Greenhouse gases are not all the same, and past methods of accounting misvalue their relative contributions over time to climate change, according to a new study by SFI External Professor Jessika Trancik and graduate student Morgan Edwards, both of MIT, who propose more nuanced metrics for methane and carbon dioxide.
Listen to Trancik's interview on NPR's Living On Earth (June 13, 2014))
The standard figure used for emissions trading and technology evaluation, the global warming potential (or GWP), says that, gram for gram, methane is about 30 times as potent a greenhouse gas as carbon dioxide. The result: After six or seven decades, the impact of the two gases is about equal, even though methane is afforded a great deal less regulatory pressure under new Clean Air laws.
“With carbon dioxide, one cares about the cumulative emissions,” Trancik says. “But with methane, the timing of emissions matters.”
In their paper, which appeared in Nature Climate Change, the authors propose two new metrics that incorporate limited information about the future but don’t require knowledge about the exact climate scenario to be followed: the instantaneous climate impact (ICI) and the cumulative climate impact (CCI); the latter is more conservative in earlier years.
The improved metrics can help policy makers weigh methane-emitting technology options such as natural gas, and make better decisions about energy technology and greenhouse gas emission tradeoffs, they say.