US electric power plants are doing something they haven't done since the 1970s

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U.S. electric power plants are emitting less carbon dioxide than the nation's transportation sector for the first time since the late 1970s, according to analysis by the Energy Information Administration.

The feat, achieved between October 2015 and September 2016, comes as cleaner burning natural gas displaces coal-generated power, more solar and wind energy feed the grid, and regulations enacted by former President Barack Obama rein in pollution from smokestacks.

But efforts to further reduce carbon emissions from the electric power sector could hit a road bump as Donald Trump, who begins his presidency on Friday, aims to increase coal production and get rid of Obama-era environmental regulations.

Emissions from the power sector have been declining since 2007, EIA said. Carbon dioxide accounts for about 80 percent of all U.S. greenhouse gas emissions from human activity, according to the Environmental Protection Agency.

The U.S. electric power sector consumes more energy than the transportation sector, which includes automobiles, planes and trains. Still, carbon emissions from power plants have managed to fall below those from transportation because carbon intensity from electric power generation has fallen faster.

That is largely due to the growing use of natural gas and the declining reliance on coal to generate electricity. Coal is responsible for 61 percent of carbon emissions from the energy sector, while natural gas contributes 31 percent, EIA said.

U.S. natural gas output has boomed in the last decade thanks to a revolution in drilling technology that allows producers to free the fossil fuel from shale rock.

Natural gas facilities are also cheaper to build and operate, according to Robert Godby, director of the Center for Energy Economics and Public Policy at the University of Wyoming. New rules that require plants to install expensive technology to mitigate emissions, as well as the uncertainty of future regulations, have also dissuaded utilities from building new coal-fired plants, he told CNBC in an interview in December.

In its latest annual outlook, EIA said it expects carbon emissions to fall at a slower pace between 2016 and 2040 than they have over the last decade. However, it projects those emissions would fall more dramatically if President Obama's plan to set carbon standards for power plants and require states to create plans to achieve them goes into effect.

The D.C. Circuit Court of Appeals is reviewing a legal challenge by two dozen states to the Clean Power Plan, Obama's policy aimed at combating global warming. But even if the court rules in its favor, Trump has threatened to scrap the plan.

Two other policies that have contributed to carbon emissions reductions appear to be on surer footing.

The Supreme Court last year left intact the EPA's Mercury and Air Toxic Standards, which have expedited the closure of older, inefficient coal-fired plants. Congress also extended tax credits for solar and wind power at least through 2020.

While Trump has vowed to revive the U.S. coal industry, energy economists say that goal will be hard to achieve given another of his campaign promises: to boost the nation's natural gas production.