Transportation is a major contributor to carbon emissions, but historically it has not been the largest source.

However, that has apparently changed.

Transportation-related sources now account for the largest share of U.S. carbon emissions.

DON'T MISS: How much do electric cars really pollute, even without tailpipes?

Earlier this year, transportation overtook all other sectors of the economy—including electric power, industrial, residential, and commercial—to earn that dubious distinction, according to the U.S. Public Interest Research Group (U.S. PIRG).

This is the first time in nearly 40 years that this has happened, the group says.

It cites U.S. Energy Information Administration (EIA) data showing the total carbon pollution of each economic sector over the previous 12 months.

Gas pump

Gas pump

The 12-month total covering May 2015 to April 2016 (the most recent available) shows transportation producing the highest levels of carbon emissions.

That was also the case for the 12-month periods ending in February 2016 and March 2016, U.S. PIRG notes.

The group attributes this to growth in transportation-related emissions, but also to decreases in other areas, such as electric power.

ALSO SEE: Peabody, world's largest private coal company, files for bankruptcy

Coal-fired power plants are being retired at an increasing rate in North America, replaced by a combination of natural gas and renewable-energy sources.

A natural-gas boom in North America has brought down prices, making it a more economically-attractive fuel than coal for many utilities.

That, along with anticipated pressure from regulators on power-plant emissions, has led to a decline of the coal industry.

Portable Emissions Measurement System (PEMS) (Photo by Millbrook Proving Ground)

Portable Emissions Measurement System (PEMS) (Photo by Millbrook Proving Ground)

Earlier this year, Peabody Energy—the world's largest private coal company—filed for bankruptcy.

Meanwhile, the long-term future of U.S. Corporate Average Fuel Economy (CAFE) standards for cars is in doubt.

MORE: Battle over CAFE standards begins now: will they change for 2022-2025?

Regulators now believe carmakers may miss the original target of 54.5 mpg (equivalent to about 40 mpg on the window sticker) by 2025, due to a combination of low gas prices and high demand for SUVs.

That may give carmakers grounds to challenge the standards for 2022 to 2025, creating the makings of a showdown between them and regulators.

[hat tip: Brian Gluckman]


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