US carbon emissions fall to 18-year low. What's behind it?

US carbon emissions fell in 2012 – again – after peaking in 2007. The Great Recession and a boom in cleaner natural gas are widely credited as driving the reduction, but broader, longer-term shifts are also changing the way Americans use energy.

The US economy is expanding. Population is growing. But carbon emissions continue to decline.

That's the good news. If the trend continues, it would suggest that post-industrial nations can grow while keeping emissions in check. The bad news is that the reduction is limited to the United States whereas the threat of increasing global emissions is global. 

Energy-related carbon emissions in the US dropped 3.8 percent in 2012 to levels not seen in 18 years, according to a report released Monday by the US Energy Information Administration (EIA). It's the second largest annual decrease in carbon emissions since 1990, behind the recession year of 2009, and it happened in a year where gross domestic product grew by 2.8 percent and population grew by 0.7 percent. 

The Great Recession and a shift from coal power to cheaper, cleaner-burning natural gas are widely credited as driving the reduction in US emissions. But Monday's EIA report points to broader, longer-term shifts that underpin an evolution in American energy use.

If emissions continue to decline amid economic recovery, it suggests the limited progress on carbon pollution is more structural than it is a result of the temporary cutbacks inherent to a recession or a boom in cleaner natural gas.  

Over the past five decades, the service industry has supplanted energy-intensive manufacturing as a dominant engine of growth, and it typically produces far fewer emissions. Innovation and government standards have dramatically driven up the efficiency of cars and appliances. Utilities, consumers, and communities have more information about their energy use than in any time in history.

Those structural changes have a big impact, says Perry Lindstrom, an EIA analyst. "If you’re making software instead of steel, for example, it requires much less energy," Mr. Perry says in a telephone interview.

An industrializing country is very energy-intensive, Lindstrom says, but in a post-industrial age it takes less energy to do the everyday things that fuel an economy. "You don’t have to hop into a car to buy a book anymore."   

The bad news for CO2 emissions is that more and more people across the world are hopping into cars. The post-industrial US is an outlier in an industrializing world led by booming middle classes in China, India, and elsewhere. Worldwide carbon dioxide emissions rose by 1.4 percent to 31.6 billion tons in 2012, according to the International Energy Agency. 

Even in the US, CO2 emissions are forecast to tick back up in the short term. Coal use will bounce back as natural gas prices increase, EIA projects, driving a 1.7 percent emissions increase in 2013 and a 0.9 percent increase in 2014. Weather plays a large role, too. Half of last year's decline was from the residential sector, according to EIA, where a very warm first quarter of the year lowered energy demand and emissions.

Despite short-term trends and anomalies, Monday's report points to long-term fundamentals that have been and could continue to keep a cap on carbon emissions in the US.

Energy intensity – energy consumption per unit of GDP – has declined in 11 out of the past 12 years, reflecting a more efficient use of energy. Carbon intensity – carbon dioxide per unit of GDP – has declined in four out of the past five years, reflecting a cleaner portfolio of fuels.

[Editor's note: A previous version of this article stated that energy use is growing in the US. Energy use actually fell by 2.4 percent in 2012, according to EIA.]