Natural Gas Needs Tighter Production Practices to Reduce Global Warming Pollution
Posted April 12, 2011
A paper about the global warming pollution from natural gas production, due to be published online this week, is already making news and is sure to make a lot more. That’s because the analysis by Robert Howarth and colleagues at Cornell calls into question the image of natural gas as a cleaner “bridge fuel” to the truly clean energy system we need to solve global warming.
We already knew that natural gas is a cleaner burning fuel than coal. That means when it’s burned at a power plant to generate electricity for our homes, it emits far less carbon dioxide pollution than coal - as well as far less mercury and other forms of air pollution.
But the production of natural gas – including its extraction process and transport to power plants can result in significant air and water pollution for surrounding communities and our atmosphere. For example, extraction releases toxic pollutants, such as benzene and formaldehyde, which can cause cancer and other serious illnesses. And there are significant methane emissions from extraction and the leaky equipment and pipes that transport natural gas. Methane is a global warming pollutant like carbon dioxide – only much more potent.
The new study raises important questions about the overall global warming pollution from using natural gas, and reaffirms two things NRDC has made abundantly clear all along:
- Efficiency and renewable energy sources such as wind and solar are the clear winners and need to be the top priority for our clean energy future.
- If the natural gas industry wants to sell its product as a “bridge fuel,” it will need to clean up its act with respect to both these methane emissions and the wide range of other environmental and health impacts associated with its current production methods.
So what’s new in the new report? Here is my assessment:
The key issue highlighted by Howarth is how much methane (the primary constituent of natural gas) is intentionally vented or leaks into the atmosphere during natural gas production, transportation, and distribution. This is important because each molecule of methane traps far more heat in the atmosphere than a molecule of carbon dioxide. The basic chemistry of natural gas ensures that burning it releases less carbon dioxide than burning other fossil fuels, such as oil or coal, but this advantage is eroded to at least some extent by methane venting or leaks. The questions are how significant is this problem and what can be done about it?
Methane emissions from natural gas production and distribution have long been recognized as a source of heat-trapping pollution, but until recently these emissions have been assumed to be relatively small—perhaps about one percent of natural gas production. Recently EPA has revised its estimates upward, more than doubling its estimate of methane emissions from natural gas production and bringing estimated total methane emissions from the natural gas system to about 2.5 percent of national natural gas use. Howarth argues that emissions could be even higher under current industry practices: 1.7-6.0 percent for conventional gas production and 3.6-7.9 percent from unconventional gas production. Hydraulically fractured wells release more methane during completion than conventional wells due to emissions of gas brought to the surface with the flow back of hydraulic fracturing fluids. (More information on hydraulic fracturing is available here.)
These are large uncertainty ranges and that is a key point. Howarth’s (and EPA’s) estimates of methane emission rates are based on very limited data. Relatively few actual observations were used to estimate “emission factors,” which were then extrapolated to estimate emissions from the system as a whole. This approach would be OK if we were sure that the overall emissions were small, but it’s not a sufficient basis to reach firm conclusions given recent indications that emissions could be much more significant. Howarth has done a great service by shining a spotlight on the need for better data and is the first to say that his paper is not definitive, telling the New York Times “I think this is just the beginning of the story, and before governments and the industry push ahead on gas development, at the very least we ought to do a better job of making measurements.”
More issues arise when trying to compare the global warming impact of using coal to that from using gas. This requires a way to add up the impact of carbon dioxide emissions and more powerful, but shorter-lived, methane emissions. The Intergovernmental Panel on Climate Change (IPCC) uses a weighting factor called the Global Warming Potential (GWP) for this purpose, which measures the relative impact of emitting one ton of methane (or other greenhouse gas) to one ton of carbon dioxide. Because different pollutants stay in the atmosphere for different lengths of time, this factor depends on what time horizon is considered. There is no one right answer for the appropriate time horizon to consider, so the IPCC publishes GWP values for 20, 100, and 500 years. For methane the IPCC values from its most recent report are 72, 25, and 7.6, respectively.
Howarth does not consider the 500 year GWP and relies on a more recent study by Shindell et al. that suggests that the indirect warming impact of methane (through chemical interactions in the atmosphere) could raise its GWP to 105 over 20 years and 33 over 100 years. While these higher figures were produced by well respected researchers, they have not yet been subject to the level of review and scrutiny conducted by the IPCC for its estimates. Moreover, while I can see an argument for using a time horizon shorter than 100 years, I personally believe that the 20-year GWP is too short a period to be appropriate for policy analysis because it discounts the future too heavily. I calculate that over a 50 year period, the GWP of methane would be in the range of 42-56, based on the IPCC and the Shindell et al. analyses.
Another factor that has to be considered (which is acknowledged in Howarth’s paper, but not reflected in the main figure or findings) is the efficiency with which natural gas can be used relative to coal for any particular application. In the United States, coal is used almost exclusively for generating electricity, with an average efficiency of about 33 percent, whereas new natural gas combined cycle power plants can achieve an efficiency of 50 percent or more. This is the most relevant comparison because many power companies are considering whether to reduce their reliance on old coal-fired power plants by ramping up generation at relatively new gas plants.
A third factor that is critically important is the ability to implement cost-effective technologies to greatly decrease methane emissions at each step in the process. Howarth’s paper does not address these opportunities, and the resulting implications for the comparison between coal and natural gas.
So what’s the bottom line?
First, as mentioned above, NRDC has long advocated that energy efficiency and renewables should be our first choice, rather than gas or coal, because they are the fastest, cheapest and cleanest solutions to global warming. Howarth’s analysis certainly underscores that priority.
Ultimately, we need to move away from dirty fossil fuels altogether and transition to sources such as wind and solar that can’t leak, spill, pollute the air or run out. Until we get there, natural gas will play a role in our energy mix (but not a dominant one) and that's why it's so crucial for the industry to clean up its act.
Second, we need much better data about methane emission rates before firm conclusions can be reached about the climate impact of natural gas relative to coal. The good news here is that companies are now required to measure and report on these emissions, so we will have more accurate data in the years to come.
More importantly, we need stronger regulation of natural gas upstream operations to address the full array of harmful impacts from inadequately regulated production, including requirements for the use of best practices to minimize releases of methane and other contaminants. As documented by the EPA’s Natural Gas Star program, there are proven, cost-effective technologies and practices that improve the operational efficiency of natural gas production and reduce methane emissions. And less methane leakage means more natural gas to sell, so tightening up the production process can also increase profits. Fortunately, some companies have already adopted these practices; the rest of the industry should be required to follow suit.
The ready availability of techniques to cut upstream emissions from natural gas is a key point distinguishing it from new investments in coal power plants. Building a new coal plant runs a real risk of locking in high pollution loads for decades. By contrast, the emissions from generating electricity with natural gas can be cut substantially at any point by cutting upstream emissions – something that can and should be done without delay.
Howarth’s paper is sure to stimulate a lot of debate and controversy over the days and months ahead for the reasons discussed here, among others. If that debate leads to better data on methane emissions and better regulation of natural gas production, the paper will have done a valuable service indeed.
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