April 13th, 2011 by David McCabe, Atmospheric Scientist
A paper by Robert Howarth and co-workers comparing the climate impacts of natural gas to coal has made a huge splash this week, by arguing that natural gas may have a bigger climate footprint than coal for generating power—a finding that flies in the face of conventional wisdom that natural gas is the cleanest fossil fuel. Howarth argues that it’s mainly leaks and venting of methane, the main component of natural gas, that makes gas impacts so high.
We’re very concerned about methane leaks from all fossil fuel extraction, especially natural gas. But this paper hasn’t convinced us that natural gas-fired power is worse than coal, for reasons discussed below.
Moreover, lost in the controversy over the Howarth paper are several bigger points:
1. Over time, we really need to be driving towards zero emissions of carbon from power generation, whether we’re using coal or gas. And we will need to use coal and gas for a long time around the world. We just can’t ramp up renewables and nuclear fast enough to avoid the worst climate damage. That means using carbon capture and sequestration. We know how to do this, and have done so around the world. We need to start applying these technologies now at commercial scale.
2.In the meantime, we can clean up excess emissions of methane from gas, oil, and coal extraction immediately at relatively low cost, making much of the coal versus gas “excess methane” discussion irrelevant. EPA and the states should take appropriate actions to make this happen.
3. Our data on these emissions is generally terrible, so much of this debate won’t be resolvable for some time. EPA needs to focus on improving that data, and the natural gas industry needs to stop fighting those efforts and help out. In the meantime we can avoid the pollution that we know is occurring.
First, Howarth’s paper:
This paper is selective in its use of some very questionable data and too readily ignores or dismisses available data that would change its conclusions. Our particular concerns include:
- The paper ignores an important question about whether natural gas is flared or vented when a shale gas well is finished. Fluids are injected under pressure (fracking) to release gas from shale. Drillers then let the natural gas push the fluids out of the well, and that natural gas can either be burned off in a flare or vented to the air. Howarth et al. ignore reports that flaring is common1 and assume, without providing support for their assumption or even stating it, that this gas is never flared from shale gas wells. We’re not fans of flaring, which still pollutes, but it is better than venting and reduces methane emissions a lot.
- The authors’ choice as to which data to use for leaks from natural gas processing, transmission, and storage systems is also a big concern. They recognize the limitations of EPA data, then use those limitations to dismiss the EPA work, and then use higher values, which come from even more questionable sources.2
These two decisions alone result in unrealistically high emissions.
On the positive side, as noted above, the splash from this paper has brought needed attention to a critical issue — methane leaks from production of natural gas, oil and coal. These leaks are very large and very poorly measured sources of U.S. climate pollution. We need to measure these leaks accurately and then turn them off. While many needed climate mitigation policies will take years to implement, cleaning up natural gas and coal mine leaks can happen right away.
Methane emissions from venting and leaking in gas, oil, and coal systems are a real threat to our climate. IPCC’s most recent assessment report found that methane is responsible for nearly 30% of the warming to date, probably second only to carbon dioxide. Methane only lasts about a decade in the atmosphere (CO2 lasts for centuries), but while there, it is a much more potent warmer. Pound for pound, including the effects that methane has on ozone and the stratosphere, methane warms the Earth 72 times more than CO2 over a 20-year period, according to IPCC. More recent work suggests even greater warming, by estimating how methane interacts with haze (Howarth et al cite this work).
Methane comes from a number of sources, many of which are difficult to address. This is another reason why focusing on fossil fuel methane makes so much sense. Extraction and production of gas, oil, and coal all result in methane releases into the air. Natural gas is mostly methane, so leaks and intentional venting of wells are probably the largest source of methane from fossil fuels. Globally, natural gas systems produce roughly 16% of anthropogenic methane. And that number could be even higher.
A few months ago EPA alarmingly raised the estimates of methane leaks from natural gas operations in the U.S. by 120%3. These leaks cause a lot of warming: annual methane emissions from just the U.S. natural gas sector (which includes production for electric power as well as direct heating and industrial use) warms the climate as much as 760 million tons of CO2, which is more than 40% of the CO2 emitted by U.S. coal-fired power plants in a year4, based on the 20-year methane Global Warming Potential (GWP) of 72.
EPA’s estimates of methane from natural gas remain crude. We badly need better assessments of these emissions, and we need them soon. The natural gas industry, which has promoted the environmental benefits of their product, should cooperate with EPA and others to provide accurate inventories of these leaks. EPA needs to focus on getting these measurements right. And while leaks from shale gas have received tremendous attention recently, including from Howarth et al., leaks from “conventional” gas wells that don’t require fracking may be just as worrisome5, and are also very poorly measured.
Regulating and reducing emissions of methane from natural gas production are imperative to protect climate and air quality (natural gas leaks also lead to ozone smog). The fixes are relatively low-tech and cost effective, since the captured gas can be re-used for energy.
Clean Air Task Force is working to better understand these emissions to get clearer and more accurate inventories and to clean up these leaks as quickly as possible. This strategy will both reduce climate pollution and improve local air quality. Let’s get on it.
1 EPA rather simplistically assumes that ~half of all well completions are flared, with the rest vented. See http://www.epa.gov/climatechange/emissions/downloads10/Subpart-W_TSD.pdf, p. 88. This estimate is crude, but venting is banned in at least a few states. Even there compliance may not be perfect, but it is clear that some shale gas wells are flared; Howarth et al. implicitly assume this never happens.
2 For their lower limit for leaks from U.S. natural gas transmission, storage, and distribution, Howarth et al use a value of 1.4% from a study of Russian systems. (EPA inventories of US systems, which Howarth dismisses, put the number a bit under 1%). For their upper limit, they use 3.6%, an average of the values they quote for 2000 and 2007 data mentioned in a non-peer reviewed magazine article, which in turn references a consultant’s study. It is not clear how the consultant’s study can be obtained for review. Howarth et al. misstate the value in the magazine article for 2000 (quoting a value lower than that in the magazine article), and it appears from that article that these values might also include leaks during gas production and processing, in addition to transmission, storage, and distribution.
3 See 2011 Draft US GHG Inventory, Chapter 3 (PDF) pages 3-43 – 3-47.
4 Ibid, p. 3-8.
5 Recent EPA documents suggest that venting from conventional wells is currently a much larger source than venting from unconventional wells. See http://www.epa.gov/climatechange/emissions/downloads10/Subpart-W_TSD.pdf, p. 84 – 90. While conventional gas production is larger than unconventional production, the large total emissions from conventional wells must not be ignored.
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