15.3: Mitigating Methane

Methane (chemical formula CH₄) is a particularly important warming agent because it affects global temperatures in four different ways. First, it is a powerful greenhouse gas in its own right, with a warming effect that is 30 times more powerful than carbon dioxide over 100 years and more than 85 times more powerful over 20 years. Second, although methane has a relatively short lifetime in the atmosphere (about 12 years), it decomposes to carbon dioxide, a significant fraction of which will remain for hundreds to thousands of years. Third, methane can react with other chemicals to produce ozone, which is also a significant greenhouse gas. Finally, methane in the upper atmosphere can react with hydrogen to produce water vapor, yet another greenhouse gas.

Because of this “quadruple threat,” methane mitigation can have a significant impact in bending the warming curve. Due to its short atmospheric lifetime, the benefits of methane reduction will begin to appear relatively quickly, within a decade or two.

Studies indicate that full implementation of the methane mitigation strategies discussed in this section could bend the global warming curve by 0.4°C by 2050. Moreover, implementing both the methane and black carbon mitigation measures discussed in the previous section could save over 2 million lives, 50 million tons of crops, and $5 trillion annually by 2050.

Sources of methane

Data from ice cores show that methane concentrations in the atmosphere were relatively steady for the past few thousand years but began to increase dramatically around the beginning of the Industrial Revolution, rising from just over 700 parts per billion (ppb) in the preindustrial era to more than 1,800 ppb today. Studies have confirmed that this rise is primarily due to human activities. The primary anthropogenic sources of methane are shown in Figure 15.3.1.

Figure 15.3.1 Methane percentage contribution from various sources, 2010. Global annual methane emissions are in the range of 558 million to 736 million metric tons CH₄ per year from the 2003–2012 decade. (Data for this range are from Saunois et al. 2016.) Adapted from the Global Methane Initiative.

One major source of emissions is the exploitation of fossil fuels. The same geologic processes that produce coal and oil also generate methane, which is often found in association with coal beds and oil fields. Natural gas is primarily composed of methane (around 90%). Anthropogenic methane emissions associated with fossil fuel extraction and use include the following:

• Methane leaks from natural gas production, processing, and pipeline distribution systems (sometimes called “fugitive emissions”)
• Methane escape during completion of oil wells and oil production
• Methane leaks from active and inactive coal mines

A second source of methane is bacterial decay of organic matter in the absence of oxygen. This occurs in underwater or underground environments such as wetlands, swamps, or landfills and is referred to as anaerobic decomposition. Bacteria in the digestive tracts of livestock can also produce methane through a process known as enteric fermentation. Anthropogenic sources of methane from anaerobic decomposition and fermentation include the following:

• Livestock—before digesting their food, ruminants such as cattle, goats, and sheep ferment the plant material they eat in a specialized stomach (called a rumen). This enteric fermentation process produces significant quantities of methane.
• Manure—decomposition of waste from livestock and poultry releases methane.
• Wet rice agriculture—flooded rice fields create anaerobic conditions similar to those in a natural wetland.
• Waste—decomposition of organic food waste in landfills and human waste in wastewater systems produces methane.

Methane mitigation strategies

As in the case of black carbon, there is a range of technologies already available to reduce anthropogenic methane emissions. Many of these technologies involve capturing methane and burning it for heat or electric power generation. Although burning methane converts it to carbon dioxide, this significantly reduces its warming effect, since CO₂ is a much less potent greenhouse gas.

Methane seeps naturally from coal beds. Coal mines create openings that allow the methane to escape into the atmosphere. Both active and abandoned coal mines are significant sources, but degasification pump stations have proven effective in removing and collecting methane. Coal mines are not usually near natural gas distribution facilities, so the methane captured is typically burned on-site and could be used for heating or generating electricity.

Oil drilling often brings natural gas to the surface along with the oil, and the gas must be vented to the atmosphere to maintain safe pressure in the well. Sometimes this gas can be stored and sold, but where gas distribution facilities are not available nearby, it is often released to the atmosphere. Burning the methane instead (this is referred to as flaring) would significantly reduce its greenhouse effect. The mitigation effect could be further enhanced by capturing the resulting CO₂, which can be pumped back into the ground and stored or pumped into a depleted oil field to enhance oil recovery (which, of course, will produce still more oil).

Figure 15.3.2 To the naked eye, no emissions from an oil storage tank are visible (left), but with the aid of an infrared camera, escaping methane is evident (right). Reproduced with permission from UNEP and WMO 2011.

