The Associated Press reported last week that U.S. greenhouse gas 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.

via Energy, Security, and Climate » The Climate Change Limits of U.S. Natural Gas.