Written by Frank Ackerman
February 18, 2019
Third in a series of posts on climate policy.
Carbon dioxide (CO2) represents most, but not all, greenhouse gas emissions. In EPA’s Greenhouse Gas Inventory for 2016, CO2 represented 82 percent of gross U.S. GHG emissions, while methane represented 10 percent (measured as CO2-equivalents). The top three sources of methane are agriculture, the energy industry, and waste management.
As fascinating as some of us may find such details, the general public has a short attention span for new information about climate change. Within that constraint, what do we want to communicate? For methane, there are two choices, an introductory and an advanced message.
The introductory message emphasizes that methane, the principal component of natural gas, is an important cause of global warming under any version of the data. It is therefore crucial to reduce and eliminate all fossil fuels, gas included, as soon as possible, replacing them with efficiency, renewables and energy storage.
At a more advanced level, some new research suggests that conventional data understate methane emissions. And a different way of comparing methane to CO2 implies that methane should have a higher CO2-equivalent value, raising its relative importance in GHG accounting. Some combination of these factors might even make natural gas as bad as coal, from a global warming perspective.
The introductory message is the one that matters for public communication. It focuses discussion on the simple fact that natural gas, like other fossil fuels, has got to go: it is part of the problem, not the solution. The advanced message, in contrast, emphasizes technical controversies and interpretation of recent research. It tends to produce eyes-glazed-over responses along the lines of “I wasn’t that good at science in school.”
But if you’re reading this, you can probably follow the technical debates, at least partway down the rabbit hole. Consider three chapters of the story of methane: the time span for calculating CO2-equivalents; the issue of gas leaks; and the gas vs. coal comparison.
Thinking about tomorrow
Methane is a much more potent heat-trapping gas than CO2, but CO2 remains in the atmosphere and traps heat for much longer than methane. On balance, how much does a ton of methane emissions contribute to warming, relative to a ton of CO2?
The answer depends on the time period under consideration. Methane has an atmospheric lifetime of 12 years. CO2 is affected by several processes that operate at very different speeds: 50 percent of CO2 is removed from the atmosphere within 30 years of emission, while 20 percent persists in the atmosphere for thousands of years.
Zooming in on a timespan as short as 20 years after emissions means focusing mainly on years when methane is still present in the atmosphere, trapping a lot of heat. Over a longer interval such as 100 years after emissions, most of the years are ones when methane has faded away, while a significant fraction of the CO2 emissions remains in the atmosphere. As a result, the CO2-equivalent value of methane is 84 over a 20-year period, compared to only 28 over a 100-year period.
Early IPCC and other technical reports tended to standardize on the 100-year CO2-equivalent value, implying that methane is 28 times as bad per ton as CO2. More recent studies have often highlighted the 20-year CO2-equivalent value, making methane 84 times as bad.
Neither one or the other is the correct value. Climate change is a problem of both short-term urgency and long-term consequences, of 20-year impacts, 100-year impacts, and beyond. This produces an awkward situation for research and reporting on greenhouse gases: the “exchange rate” between the two leading gases is either 28 or 84. It is less of a problem for public policy, where either of the CO2-equivalent values for methane is enough to make the case: a low-carbon economy must eliminate methane emissions, not rely on natural gas as a bridge to anywhere we want to go.
Counting the leaks
Natural gas leaks from wells and pipelines, increasing the lifecycle methane emissions associated with gas-fired heating or electricity generation. EPA estimated that methane leaks represented 1.4 percent of gas production nationwide in 2015. But a new study based on extensive field measurement found that methane leaks were 2.3 percent of gas production that year. Other studies have reported even higher leakage rates in areas where fracking is widespread.
It would be a mistake, however, to pin the critique of natural gas solely to high levels of leaks. The same study that found leaks of 2.3 percent also found that “the higher estimates stem from a small number of so-called superemitters … most tied to [malfunctioning] hatches and vents in natural gas storage tanks at extraction wells.”
It is not hard to imagine the industry, under pressure from regulators, fixing the malfunctioning hatches and vents, and developing better ways to seal leaks in general. This would increase the amount of gas that could be delivered to customers, potentially increasing industry profits. The International Energy Agency, which estimates gas leaks of 1.7 percent worldwide, also finds that 40 to 50 percent of current methane emissions could be avoided at no net cost.
The key point about methane is not the current high levels of leaks. Rather, reducing methane emissions, from leaks and other sources, is one of the most cost-effective strategies for greenhouse gas mitigation.
Different shades of bad
Some environmental advocates now claim that burning gas is just as bad for the climate as burning coal. There are several strong counterarguments, which do not undermine the case against gas.
Above all, coal is an environmental disaster, causing havoc throughout its life cycle. Coal mining devastates local communities, on a level that equals or surpasses anything done by fracking. It even releases methane from coal mining, equal to 8 percent of U.S. methane emissions according to the 2016 greenhouse gas inventory. The canaries in the coal mines, back in the day, were brought in to detect carbon monoxide and methane, the deadly gases that threatened miners.
Coal combustion gives rise not only to CO2, but also to many toxic pollutants which kill people near the plants. Since coal plants are usually located in low-income and minority neighborhoods, plant siting raises issues of environmental justice. After combustion, coal ash must be disposed of, creating a whole new set of toxic risks and environmental justice concerns in the siting of these impacts. Gas does not cause local toxic emissions or leave ash behind when it burns.
Even restricting attention to greenhouse gas emissions, an extraordinary level of leakage is required to make gas as bad as coal from a 20-year perspective, let alone a 100-year perspective. The IEA has a graph displaying the relationship between leak rates, time horizon, and climate impacts from coal vs. gas.
Finally, consider the emotional meaning of the statement that gas is as bad as coal. It often seems as if activists feel the need to show that gas is as bad as it gets, in order to support opposition to gas-fired power plants. This is surely a mistake.
It is not necessary to make something the worst ever, in order to establish that it is bad. George W. Bush was a bad president, for the environment and so much else; now it turns out that he was not the worst possible president. There is no reason to claim that Bush was as bad as Trump – or that a return to Bush-era policies would be a bridge to the future. It’s just a different shade of bad.
A gas-fired electric grid is different from a coal-fired one. But from a climate perspective, they are different shades of bad: both involve carbon emissions far above a sustainable, climate-friendly level. The need for a carbon-free alternative is the conclusion that matters, independent of the latest research details on methane.