The Environmental Protection Agency is drastically undervaluing the potency of methane as a greenhouse gas when the agency compares methane’s climate impact to that of carbon dioxide, a new study concludes.
The EPA’s climate accounting for methane is “arbitrary and unjustified” and three times too low to meet the goals set in the Paris climate agreement, the research report, published Wednesday in the journal Environmental Research Letters, found.
The report proposes a new method of accounting that places greater emphasis on the potential for cuts in methane and other short-lived greenhouse gasses to help limit warming to 1.5 degrees Celsius above pre-industrial levels.
“If you want to keep the world from passing the 1.5 degrees C threshold, you’ll want to pay more attention to methane than we have so far,” said Rob Jackson, an earth system science professor at Stanford University and a co-author of the study.
Methane is the second-leading contributor to climate change after carbon dioxide but is a far more potent greenhouse gas.Unlike carbon dioxide, which can remain in the atmosphere for centuries, methane is a “short-lived climate pollutant” that stays in the atmosphere for approximately 12 years.
The vastly different atmospheric lifetimes of methane and carbon dioxide make comparing the climate impact of the two gasses difficult.
The EPA, following guidance by the United Nations Framework Convention on Climate Change (UNFCCC), quantifies how equal amounts of different climate pollutants like carbon dioxide, methane, and nitrous oxide contribute to warming the planet over a 100-year period.
The comparison allows government agencies and the private sector to weigh the relative impacts of different greenhouse gasses and then determine how much emphasis to place on reducing their emissions. However, the use of the 100-year yardstick results in a greater emphasis on pollutants like carbon dioxide that remain in the atmosphere for a relatively long time and downplays the contribution of short-lived pollutants like methane, even though they do far more, on a metric ton-for-metric ton basis, to warm the atmosphere in the short-term.
Sam Abernethy, a Stanford doctoral student and the lead author of the study, said he became interested in the “global warming potential” of methane after looking into why the United States and other countries use the 100-year time frame.
Abernethy found that the period of 100 years was an “arbitrary and unjustified” choice adopted by the Kyoto Protocol, the first binding international climate agreement, in the 1990s, and used in international reporting and agreements ever since.
The 100-year measure was selected for the Kyoto agreement because it was the middle ground between two other possible time frames—20 years and 500 years—provided in early reports by the Intergovernmental Panel on Climate Change (IPCC).
“I was confused at how something so arbitrary could be underpinning so much of climate policy and how we think about different greenhouse gasses,” Abernethy said.
Over a 100-year period, methane is 28 times more potent than carbon dioxide as a greenhouse gas. However, over a 20-year period, a yardstick that climate scientists have previously suggested would be a more appropriate timeframe, methane is 81 times more potent than carbon dioxide.
“It’s a huge swing in how much we value methane, and therefore how many of our resources go towards mitigating it,” Abernethy said.
However, the use of either time frame remains largely arbitrary.
To determine a “justified” time frame, the Stanford researchers took the Paris climate goal of limiting warming to 1.5 degrees Celsius as a starting point, and then calculated the most appropriate time frame to meet that goal.
Based on climate models using scenarios where global warming is limited to 1.5 degrees, they determined the planet would reach 1.5 degrees of warming above pre-industrial levels in approximately 24 years.
“If that’s the case, and you’re using a 100-year frame for methane, then you’re not going to put enough value on reducing methane emissions compared to other greenhouse gasses,” Jackson said..
Over a 24 year time period methane is 75 times more potent than carbon dioxide as a greenhouse gas. This is three times higher than 25, the current value that the EPA uses for methane.
“It’s not inherently wrong,” Abernethy said of the 100-year time frame. “It’s just not aligned with our current [climate] goal.”
Jackson said that carbon dioxide remains the most important greenhouse gas. But he added that additional attention must be paid to methane if the world is to limit warming to 1.5 degrees Celsius.
Taylor Gillespie, a spokeswoman for the EPA, defended the agency’s use of a 100-year time frame and said the relative values they give to different greenhouse gasses “is separate from the choice of temperature targets.”
The agency’s use of a 100-year time frame is based on international guidelines set by the UNFCCC, which seeks to ensure uniformity between countries so that national greenhouse gas inventories reported by all nations are comparable, Gillespie said.
The UNFCCC did not respond to a request for comment about the time frame it uses for comparing the relative climate impacts of greenhouse gasses.
The UNFCCC and international agreements like the Paris climate agreement, have historically placed greater emphasis on reducing carbon dioxide to meet climate goals. However, efforts to reduce methane emissions played a leading role in the UNFCCC’s international climate conference in Glasgow in November.
In the runup to Glasgow, many environmentalists and officials in the Biden administration argued that a focus on reducing methane emissions would have a much greater impact in the short-term than the focus on carbon, given how much more potent methane is, and how much longer carbon stays in the atmosphere. Focusing on methane, they said, would give governments the ability to slow global warming in the near-term, while buying additional time to tackle long-term warming driven by carbon dioxide..
Steven Hamburg, chief scientist for the Environmental Defense Fund, said he sees a growing realization within the UNFCCC of the need to place greater emphasis on tackling methane emissions alongside carbon dioxide. What’s lacking, however, is an effort to revise the metrics used to measure the gas’s relative climate impact, he said.
A 2017 study co-authored by Hamburg and published in the journal Science called for the adoption of a dual 100-year and 20-year “Global Warming Potential” or time frame for reporting the impacts of different greenhouse gasses.
“I think there is no perfect metric,” Hamburg said of the current study and its call for a 24-year timeframe synched to limiting warming to 1.5 C. “But they’re trying to continue to draw attention to the bias that’s inherent to GWP 100. And that is very real, and very problematic.”
The EPA’s current global warming potential figure for methane is too low not only because it uses a 100-year time frame but also because the figure relies on outdated science, Hamburg said.
IPCC reports released in 2014 and 2021 placed the 100-year climate impact of methane at 28, while the EPA still relies on a 2007 IPCC report which calculated a slightly lower value of 25.
“At a minimum, they should update the numbers,” Hamburg said.
Gillespie, the EPA spokeswoman, said the agency will begin using a value of 28 for methane, a 12 percent increase in methane’s climate impact, in 2024, in line with the UNFCCC international guidelines.