Leaks in the natural gas production and distribution system should be relatively straightforward to address. Loss of gas from the system represents a loss of profits, and significant leaks can present a safety risk. Companies involved with natural gas production, storage, and distribution are generally motivated to locate and address leaks. A wide range of portable methane detectors are now available to help with this task (Figure 15.3.2).

The Oil and Gas Climate Initiative (OGCI), which includes several of the world’s largest oil and gas producers, has set a target of reducing the methane intensity of its member companies by 20% by 2025, a very modest goal but a start nonetheless. In contrast, the International Energy Agency estimates that the oil and gas industry can reduce its worldwide methane emissions by 75%—and up to two-thirds of those reductions can be realized at zero net cost.

Methane emissions associated with oil and gas extraction are an important consideration in evaluating the impacts of hydraulic fracturing (often called hydrofracking, or fracking for short). In the United States, hydrofracking over the past decade or so has significantly reduced the cost of natural gas, resulting in a significant shift from coal to oil in electric power production. Because natural gas emits only about half as much CO₂ per unit of energy produced compared with coal, this shift could significantly reduce greenhouse gas emissions associated with electricity generation. However, the leakage of as little as 3% of the methane during well completion or production could reduce or even negate the climate benefits of reduced CO₂ emissions and be as greenhouse-gas intensive as burning coal. Studies by different groups have resulted in published estimates of methane leakage rates of between 1.4% and 2.3% of production industry-wide, but emissions from individual facilities can be significantly higher. The extent of methane emissions from hydrofracking is currently a topic of serious scrutiny.

About 10% of anthropogenic emissions are due to wet rice agriculture, in which rice is grown in flooded fields. Studies have shown that periodic short-term draining of the rice fields to expose the soil to oxygen, known as intermittent aeration, can significantly reduce methane emissions.

Methane emissions from livestock and poultry manure can be addressed using covered anaerobic digesters, which accelerate the decomposition process and capture the resulting methane, rather than allowing it to escape to the atmosphere. Burning the methane can generate heat or electricity for on-farm use or for sale. California agencies are funding pilot projects to demonstrate the collection and concentration of methane from dairy digesters for injection into natural gas pipelines. Industrial-scale hog farms, meanwhile, are expanding their efforts to reduce methane—partly because such efforts often yield co-benefits in terms of a reduction in offensive odors—after being sued dozens of times and being required to pay neighbors damages for the offensive odor. In a recent North Carolina lawsuit against the largest pork and hog producer, Smithfield Foods, a jury awarded $473.5 million in damages, reduced to $94 million by a North Carolina law capping punitive damages.

Similar to degasification pumps for coal mines, landfill gas wells can capture methane from landfills to be burned for heat or energy. Wastewater can undergo anaerobic wastewater treatment with installations that use technology similar to anaerobic digesters, but typically on a larger scale, with the methane captured and used for energy.

Although this chapter focuses primarily on technological mechanisms to remove methane, there are other mitigation strategies that involve societal transformations or changes in ecosystem management. For example, a reduction in meat consumption, particularly lamb and beef, would reduce the associated methane. Similarly, we’ll see in the next chapter how reducing food waste could substantially lower methane emissions from landfills as well as the CO₂ emissions associated with the energy required for food production, transportation, and storage.

Table 15.3.1 Methane mitigation measures

Sector	CH4 Mitigation Measures
Extraction and transport of fossil fuel	Extended pre-mine degasification and recovery and oxidation of CH4 from ventilation air from coal mines
Waste management	Extended recovery and utilization, rather than venting, of associated gas and improved control of unintended fugitive emissions from the production of oil and natural gas Reduced gas leakage from long-distance transmission pipelines Waste management Separation and treatment of biodegradable municipal waste through recycling, composting, and anaerobic digestion as well as landfill gas collection with combustion/utilization Upgrading primary wastewater treatment to secondary/tertiary treatment with gas recovery and overflow control
Agriculture	Control of CH4 emissions from livestock, mainly through farmscale anaerobic digestion of manure from cattle and pigs Intermittent aeration of continuously flooded rice paddies

Source: Reproduced with permission from United Nations Environment Programme and World Meteorological Organization. 2011. Integrated Assessment of Black Carbon and Tropospheric Ozone. UNEP, Nairobi, Kenya.

Figure 15.3.3 Seven measures that aim to reduce methane emissions. Reproduced by permission from Climate and Clean Air Coalition; data from UNEP and WMO 2011.

Table 15.3.1 and Figure 15.3.3 summarize measures that aim to reduce methane emissions.