For the first time in five years, power plants across New England are producing more carbon emissions, dealing a setback to Massachusetts’ legally mandated efforts to reduce greenhouse gases and raising concerns that reduced production of nuclear energy will undercut environmental gains.
Last year, the region’s power plants released 5 percent more carbon dioxide than the year before, the first year-to-year increase since 2010, according to ISO New England, an independent company in Holyoke that operates the region’s power grid.
The uptick comes as Massachusetts works to curb carbon emissions in nearly every sector of its economy, in hopes of reaching its 2020 targets.
Massachusetts is legally required to reduce greenhouse gases 25 percent below 1990 levels by that date — part of a national effort to stave off global warming.
“We need this part of the puzzle to continue to fall precipitously in order to have a chance of meeting the 2020 goal,” said Ian Bowles, who served as energy and environmental affairs secretary during the Patrick administration and now helps finance renewable energy projects. “We need steep, sustained declines in emissions here, and not a step back.”
State officials acknowledge the need for greater reform, and have urged lawmakers to pass legislation that would substantially increase the amount of hydroelectric power that is fed into the region’s grid.
“Action is needed on existing policies . . . to further diversify our energy portfolio and meet the goals set forth in the Global Warming Solutions Act,” said Katie Gronendyke, a spokeswoman for the state’s Executive Office of Energy and Environmental Affairs. “The administration continues to work toward our state’s emission reduction goals.”
The cold winter of 2015 may have contributed to the state’s unexpected rise in carbon emissions by increasing the use of electric heaters — which run on power produced by plants that, in turn, contribute to carbon emissions.
But the bigger factor was probably the 2014 closing of the Vermont Yankee nuclear plant, specialists said.
Nuclear power is largely carbon-neutral, and the loss of Vermont Yankee spurred the need for replacement energy, resulting in a 13 percent increase in the use of natural gas-generated electricity.
Such plants last year provided about half of the region’s electricity, according to ISO New England.
While natural gas is thought of as a clean energy source that produces fewer greenhouse gases than coal or oil, burning it still releases substantial amounts of carbon dioxide and methane into the atmosphere.
A second nuclear plant, the Pilgrim Nuclear Power Station in Plymouth, is scheduled to close in 2019; that will again cost the state a carbon-neutral source of electricity.
Overall, carbon emissions in New England have declined by about 25 percent in the past 15 years, and state officials released a report in January that suggested Massachusetts was still on course to meet its climate goals, set forth in a 2008 law that created a framework for reducing heat-trapping emissions.
But the report acknowledged that a significant amount of projected emissions declines depend on the expansion of cleaner energy sources.
Over the next few weeks, lawmakers will debate whether to compel utilities to enter into long-term contracts to buy hydroelectric power from Canada, or other renewable energy from outside of Massachusetts.
For much of the past decade, power plants contributed about 22 percent of the region’s carbon emissions, substantially less than cars and other forms of transportation, which emit more than 40 percent.
Most of the rest comes from homes, businesses, and industrial sources.
While not the largest source of greenhouse gases, the power sector offers the best opportunity to make drastic reductions, officials and advocates say.
“The electricity sector is by far the lowest-hanging fruit,” said Ken Kimmell, who served as commissioner of the state’s Department of Environmental Protection during the Patrick administration. “We need to make disproportionately large cuts there to meet our overall goals.”
Kimmell, now president of the Union of Concerned Scientists, said the region will have to make fundamental changes in its electricity production to reduce its carbon footprint, especially as more residents drive electric vehicles.
“We need to be supplying that electricity through renewable energy,” Kimmell said.
In the state’s January report, officials estimated that nearly 17 percent of required emissions cuts would come from hydropower, or other clean energy imports.
An additional 23 percent would come from increased energy efficiency, and nearly 16 percent would come from new fuel-economy standards.
But environmental advocates have raised doubts that the state can reach the 2020 target thresholds.
In January, lawyers for the Conservation Law Foundation, an environmental advocacy organization based in Boston, argued before the state’s top court that the Baker and Patrick administrations had violated the law by failing to enact the policies necessary to meet the mandated levels.
Advocates have insisted that the state has fallen behind and needs a major course correction — well beyond action on the hydro plan — to meet the law’s requirements.
Even if lawmakers pass a hydroelectric bill, the expensive power lines might never get built, they say.
Some would be likely to pass through New Hampshire, where the proposal remains highly controversial, and others would require an expensive extension into Vermont.
In 2014, before Pilgrim’s owners announced that it would be closed, a collaborative effort by local environmental groups called the Global Warming Solutions Project projected that Massachusetts was on pace to reduce emissions by 20 percent below 1990 levels, well short of the goal.
A more recent report by the Conservation Law Foundation, which factors in the closing of Pilgrim, estimated that the state is more likely to cut its emissions between 16 and 19 percent without major policy changes.
That begins with becoming less reliant on natural gas, environmental advocates say — even though it is cleaner than oil and coal.
“We are over-dependent on natural gas for power in New England,” said Caitlin Peale Sloan, a staff attorney at the Conservation Law Foundation. “Natural gas is no longer a part of the solution in the fight against carbon emissions in New England, but part of the problem.”
Jack Clarke, the director of public policy at Mass Audubon, said the state is running out of time.
“Going backward is a major concern,” he said.
David Abel can be reached at email@example.com. Follow him on Twitter @davabel.