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For the first time in 40 years, power plants are no longer the biggest source of U.S. greenhouse gas pollution. That dubious distinction now belongs to the transport sector: cars, trucks, planes, trains and boats.

Whatever federal authorities may or may not do to alter parts of the U.S. Phase 2 regulation of greenhouse gases from trucks, California will keep regulations in place that are at least as tough as the October 2016 final rule, and might even get more strict and encompass aspects of nitrogen oxide emissions.

A notice from the U.S. Environmental Protection Agency to revisit parts of the Phase 2 final rule on greenhouse gas emissions from heavy- and medium-duty trucks isn’t likely to create changes quickly, as the agency has not presented plans for expedited action and environmental groups are vowing to pursue a challenge in a federal appeals court — once the case is no longer being held in abeyance.

In a move to respond to concerns by the trailer and glider industry, the U.S. Environmental Protection Agency announced it plans to “revisit” its Phase 2 provisions on greenhouse gas emissions and fuel efficiency for medium- and heavy-duty engines.

MADRAS, Ore. — Fleet owners should not expect the truck and engine sections of the federal Phase 2 rule on greenhouse gas emissions to change before implementation starts in 2021, said Roger Nielsen, the CEO of Daimler Trucks North America.

The Truck Trailer Manufacturers Association has expanded its offensive against the part of the federal Phase 2 greenhouse gas rule that concerns trailers by asking the U.S. Environmental Protection Agency to consider a Trump administration executive order intended to prune burdensome regulations.

The diesel engines built this year are more efficient as the second step of greenhouse-gas Phase 1 standards takes effect. These power plants also offer improved reliability, advanced engineering, reduced maintenance and performance improvements that fleet managers and truck drivers will notice, industry experts said.

California is proceeding with its Phase 2 greenhouse-gas emissions rule for trucks — designed to be “harmonized” with the similar fed-eral rule released last year — no matter the fate of the U.S. regulation.

Eight states and five environmental groups filed motions with a federal appellate court on Jan. 23, asking to intervene in a lawsuit brought by the Truck Trailer Manufacturers Association against two federal agencies over the Phase 2 greenhouse gas rule on truck emissions.

Two trade associations filed petitions for review of the Phase 2 final federal rule on greenhouse-gas regulations for trucks, and the U.S. Court of Appeals for the D.C. Circuit has combined them into a single case to be heard by a panel of three judges to be chosen later.