The Climate Change Limits of U.S. Natural Gas

The Associated Press reported last week that U.S. greenhouse gas carbon dioxide emissions have dropped to a twenty-year low on the back of abundant natural gas. “The question,” it correctly observed, “is whether the shift is just one bright spot in a big, gloomy [climate change] picture, or a potentially larger trend.”

I’ve argued repeatedly in the past that surging supplies of natural gas are good news for climate change. But there are important limits to what U.S. natural gas can do. This post is going to illustrate those with some simple numbers.

Let’s start with a reference point. In 2009, in advance of the Copenhagen climate summit, the United States pledged to reduce (PDF) its greenhouse gas emissions to 17 percent below 2005 levels by 2020. It also repeatedly emphasized its intention to reduce those emissions to 30 and 42 percent below 2005 levels by 2025 and 2030 respectively.

How far down that road could a shift from coal to gas get the United States?

I’m going to focus on carbon dioxide emissions from energy. The EIA currently projects that U.S. emissions will be 5,429 million metric tons of carbon dioxide (MtCO₂) by 2020, assuming that currently pending fuel economy rules for 2017-25 go ahead as planned. 1,787 MtCO₂of that total would come from coal; 1,371 would come from natural gas.

That already reflects a gradual substitution of gas for coal. But what would happen if natural gas completely replaced coal? Assume that the emissions from gas are about half those from coal. Then U.S. emissions would drop to 4,536 MtCO₂. That’s 24 percent below 2005 levels.

That leads to our first conclusion: substituting natural gas for coal has the theoretical potential to get us to our 2020 carbon goals. But, unless we deploy it with carbon capture and sequestration, it cannot get us to our 2025 or 2030 goals. (The 2025 and 2030 comparisons require a little bit of extra math that I won’t go through here.) One can push this a bit farther, supposing that natural gas completely replaced oil in residential, commercial, and industrial applications. Oil use in those three sectors is projected to generate 462 MtCO₂ in 2020; replacing oil with natural gas could in principle reduce those emissions by somewhere around 150 MtCO₂. That doesn’t change our bottom-line conclusions.

But we’re not done. These figures are extreme limits that assume spectacular gains in natural gas use. Alas those gains aren’t practical.

Focus on the coal-to-gas shift. I estimated that a complete replacement of coal with natural gas could slice 894 MtCO₂ off of U.S. emissions. You need to burn about 18.2 Mcf (thousand cubic feet) of natural gas to generate a ton of greenhouse gas emissions. This implies that completely replacing U.S. coal with natural gas would require roughly 16 trillion cubic feet (Tcf) of additional natural gas. That’s a 60 percent increment to projected natural gas supplies in 2020. Put another way, it’s more than double the amount of natural gas currently used in U.S. power plants.

This is almost certainly not a practical addition to U.S. natural gas production. Perhaps a more reasonable (but still challenging) outer limit would see half of the U.S. coal use currently anticipated for 2020 replaced with natural gas. That would result in U.S. emissions 17 percent below 2005 levels, meeting the strict part of the Copenhagen commitment but leaving a big lift for other shifts to deliver on the follow-on targets.

The bottom line? Natural gas can do a lot to bend the U.S. emissions curve over the coming years. In even the medium run, though, simply moving from coal to gas is not a substitute for broader policy, at least not if the United States wants to realize the sorts of emissions cuts that both Barack Obama and John McCain talked about only four years ago. Best to think of gas as a climate opportunity – to forestall construction of long-lived and highly polluting infrastructure,  to make carbon capture and sequestration cheaper, to balance intermittent renewable sources – rather than as a solution in itself.