WASHINGTON – Heat-trapping pollution from U.S. power plants hit a 27-year low in April, the Department of Energy announced Wednesday.

A big factor was the long-term shift from coal to cleaner and cheaper natural gas, said Energy Department economist Allen McFarland. Outside experts also credit more renewable fuel use and energy efficiency.

Carbon dioxide – from the burning of coal, oil and gas – is the chief greenhouse gas responsible for man-made global warming.

“While good news for the environment, we certainly would not want to assume that this trend will continue and that we can simply relax,” said John Reilly, co-director of MIT’s Joint Program on the Science and Policy of Global Change.

Electric power plants spewed 141 million tons of carbon dioxide in April, the lowest for any month since April 1988, according to Energy Department figures. The power plants are responsible for about one-third of the country’s heat-trapping emissions.

April emissions peaked at 192 million tons in 2008 and dropped by 26 percent in seven years.

Carbon pollution from power plants hit their peak in August 2007 with 273 million tons; summer emissions are higher because air conditioning requires more power.

In past years, experts said the U.S. reduction in carbon dioxide pollution was more a function of a sluggish economy, but McFarland said that’s no longer the case.

“You don’t have a 27-year low because of an economic blip,” McFarland said. “There are more things happening than that.”

The price of natural gas has dropped 39 percent in the past year, he said. Federal analysts predict that this year the amount of electricity from natural gas will increase 3 percent compared to last year while the power from coal will go down 10 percent.

Those reductions were calculated before this week’s announcements of new power plant rules. The new rules aim to cut carbon pollution from electricity generators another 20 percent from current levels by 2030.

The pollution cuts in April are because efficiency has cut electricity demand and energy from non-hydropower renewable sources has more than doubled, said Princeton University professor Michael Oppenheimer.

“A factor behind all these trends is that the writing is on the wall about the future of coal and thus the future of U.S. carbon dioxide emissions,” Oppenheimer said in an email. “The regulatory noose is tightening and companies are anticipating a future with lower and lower dependence on fossil fuels and lower and lower carbon dioxide emissions.”