A new study projects U.S. carbon emissions to fall nearly 17 percent by 2020. That happens to be the target President Obama pledged at the Copenhagen climate conference three years ago.
How's that happening? The study cites a combination of federal regulation, changing electricity and auto markets, energy efficiencies, and state carbon regulation.
Cars are guzzling less these days, due to mileage regulations and shopping preferences, says study author Dallas Burtraw at Resources for the Future.
"A new car built about a decade from now will be twice as energy efficient as one built and sold around 2010," Burtraw says.
Burtraw also credits energy efficiencies -- everything from industrial energy savings to new appliances to LED lights.
"The flagship example is the lightbulb," Burtraw says. "Improvement in the efficiency of lighting has contributed substantially to reduced energy use in the home."
Nationally, power plants are increasingly also burning less coal and more cheap natural gas -- which emits half the carbon-dioxide per unit of energy.
The U.S. emissions projection also assumes the implementation of more EPA air rules on power plants. And to some, that may be a rosy assumption, given the politics of today's regulatory process.
"The variables here are the stringency of the regulation, the pace of the regulation, and how readily EPA can fend off legal challenges," says Elliot Diringer at the Center for Climate and Energy Solutions.
So, a 16 or 17 percent cut in U.S. emissions by 2020 is far from a certainty.
More broadly, Rick Piltz at Climate Science Watch notes that warming is a global issue.
"If you eliminated all the emissions from the industrialized western countries and didn't do anything with the developing countries," Piltz says, "that would be enough to push global warming past dangerous tipping point. Or if you eliminated all those emissions in China and other developing countries, there would be enough in the West over time to push climate over a tipping point."
At the very least, Diringer notes the U.S. hitting its target could put some planetary peer pressure on other countries.