Proponents of natural gas point out that it releases far less pollution than coal per BTU and could be a cheap source of energy that will reduce health problems and help millions of people worldwide escape from poverty. When combusted, natural gas emits only 117 lbs of carbon dioxide per million BTU of energy produced, whereas coal emits 210 lbs, as well as much larger concentrations of harmful mercury, sulfur, nitrogen, and particulate pollutants. Many hope that natural gas will be a “bridge fuel,” replacing dirty coal-fired electricity generation for the next few decades, while the world develops the technology needed to make renewable energy cheap enough that it will replace fossil fuels.
However, in recent years, scientists have identified a number of potential concerns about the relative benefits of natural gas and unconventional petroleum recovery (one step of which is hydraulic fracturing of deep, low-permeability reservoirs). Chief among these is the worry that the total lifecycle greenhouse gas emissions of unconventional natural gas—production, transportation, refining, distribution, and combustion—are high enough that it doesn’t have a significantly lower climate impact than coal. When lost to the atmosphere before being combusted, methane (the main energy-releasing compound in natural gas) is a significantly worse greenhouse gas than carbon dioxide, trapping about 28 times more heat than carbon dioxide during its first 100 years in the atmosphere.
Despite our good understanding of how “fugitive methane” emissions happen, there is wide uncertainty about the severity of the problem, how much different industry sectors and parts of the natural gas lifecycle contribute to emissions, and how best to reduce the leaks. On one side of the argument is a group of scientists at Cornell, who estimate that 2.2–4.3% of methane recovered from unconventional reservoirs escapes at the well site (plus additional leakage during transportation, refining, storage, and distribution). To put these numbers in context, switching from the best coal-fired power plants to natural gas should produce climate benefits as long as total lifecycle methane leakage remains under about 3.2%.
Meanwhile industry-funded research by the Environmental Defense Fund has found that only 0.42% of methane escapes during unconventional production (with additional leakage after production). Wildly different estimates such as these have made it difficult to calculate the benefits of switching more fully to natural gas power, and they create complications for policymakers and researchers seeking to regulate energy production and to allocate research and development funds. Unsurprisingly, those in favor of natural gas tend to cite studies finding lower fugitive methane emissions during production, while their opponents tend to cite studies showing greater emissions.
This is why a comprehensive recent paper in Science led by Adam Brandt of Stanford is a welcome addition to the growing literature about industrial methane leakage. Brandt and colleagues studied over 200 journal articles reporting measurements of methane emissions from natural gas systems in Canada and the United States for the past 20 years, at scales ranging from individual emitting devices to the atmosphere above the entire United States. They then compared these measured values against official EPA inventories of methane emissions. The EPA inventories are important because they form the basis for official estimates of how much greenhouse gas is emitted across the entire United States each year, and they are often used for estimating the total life-cycle emissions of various processes or products, such as natural gas.
Brandt and his colleagues found that the measured emissions average 1.5 times greater than these official estimates, meaning that the United States and Canada may consistently underestimate their contribution to anthropogenic greenhouse gas emissions, and that some life-cycle analyses of products that consume natural gas during their lifetime may also underestimate greenhouse gas contributions.
Abandoned oil and gas wells are one of several types of sources that exceed the official emissions estimates. With nearly half a million natural gas wells (both conventional and unconventional) now operating in the United States, abandoned wells may represent an increasingly important future contributor to greenhouse gas emissions and other pollution. Furthermore, distribution of natural gas through pipelines and other infrastructure is probably a major source of emissions in excess of EPA estimates. This suggests that an important component of reducing methane leaks in the United States will be to improve the aging natural gas distribution system. Interestingly, the new paper also supports earlier arguments that a few “superemitters” may be responsible for a large portion of total methane emissions. If true, this is probably good news for the climate because it means that a large amount of fugitive emissions might be stopped relatively cheaply and quickly.
Perhaps most importantly, the authors conclude that even though methane leakage is probably higher than official estimates, a complete shift from coal to natural gas electricity generation would still create significant climate benefits. Although the Brandt study doesn’t specifically estimate the rate of methane leakage at well sites, it concludes that emissions from unconventional natural gas production represent a low proportion of the total. However, despite recent calls to replace diesel trucking with natural gas, Brandt and colleagues argue that diesel is probably a lower-emissions fuel for transportation. Even converting more passenger cars from gasoline to natural gas fuel will probably have negligible benefits for the climate.
Other research supports this conclusion, finding that replacing gasoline-powered vehicles with natural gas engines would produce higher greenhouse gas emissions for the first 80 years; replacing diesel vehicles would have climate costs for 280 years. (Trains, which are typically powered by diesel-electric engines, remain significantly more efficient than even diesel trucks for shipping).
Hopefully this new review will go a long way toward resolving the considerable disagreement about the relative climate impacts of natural gas, so that the world can make well-informed choices about the energy mix we will need to reduce climate change and sustain a quickly growing population.
Image courtesy Wikimedia Commons featuring the Apollo Beach power plant.