The reduction in the United States’ carbon dioxide emissions between 2007 and 2013 was driven mainly by the economic recession, reports a paper published this week in Nature Communications. The shift to less carbon-intensive energy sources, such as natural gas, played a comparatively small role in reducing emissions during this period.

Between 2007 and 2013, US fossil fuel CO2 emissions dropped by about 11%. This decline has been attributed to innovations in hydraulic fracturing technology, which has dramatically increased the country’s supply of natural gas since 2007. However, the factors that contributed to the decreased emissions have not yet been quantified, and thus the role of the “gas boom” remains speculative.

Klaus Hubacek and colleagues report the sources of change in US CO2 emissions over periods of mostly increasing emissions (1997-2007) and mostly decreasing emissions (2007-2013). They find that although increasing emissions prior to 2007 were driven by economic and population growth, 83% of the decrease between 2007 and 2009 was a result of decreased consumption of goods and services. Only 17% of the decrease was related to changes in the fuel mix. During the 2009-2013 economic recovery, factors such as high gasoline prices, mild winters in 2012, and more energy-efficient manufacturing were more important in keeping emissions from rising.

The authors suggest that further increases in the use of natural gas may be of limited benefit in reducing emissions as the US economy recovers and grows, and that future reductions will depend on policies like the Environmental Protection Agency Clean Power Plan.