Last week, Cornell University professor Robert Howarth released a preliminary assessment questioning claims that natural gas is cleaner than coal when lifecycle emissions are taken into account. “When the total emissions of greenhouse gases are considered,” Howarth argues,  “[hydraulic fracturing]-obtained natural gas and coal from mountain-top removal probably have similar releases and in fact the natural gas may be worse in terms of consequences on global warming.”

Although Howarth does not provide much detail about his methodology, what he does include in his two-page draft left me extremely skeptical.  At Worldwatch, our optimism about natural gas’s ability to facilitate a swifter transition to a low-carbon economy is predicated on the assumption that consuming natural gas generates significantly fewer greenhouse gas emissions than coal, so I agree that it’s essential to consider climate and other environmental impacts over the fuels’ entire cycle. But in performing a lifecycle assessment, gas and coal must be held to the same standard, and it’s not clear that Howarth is doing this in his analysis.

Here’s how he gets his numbers, as far as I can tell:

He begins by using the carbon emission factors of coal and natural gas to estimate the direct greenhouse gas (GHG) emissions that the fuels release during combustion. Next, he estimates that the additional indirect GHG emissions released during the production, processing, and transport of natural gas are equivalent to about one-third of the emissions released during combustion. Finally, he estimates the quantity of methane leakage that occurs during the production of natural gas and calculates the impact this has on climate forcing, using a 20-year global warming potential (GWP). When the numbers are added up (see Table), natural gas appears to have slightly higher lifecycle emissions than coal.

Emissions (grams carbon per million joules)

Natural Gas Coal
Direct 13.7 24.0
Indirect 4.5 7.9
Methane Leakage 14.8
Total 33.0 31.9
Source: Based on Howarth, “Preliminary Assessment of the Greenhouse Gas Emissions from Natural Gas Obtained by Hydraulic Fracturing.”

The key to Howarth’s finding is a sizable climate forcing effect  – equal to 45 percent of the total climate impact from natural gas – from methane leaked during the production, processing, and transport of natural gas. Methane, the main component of natural gas, is a greenhouse gas about 25 times more powerful than carbon dioxide on a 100-year time frame, and 72 times more powerful than CO2 on a 20-year timeframe, according to the Intergovernmental Panel on Climate Change’s Fourth Assessment Report.

Using a 20-year time frame, as Howarth chooses to do, significantly increases the numbers attached to methane’s climate impact. For example, the 4.6 Teragrams of methane leaked in 2008 from natural gas systems (according to EPA estimates) are equivalent to 330.6 Teragrams of carbon dioxide, or just under the amount of CO2 that the U.S. residential sector emitted last year.

But Howarth’s analysis seems to ignore the amount of methane emitted during coal mining (see table above). As last Monday’s tragic accident at a Massey coal mine in West Virginia illustrates, large amounts of methane are associated with coal seams. In underground mines, mining operations routinely vent methane to the atmosphere in order to avoid buildups that can cause air-quality problems and explosions. Surface coal mines can also release methane as overlying sediments are removed and coal seams are exposed. According to the draft EPA Greenhouse Gas Inventory for 2010, coal mining in 2008 released 3.2 Teragrams of methane – only about 30 percent less than the methane released from natural gas.

The methane emissions associated with underground coal mining (which are estimated based on measurements taken by mine ventilation systems and are therefore less uncertain than other emissions figures) are actually higher than those associated with natural gas systems on a per-Btu basis. Omitting methane leakage from his lifecycle analysis of coal is likely what enabled Howarth to conclude that natural gas’s emissions even come close to those for coal.

As for surface mining, including the mountaintop removal mining that Howarth compares favorably with gas produced from hydraulic fracturing, methane emissions are lower. But the other environmental impacts – including large-scale deforestation, biodiversity loss, water pollution, and landscape destruction – are so high that it’s hard to see how an ecologist could publish anything that might be used as a case for coal over gas.

Methane leakage is a serious challenge that both the natural gas and coal industries have begun to address in the last 10 years. Many in the gas industry in particular have adopted technologies and practices that reduce methane loss through voluntary partnership with EPA’s Natural Gas STAR program, which estimates that in 2008, voluntary actions taken by its partners reduced emissions by more than 114 billion cubic feet, or 2.2 Teragrams of methane. A proposed rule announced by EPA Administrator Lisa Jackson on March 22 would require natural gas industry facilities to report their vented and fugitive methane and carbon dioxide emissions. Methane emissions per Btu of natural gas produced have been on the decline since 1990.

Worldwatch’s Natural Gas and Sustainable Energy Initiative will continue to examine the impacts of methane leakage in the natural gas sector. While we share Dr. Howarth’s urgency about the need to transition to a renewable-based economy, we believe based on our research that natural gas, not coal, affords the cleanest pathway to such a future. We look forward to reading a more complete version of Howarth’s findings. Until more rigorous analysis has been conducted, however, it is irresponsible to offer the coal industry more ammunition in its fight to continue U.S. dependence on what is, we remain convinced, the dirtiest fossil fuel.

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air pollution, Climate Change, coal, methane leakage, natural gas