Focus far more on methane

 From Inside Climate News

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.

Source: Wikipedia