The real reason carbon emissions had fallen

For those concerned about climate change, news that U.S. carbon emissions trended down for several years recently was welcome. They fell by around 10.5 percent from 6.02 billion metric tons in 2007 to 5.4 billion metric tons in 2013 (although 2013 showed a 2.5 percent rise over 2012). The trend toward lowering the amount of carbon dioxide, an identified greenhouse gas, added to the atmosphere meant moving in the direction of necessary change to prevent additional global warming.

The hope was that a rational shift to less carbon-intensive energy sources had done the trick. But a new paper appearing in Nature Communications this week dashed that thought. The shift to different energy sources was a contributor to the emissions drop from 2007 to 2013 -- but they were responsible for only 17 percent of it.

Instead, a slowing economy due to the global economic collapse was the primary cause, as 83 percent of the drop resulted from decreased consumption of goods and services.

"We can show that most of the increase (prior to 2007) is the level of consumption," Klaus Hubacek, an ecological economist at the University of Maryland and one of the study's co-authors, told CBS MoneyWatch. "After the crash, economic activity goes down, carbon emissions go down." The researchers were able to isolate different factors and show that economic activity was the actual driver of emissions growth.

As the global economy improved, emissions have increased again, as the 2013 rise shows. Furthermore, despite increased U.S. energy efficiency since the Great Recession, pollution is getting exported. "Even though we burned less coal in the U.S., the export of coal has increased," Hubacek said. "What you dig out of the ground, you do something (with)."

A lot of coal mined in the U.S. and Europe goes to China, which is a net importer. Less sophisticated systems there mean Chinese power plants using coal to generate electricity pollute more than equivalent plants in the West.

Also, rising use of natural gas will have limited benefit in reducing future emissions, the paper noted.

What could help? A combination of decreasing the "energy intensity of the U.S. economy" and "radical decarbonization of the energy sector" could help drastically reduce carbon emissions, according to the report. But that will depend on government policies "that can lock in the recessionary emissions reductions and ensure continued decarbonization" through more efficient and low-carbon energy technologies.