The environmental virtues of cheap, abundant and clean-burning natural gas from shale plays are offset somewhat by methane emissions that occur during gas production and transport -- but not so much as to sound alarms for Duke University researchers who have studied natural gas and climate change issues.
"Over the range of scenarios that we examine, abundant natural gas by itself is neither a climate hero nor a climate villain," said Richard Newell, director of the Duke University Energy Initiative.
The findings are published in a special issue of Environmental Science and Technology, "Understanding the Risks of Unconventional Shale Gas Development." According to the research, abundant shale gas is not likely to substantially alter total emissions without policies targeted at greenhouse gas reduction.
Most evidence indicates that natural gas as a substitute for coal in electricity production, gasoline in transport, and electricity in buildings decreases greenhouse gases. But natural gas production and consumption has higher emissions than renewables and nuclear power. The amount of emissions of methane and carbon dioxide that occur during natural gas production and transport are hard to pin down, the researchers conceded.
"We find that so far increased natural gas has mostly taken the place of coal, but looking forward there also may be increased consumption for sectors such as industry, as well as some degree of displacement of zero-emission sources such as renewables and nuclear," said Daniel Raimi, an associate in research at the Energy Initiative. "The net effect on U.S. greenhouse gas emissions appears likely to be small in the absence of policies specifically directed at greenhouse gas mitigation."
Newell and Raimi modeled two hypothetical futures: one where natural gas production and prices follow a "reference case" scenario, and another where increased shale gas production lowers prices and encourages increased consumption. They also account for a range of methane emissions scenarios, ranging from 25% below to 50% above the levels estimated by the U.S. Environmental Protection Agency (EPA).
"The fact that increased shale gas doesn't have a huge climate impact on its own doesn't mean it's not important. If broad climate policy is enacted, having abundant natural gas could be very helpful by making it cheaper for society to achieve climate goals," Newell said. "If natural gas is expensive, then it will be more costly to switch away from fuels that have higher greenhouse gas emissions, such as coal and oil. But keeping methane emissions low is essential to maximizing the potential benefits of natural gas."
Raimi also emphasized the need to cut methane emissions, but he said that "even if methane emissions from natural gas systems are significantly higher than current EPA estimates, we did not find this significantly alters the impact of abundant natural gas on long-term national or global greenhouse gas emissions pathways."
In their article, the researchers write that more investigation is needed in multiple areas, such as natural gas system methane emissions; the emissions profiles of natural gas versus electricity and oil-based heating systems; greenhouse gas implications of changes in international trade patterns due to shale gas growth; and the likely magnitude of substitution of natural gas for coal versus zero-carbon electricity, both in the United States and internationally.