The Environmental Protection Agency significantly underestimates the potency of methane as a greenhouse gas when it compares the climate impact of methane to that of carbon dioxide, a new study finds.
The EPA’s climate accounting for methane is “arbitrary and unwarranted” and three times too low to meet targets set in the Paris climate accord, according to the research report, published Wednesday in the journal Environmental Research Letters. .
The report proposes a new accounting method that places greater emphasis on the reduction potential of methane and other short-lived greenhouse gases to help limit warming to 1.5 degrees Celsius above pre-industrial levels.
“If you want to keep the world from going over the 1.5C threshold, you’ll want to pay more attention to methane than we have so far,” said system science professor Rob Jackson. terrestrial at Stanford University and co-author. of the study.
Methane is the second largest contributor to climate change after carbon dioxide, but it is a much more potent greenhouse gas. Unlike carbon dioxide, which can stay in the atmosphere for centuries, methane is a “short-lived climate pollutant” that stays in the atmosphere. for about 12 years.
The very different atmospheric lifetimes of methane and carbon dioxide make it difficult to compare the climate impact of the two gases.
The EPA, following guidelines from 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 global warming of the planet over a period of 100 years.
The comparison allows government agencies and the private sector to weigh the relative impacts of different greenhouse gases, and then determine the importance to place on reducing their emissions. However, using the 100-year criterion puts more emphasis on pollutants like carbon dioxide that stay in the atmosphere for a relatively long time and minimizes the contribution of short-lived pollutants like methane, even if they do much more, metric ton for metric ton, to warm the atmosphere in the short term.
Sam Abernethy, a Stanford doctoral student and lead author of the study, said he became interested in the “global warming potential” of methane after examining why the United States and other countries use the 100-period period. year.
Abernethy found that the 100-year period was an “arbitrary and unwarranted” choice adopted by the Kyoto Protocol, the first binding international climate agreement, in the 1990s, and used in international reports and agreements ever since.
The 100-year measure was chosen for the Kyoto accord because it was midway between two other possible timeframes – 20 years and 500 years – provided in the first reports of the Intergovernmental Panel on Evolution climate (IPCC).
“I was confused how something so arbitrary could underpin so much of climate policy and how we think about different greenhouse gases,” 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 criterion that climatologists have previously suggested would be a more appropriate timeframe, methane is 81 times more potent than carbon dioxide.
“It’s a huge shift in how much we value methane, and therefore how many of our resources go to mitigating it,” Abernethy said.
However, the use of one or the other of the deadlines remains largely arbitrary.
To determine a “justified” time period, the Stanford researchers took the Paris climate goal of limiting warming to 1.5 degrees Celsius as a starting point, then calculated the most appropriate time period to achieve that goal.
Based on climate models using scenarios where global warming is limited to 1.5 degrees, they determined that the planet would reach 1.5 degrees of warming above pre-industrial levels in about 24 years.
“If that’s the case and you’re using a 100-year framework for methane, you’re not going to place enough value on reducing methane emissions relative to other greenhouse gases,” Jackson said.
Over a 24-year period, methane is 75 times more potent than carbon dioxide as a greenhouse gas. That’s three times more than 25, the current value the EPA uses for methane.
“It’s not inherently bad,” Abernethy said of the 100-year timeframe. “It’s just not aligned with our current situation. [climate] goal.”
Jackson said carbon dioxide remains the most important greenhouse gas. But he added that extra attention must be given to methane if the world is to limit warming to 1.5 degrees Celsius.
EPA spokesman Taylor Gillespie defended the agency’s use of a 100-year timeframe and said the relative values they assign to different greenhouse gases “are separate from the choice temperature targets.
The agency’s use of a 100-year time frame is based on international guidelines established by the UNFCCC, which aims 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 on the time frame it uses to compare the relative climate impacts of greenhouse gases.
The UNFCCC and international agreements like the Paris Climate Accord have historically placed more emphasis on reducing carbon dioxide to achieve climate goals. However, efforts to curb methane emissions took center stage at the UNFCCC’s international climate conference in Glasgow in November.
As Glasgow approached, many environmentalists and Biden administration officials argued that a focus on reducing methane emissions would have a much bigger impact in the short term than a 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 chance to slow short-term global warming, while buying extra time to tackle long-term warming caused by carbon dioxide.
Steven Hamburg, chief scientist of the Environmental Defense Fund, said he saw a growing awareness within the UNFCCC of the need to put more emphasis on tackling methane emissions alongside carbon dioxide. . What is missing, however, is an effort to revise the metrics used to measure the relative climate impact of the gas, he said.
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A 2017 study co-authored by Hamburg and published in the journal Science called for adopting a dual 100-year and 20-year “global warming potential” or timeline to account for the impacts of different greenhouse gas.
“I think there is no perfect measure,” Hamburg said of the current study and its call for a synchronized 24-year delay to limit warming to 1.5°C. “But they’re trying to keep drawing attention to the inherent bias in GWP 100. And it’s very real and very problematic.
The current EPA global warming potential figure for methane is too low, not only because it uses a 100-year time frame, but also because the figure is based on outdated science, Hamburg said.
IPCC reports published in 2014 and 2021 place 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% increase in methane’s climate impact, in 2024, in line with international guidelines from the UNFCCC.