States can claim some success in decarbonizing the power sector. As for limiting tailpipe emissions? They've barely managed to move the needle.

That's one reason climate advocates are concerned about EPA's proposal to freeze fuel economy standards at 2020 levels through 2026. Where states have considerable authority to influence the power sector, by setting renewable portfolio standards or pushing for emissions reductions from power plants, they have far fewer tools to manage transportation emissions.

And what tools they do have often remain untouched for want of political will.

Washington state has rejected several carbon tax proposals along with a renewable fuel standard.

In New York, plans for congestion pricing have languished for years.

Next door in Massachusetts, lawmakers recently passed a sweeping energy bill but stripped it of a provision that would have imposed a fee on carbon emitted from transportation and other sectors of the economy.

"It's really, really hard for states to catch the slack here," said Michael Mehling, deputy director of the Center for Energy and Environmental Policy Research at the Massachusetts Institute of Technology. Investments in public transportation and electric vehicle infrastructure are perhaps the best areas for states to tackle transportation emissions, even though "that's a heavy lift," Mehling said. "The money doesn't magically appear."

Liberal states have sought to position themselves as a bulwark against President Trump on climate, pledging to cut climate-changing emissions even as the federal government rolls back programs intended to green America. Yet EPA's proposed fuel economy freeze could hinder their ability to fight back.

Transportation emissions have edged higher in recent years, even as power-sector emissions have plummeted 25 percent from 2005 levels.

The Rhodium Group, an economic research firm, estimates that emissions could grow by 321 million to 931 million metric tons, depending on oil prices, between 2022 and 2035 if the freeze holds.

The single biggest state tool for reducing transportation emissions today is fuel economy standards, and only California has the legal authority under the Clean Air Act to set stricter requirements than the federal government. Now the Golden State's ability to set its own standards is under threat from the Trump administration. That in turn has a trickle-down effect. Twelve states and the District of Columbia have adopted California's standards.

"It is important to acknowledge just how negative the impact of this would be for states in terms of emissions and pollution and clean car availability," said Carol Lee Rawn, who leads the transportation program at Ceres, a nonprofit that works with businesses and investors to promote sustainability.

Environmentalists nonetheless have some reason for optimism. A collection of Northeastern states has been laboring to develop a transportation-sector solution that could mirror the Regional Greenhouse Gas Initiative, a cap-and-trade program encompassing the power sector in nine Northeastern states. By putting a cap on transportation-sector emissions, they hope to create a revenue stream for transportation and electric vehicle infrastructure investments.

"I think there is potential there," Rawn said. "And I think there is growing recognition on the part of states that transportation is an increasing source of concern as emissions from the power sector go down and emissions from transportation goes up."

In New York, proponents of a congestion pricing plan said the federal freeze offered a fresh opportunity for Gov. Andrew Cuomo and New York City Mayor Bill de Blasio, both Democrats, to prove their climate credentials.

Both have pledged to fight the Trump administration's freeze, but the pair have been unable to strike a deal over a congestion pricing plan that would charge motorists who drive into New York City every day, said Charles Komanoff, director of the Carbon Tax Center and a longtime advocate of congestion pricing. Komanoff said he hopes the plan would motivate businesses to locate in the city, instead of the suburbs, by providing a revenue stream to revitalize New York's mass transit system.

"New York City is such a best case for congestion pricing, it is scandalous and frustrating that it hasn't happened yet," he said. "There is no major American city I know of that is doing anything meaningful to bend transportation and land use toward vastly lower emissions that would be really proactive."

The political difficulty of enacting new transportation policy helps explain why the attorneys general of 19 blue states pledged to sue EPA over the proposed freeze yesterday. In the end, the best state strategy for tackling transportation emissions may be protecting the federal government's current rules.

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.