WASHINGTON (Reuters) - U.S. emissions linked to climate change fell last year on a record drop in coal-fired power generation, but further declines are unlikely without rapid policy changes, according to an estimate released on Tuesday.

Emissions fell 2.1% in 2019 as coal-fired electricity output dropped 18% to the lowest level since 1975, according to the estimate here by independent researcher Rhodium Group, which analyzed preliminary U.S. energy and economic data.

It was a change of direction from 2018, when emissions rose between 1.5% and 2.5% due to increased heating demand and a robust economy that pushed planes and trucks to guzzle fuel, according to the group’s estimate.

Official U.S. data on 2018 and 2019 emissions have yet to be released.

“The good news is we’re making really impressive progress in reducing emissions from electricity,” Trevor Houser, the head of energy and climate at Rhodium said in an interview. “The bad news is that electricity is only 25% of total U.S. emissions, and we’re making almost no progress anywhere else in the economy.”

U.S. utilities have been shuttering scores of old and inefficient coal-fired power plants in recent years due to competition from lower-cost natural gas, incentives for solar and wind power, and concerns about climate change.

But emissions from transportation, which makes up nearly 30% here total U.S. greenhouse gas emissions, have been harder to cut. They fell only 0.3% in 2019, Rhodium said, while direct emissions from buildings rose 2.2%, and emissions in agriculture, land use, and methane leaks from oil and gas operations rose by 4.4%.

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U.S. vehicle emissions were basically flat in 2019 as an increase in miles traveled was offset by efficiency standards put in place during the Obama administration, Houser said.

President Donald Trump’s administration froze the Obama-era standards on vehicle efficiency - part of a broad campaign to slash regulations and boost output of oil and natural gas.

Houser said Trump’s vehicle efficiency policy “raises doubts about our ability to ... turn what is now flat transportation emissions into declining transportation emissions.”

The Trump administration has also started the process of removing the United States from the 2015 Paris Agreement on climate, in which Washington agreed to cut U.S. emissions by 26% by 2025.

Hitting that target will be “extremely challenging”, Rhodium Group said, because it would require cuts significantly faster than the 0.9% annual average since 2005.

Rhodium added that further big emissions reductions from coal plant retirements will be harder to achieve in the future because many of the remaining facilities are competitive.