attempted to explain the rise of methane from 2007 to present, first taking a look at fossil fuel production.
Instead of finding evidence that supported a fossil fuel-based elevation in methane levels, scientists discovered that a rare isotope, carbon-13, associated with oil and gas production had dropped “significantly” over the same time frame.
“That [isotope] drop casts doubt on one of the first explanations experts considered for the post-2007 rise: an increase in methane emitted from fossil fuels, including “fugitive” methane gas escaping during oil and natural gas drilling,” wrote Lindsey and Scott.
“Instead, the chemical fingerprints point toward agricultural and wetland emissions from the tropics,” they continued.
But a regulatory “impasse” on agricultural and wetland mitigation, according to Lindsey and Scott, “might intensify the need to control emissions from other sources, including fossil fuels.”
“If controlling methane emissions remains part of U.S. climate and air quality policy, NOAA research will help policy makers figure out where to start,” the authors continued.
Stefan Schwietzke, a researcher and methane expert at the Cooperative Institute for Research in Environmental Sciences, a collaborative effort between NOAA and the University of Colorado Boulder, told Climate.gov that attempts to link rising methane levels with the expansion of hydraulic fracturing in the United States over the past decade have produced results that are “counter-intuitive.”
In an email to Climate.gov, Schwietzke explained that his research showed that while methane was rising, the percentage of the methane increase due to fossil fuel production laden with rare, carbon-13 isotopes is falling across all parts of the globe.
“The drop seems to rule out fossil fuel emissions, wildfires, or biomass cook stoves as the reason for the post-2007 methane surge.
All those sources of methane, to a greater or lesser extent, are enriched in carbon-13, not depleted,” wrote Lindsey and Scott, saying that, “It’s a counterintuitive finding: methane from fossil fuels is higher than we thought, but it seems to be making up a smaller share of total global emissions.”
With those sources ruled out, Schwietzke wrote, “The decline in the 13-C isotope of methane in the atmosphere indicates that microbial sources must have an increasing share of total methane emissions globally.”
Ed Dlugokencky, a research chemist with NOAA’s Earth System Research Center, says that while the biogenic or microbial thesis is strong, the exact source is unclear, telling Climate.gov, “it seems like methane emissions are increasing most in the tropics and mid-latitudes of the Northern Hemisphere, and we have some ideas why, but no definite answers.”
According to research performed on behalf of NOAA by the University of Colorado Boulder’s Institute of Arctic and Alpine Research, the carbon isotope findings “are consistent with an increase in microbial emissions from both natural wetlands and agricultural sources.”
The report lists rice farming and the associated flooding of rice cultivation and livestock emissions as the possible agricultural sources of methane.
“With respect to geography, both sources are plausible. The world’s largest population of ruminant livestock is in India (Northern Hemisphere tropics), and other tropical countries in Africa and South America have large populations as well,” wrote Lindsey and Scott.
“Most rice cultivation takes place in the Northern tropics, in India, China, and Southeast Asia. The tropics of both hemispheres are home to the world’s largest wetlands.”
Euan Nisbet, a methane expert at the University of London, disagrees. Natural wetlands – not agricultural sources – are responsible for most of the global methane growth since 2007.
Nisbet told Climate.gov that the evidence points away from agricultural sources and towards natural wetlands responding to rainfall increases associated with climate factors like La Niña, which feature wetter climate patterns in the tropics.
Dlugokencky agreed with Nisbet, but told Climate.gov via email that the large drop in carbon-13 related methane emissions called for an explanation that went beyond natural wetland sourcing, and that “there is likely a contribution from agricultural sources, too.”
