For decades, the United States increased the amount of carbon pumped into the atmosphere each successive year. But recently emissions have begun to decline, dropping about 11 percent between 2007 and 2013. Some scientists and media reports attributed the change to the rise of hydraulic fracturing, or fracking, and the replacement of "dirty" coal with clean-burning natural gas.
But a new analysis of national consumption patterns finds that natural gas played only a minor role in the carbon story—the root of the decline can be found in the Great Recession of 2007.
Starting around the middle of the 20th century, the majority of U.S. electricity was generated through the burning of coal. In 2007, 50 percent of the nation’s electricity came from the fossil fuel. But by 2012, coal accounted for just 37 percent of the country's electricity generation, because it was replaced mostly by natural gas.
Burning natural gas emits half as much carbon dioxide per unit of energy as burning coal. Because the decline in carbon emissions began at about the same time that natural gas use started rising, many thought the two events must be related. But no one had actually looked closely at the reasons for the emissions decline. So Klaus Hubacek of the University of Maryland, College Park and his colleagues examined the major contributors to U.S. carbon emissions since 1997, including population growth, changes in consumption volume, patterns of consumption, the amount of energy used and the mix of fuels used to create that energy.
The team found that prior to 2007, the rise in emissions was primarily driven by increases in the amounts of goods and services consumed, as well as a growing U.S. population. Then, in mid-2007, the U.S. housing bubble burst, sparking a severe recession. Unemployment more than doubled over the next two years. Incomes dropped, and the poverty rate increased. And between 2007 and 2009, carbon emissions fell by 9.9 percent.
In effect, more than half the carbon decline was due to a drastic drop in the volume of goods consumed by the U.S. population. Almost a third of the drop could be attributed to changes in production structure, including offshoring American industries to China and other countries. Only 17 percent could be attributed to changes in the mix of fuels used to generate energy, and that wasn’t due to the rise in fracking. The shale gas boom didn’t start until 2009, the researchers note. Before that, coal had already been on the decline.
“Changes in the U.S. fuel mix from 2007 to 2009 alone would not have caused a decrease in U.S. emissions,” the team concludes in their paper, which appears this week in Nature Communications.
After 2009, as the economy began to recover and Americans started consuming goods in greater volumes, carbon emissions decreased by only 0.2 percent each year, on average. At that point, the shale gas boom began to have an effect on carbon emissions. But even then, it wasn’t the biggest factor in the decline. Changes in production and consumption dominated from 2009 to 2011, and after that, a mild winter in 2012 and high gas prices from 2011 to 2013 meant that Americans used less energy overall, emitting less carbon.
The Obama Administration has set targets for reducing U.S. carbon emissions by 17 percent in 2020 and 83 percent in 2050, relative to 1997 levels. Getting there won’t be easy. “Further increases in the use of natural gas in the U.S. may not have a large effect on global greenhouse gas emissions and warming, and further emissions reductions due to decreases in energy intensity are not inevitable,” says study coauthor Laixiang Sun of the University of Maryland.
The natural gas boom could even make the situation worse. In the short term, gas will compete with renewables, such as wind and solar, notes Hubacek. Plus, coal may be on the decline in the United States, but the nation is still exporting the fuel to China and other countries, which, in effect, exports the resulting carbon emissions around the world.