A new study in Nature Climate Change, for instance, gets at why understanding the importance of methane can be such a difficult, confusing affair. In particular, it takes issue with some of the math that has often been used to compare the consequences of emitting methane with the impact of the chief, long-lived global warming gas, carbon dioxide. And it finds that really, we may not even know how important our methane emissions are in the first place until we also know how quickly we’re able to get carbon dioxide under control.
“People are placing too much emphasis on methane,” says Raymond Pierrehumbert, a climate scientist at the University of Oxford and one of the paper’s authors. “And really, people should prove that we can actually get the CO2 emissions down first, before worrying about whether we are doing enough to get methane emissions down.”
The study was led by Myles Allen, also of Oxford, with colleagues from several other UK universities as well as institutions in Norway and New Zealand.
The key problem addressed by the study is that greenhouse gases are all different, and yet nonetheless, policymakers and analysts have a tendency to pool them all together by using a common metric, “carbon dioxide equivalents.” Thus, according to the EPA, while in 2014 the U.S. emitted 5.556 billion tons of carbon dioxide, it emitted 6.871 billion tons of carbon dioxide equivalents — the number rises because of the inclusion of emissions of methane, nitrous oxide and other greenhouse gases.
But all of these gases are different — after a pulse of methane is emitted into the atmosphere, half of it is no longer there after 8.3 years, and then only a quarter is left after another 8.3 years, and so on. That’s very different from the behavior of a pulse emission of carbon dioxide, some of which remains in the atmosphere for thousands of years.
The usual way to convert emissions of methane, black carbon, and other so-called short lived climate pollutants into carbon dioxide equivalents involves calculating their “global warming potential” over 100 years — thus, according to EPA, methane has 28 to 36 times as much warming impact as the same amount of carbon dioxide over a century. But the new study in Nature Climate Change finds that because methane has a shorter atmospheric life than carbon dioxide, the truth is that this gas – along with black carbon and various hydrofluorocarbons, or HFCs — really has an impact over 20 to 40 years, rather than 100. (The calculation is actually for all uses of the global warming potential approach, not just for methane.)
“The appearance of 100 years in the name of the metric really deceives a lot of people into thinking this is telling us something about temperature 100 years out, whereas it’s not,” says Pierrehumbert.
All of this may seem technical, but here’s why it matters. Governments are struggling to keep the global temperature from rising more than 2 degrees Celsius above pre-industrial averages, and would prefer holding it to 1.5 degrees — which means eventually bringing carbon dioxide emissions to zero. If the temperature peaks soon, say by the year 2050, then controlling the warming caused by methane, and other short-lived climate pollutants like black carbon, would mean a lower total peak temperature, the new study finds. In that case, these emissions definitely matter.
But if we don’t get carbon dioxide under control and peak emissions by 2050, the new study suggests, then today’s methane emissions become irrelevant. They simply won’t be causing warming any longer. But a significant amount of the carbon dioxide we emit today will still be in the atmosphere.
Or as the study puts it, the global warming potential approach “is an effective metric to limit peak warming if and only if emissions of all climate pollutants, most notably CO2, are being reduced such that temperatures are expected to stabilize within the next 20-40 years.”
“If we knew we were going to peak by say 2050, then there is a lot of value in putting efforts into some of the short lived stuff,” says Pierrehumbert. “But if we don’t do a good job with CO2, that means as far as temperature targets go, the money we put into methane and black carbon, as far as climate goes anyway, is wasted.”
This doesn’t mean we shouldn’t cut methane emissions – there remain many other reasons for doing so, including climate-related ones. For instance, if gas drilling operations in the U.S. cut down on accidental releases of methane then they will have more natural gas to sell in electricity markets — and if that natural gas displaces coal, overall carbon dioxide emissions will go down.
But it does mean, Pierrehumbert argues, that we need more clarity when it comes to what we are actually dealing with when it comes to methane — and this is an inherent problem when you’re comparing what his paper calls a “cumulative” greenhouse gas, carbon dioxide, with one that is “short-lived,” like methane.
In the end, while this may all seem technical, the fact is that countries of the world are embarking on a new project of lowering their emissions, and reporting the progress they have made to one another under the Paris climate agreement. And as long as confusion remains over the role of carbon dioxide versus methane and other greenhouse gases, confusion will also remain over just how close everyone is to meeting the goals.
In reaction to the new study, Duke University’s Drew Shindell, who heads the scientific advisory panel of the Climate and Clean Air Coalition, which focuses on the problem of short-lived climate pollutants like methane, said he didn’t disagree with any of the calculations — or with the point that some of the metrics used can be misleading.
The concept of global warming potential, Shindell says, was “proposed originally as an example of how difficult it is to compare things, and then it becomes the thing that everybody uses and nobody really understands.”
However, Shindell did object to the idea that this analysis is the only way to look at the benefits of reducing short-lived climate pollutants, like methane or black carbon. He pointed out that these pollutants also have other consequences outside of their impact on planetary temperature — such as local air quality problems that can make people sick or even in some cases kill them.
“There are huge societal benefits, separate from peak warming,” of reducing these pollutants, Shindell says.
The new study, of course, can also be taken to mean that methane emissions really do matter to global warming — at least if the world is otherwise really acting fast on carbon dioxide.
The new work “seems to stand for the proposition that methane and other short term climate forcing pollutants play a unique and important role in accelerating warming in the near term and that the conventional method of normalizing all climate pollutants to a 100 year global warming potential does not capture this,” adds Mark Brownstein of the Environmental Defense Fund, which has focused heavily on the methane emissions problem. “It’s another paper that argues for the importance of focusing on methane as a distinct opportunity.”
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