The more than 1.5 million cars, trucks, boats and planes in Maine account for an ever-greater share of the state’s greenhouse gas pollution, presenting major regulatory challenges at a time when emissions from other sources are falling.

Between 2002 and 2015, emissions of climate-warming carbon dioxide from power plants declined 73 percent due largely to a shift to cleaner-burning natural gas and Maine’s participation in a multistate regulatory program on greenhouse gases. Likewise, emissions from Maine’s business and residential sectors fell 37 percent and 14 percent, respectively, according to data from the Maine Department of Environmental Protection.

Meanwhile, carbon dioxide emissions from the transportation sector in Maine fluctuated but remained flat overall during that period. As a result, emissions from cars, trucks and other vehicles accounted for 52 percent of all emissions in Maine in 2015, up from roughly 40 percent at the turn of the century.

Nationally, the transportation sector accounted for 28 percent of greenhouse gas emissions in 2016 – roughly half transportation’s 54 percent share in Maine that year, according to federal data.

That juxtaposition highlights the challenge of reducing transportation-related emissions in a large, rural state where limited mass-transit options mean residents and businesses rely heavily on cars or trucks. It also comes at a time when report after report predicts that Maine’s economy – particularly the commercial fisheries and outdoor recreation industries – as well as cultural traditions face severe impacts from warming waters and temperatures.

Environmental advocates as well as Gov.-elect Janet Mills are citing those statistics as proof that Maine needs to aggressively expand the number of electric vehicles and access to mass transit in the state.

“It really is obvious that transportation needs to be an important part of our climate solution,” said Dylan Voorhees, climate and clean energy director at the Natural Resources Council of Maine.

In many ways, Maine has been among the national leaders in seeking to address climate-warming gases.

It is part of the nine-state Regional Greenhouse Gas Initiative, which established the first cap-and-trade regulatory program on power plants in order to require emissions reductions. That program is partly credited with lowering carbon dioxide emissions across the Northeast while generating more than $3 billion for states from the sale of emissions “allowances.” Most of Maine’s share of that money has flowed to energy-efficiency and weatherization programs.

Maine also hitched its vehicle emissions standards to the stricter limits required of new cars sold in California, although the Trump administration is seeking to undercut the ability of California (and therefore other states) to exceed federal pollution standards.

Yet state and federal data show that Maine has made little to no progress – at least in sheer numbers – in addressing transportation-related emissions in recent decades.

In 1990, cars, trucks and other vehicles in Maine emitted an estimated 8.2 million metric tons of carbon dioxide into the air, according to data from the U.S. Energy Information Administration. Transportation-related emissions gradually declined to 7.5 million metric tons five years later but began to creep back up until hitting record highs of 9.3 million tons in 2005 and 2006 during the period of national economic expansion.

In 1997, the transportation sector surpassed businesses as the top contributor of greenhouse gases in Maine – a position it has not only held but widened since then.

Cars, trucks and other vehicles pumped an estimated 8.8 million and 8.9 million metric tons of carbon dioxide into the atmosphere in Maine in 2015 and 2016, respectively. The business sector, which two decades ago was the top source of emissions, emitted half of that amount in 2015, according to the federal data.

Kathleen Meil, Maine policy advocate for the nonprofit Acadia Center, which focuses on clean-energy issues, called the transportation sector’s contribution to the challenge “astounding.”

“As other sectors become less carbon-intensive, the piece of the pie for the transportation sector has grown,” said Meil. “The other part of it is we have not taken the (concrete) initiatives with transportation emissions that we have with other sectors.”

In a recent report titled “Building a Stronger Maine: Memorandum to the Next Governor,” the Rockport-based Acadia Center said modernizing Maine’s transportation system could create up to 8,700 new jobs with more than $1 billion in new wages. The Acadia Center has recommended Maine work toward goals of reducing greenhouse gas emissions by 45 percent by 2030 by, among other things, using 500 electric-powered buses and moving toward 17 percent of passenger cars running on electricity.

Meil said the nine-state Regional Greenhouse Gas Initiative that regulates power plant emissions is a “great model” for the transportation sector. And both Meil and NRCM’s Voorhees hope that, under Gov.-elect Mills, Maine will take a more active role in the 13-state Transportation and Climate Initiative that is attempting to take a regional approach to transportation-related emissions.

A core group of the initiative – not including Maine – is expected to release new recommendations and policy statements in the coming days.

“Although many of the states active in this right now have rural populations, they are also states that are dominated by large population centers,” Meil said. “We’re not (in Maine) and we need to make sure rural areas are represented.”

Mills made addressing climate change a major focus of her campaign – something that has been lacking in statewide elections for several cycles in Maine. Speaking at an Acadia Center clean-energy event last week, Mills said that while the issue of climate change is not new, her administration will “take it seriously and welcome new and innovative approaches to prevention and mitigation at all levels.”

Mills mentioned using rebates and incentives offered through the Efficiency Maine program – partly funded by RGGI proceeds – to “reduce home heating costs and to encourage the use of electric vehicles.” She also pledged to “promote a 21st-century transportation infrastructure and system that supports and encourages all modes of transportation, especially those that will reduce carbon pollution, like electric vehicles.”

“I stand here today as the next governor of Maine to tell you that under my administration, Maine will be open for business for renewable energy once again and that this state will again be committed to investing in innovative technologies to create jobs, cut costs, and reduce emissions,” Mills said, according to her prepared remarks.

Voorhees said electric vehicles could be a major part of Maine’s effort to reduce transportation-related emissions, especially as automobile manufacturers expand their electric offerings and businesses offer more recharging stations. But he also said improving mass transit, ride-sharing or other infrastructure in Maine – and not just in more urban areas – is also critical.

“It is going to take time,” Voorhees said. “You have to work hard at it because no matter what you do, you can’t bend the (emissions) curve quickly … and it’s not slow-and-steady. We don’t have time to be slow. But we have to be steady” in addressing transportation emissions.