In a decision environmental advocates hailed as a landmark, the state’s highest court ruled Tuesday that Massachusetts regulators must set specific limits on various sources of greenhouse gases to comply with the legal obligation to reduce emissions linked to climate change.
For years, environmental groups have argued that both the Patrick and Baker administrations have not done enough to meet the mandates of the state’s 2008 Global Warming Solutions Act, which requires the state to cut its greenhouse gases 25 percent below 1990 levels by 2020.
State officials have countered that the law only requires the state Department of Environmental Protection to set emissions targets, not hard caps, and that it gives the agency broad discretion over how to reach them.
In a rebuff to the state, the Supreme Judicial Court sided with the environmental groups and in a unanimous decision ordered the Baker administration to enact specific policies to carry out the required emissions cuts.
“The purpose of [the law] is to attain actual, measurable, and permanent emission reductions in the Commonwealth, and . . . to ensure that legally mandated reductions are realized by the 2020 deadline,” Justice Robert J. Cordy wrote, overturning a Superior Court ruling.
Cordy described the language of the Global Warming Solutions Act as “unambiguous” and said it requires the state to establish “limits on multiple greenhouse gas emissions sources . . . [that] must decline on an annual basis.”
Recent state policies, such as promoting electric vehicles through tax incentives and coordinating with other states to curb carbon emissions, “fall short of complying with the requirements,” Cordy wrote.
The decision ordered the state to limit overall emissions from specific sources, such as vehicles and power plants, set specific annual limits on those emissions, and require them to decline each year.
In addition to the 2020 threshold, the law also requires the state to cut emissions 80 percent below 1990 levels by 2050.
Existing regulations “fail to ensure the type of mass-based reductions in greenhouse gases across the sources,” Cordy wrote.
Environmental advocates described the ruling as historic.
“Today, our highest court declared clearly and unequivocally that our leaders can no longer sit on their hands while Massachusetts communities are put at risk from the effects of climate change,” said Jenny Rushlow, an attorney who argued the case on behalf of the Conservation Law Foundation, a Boston advocacy group that sued the state in 2014.
She urged the state to begin drafting new regulations immediately.
“Thanks to this landmark decision, our role as a national leader in battling climate change has only been stalled, but not sacrificed,” Rushlow said.
State environmental officials said they are reviewing the ruling.
“The Baker-Polito Administration . . . remains committed to meeting the Global Warming Solutions Act goal of reducing greenhouse gas emissions . . . as well as achieving greater reductions for 2030 and beyond,” Katie Gronendyke, a spokeswoman for the Executive Office of Energy and Environmental Affairs, said in a statement.
Dan Dolan, president of the New England Power Generators Association, a Boston-based trade group, said he hoped the court’s ruling would “provide certainty about what will be the expectations for reductions from power plants, transportation, and other sectors.”
At the same time, he urged officials and lawmakers to act cautiously before enacting policies to comply with the court’s ruling, such as passing a law to import large amounts of hydroelectric power from Canada.
“Given that none of those expectations have been spelled out, it is premature to pass new legislation that would carve up the electricity market for provincially owned hydropower or other resources,” he said.
The ruling came a day after the Globe reported that the region’s power plants last year produced 5 percent more carbon emissions than the year before, the first increase since 2010, according to ISO New England, an independent company in Holyoke that operates the region’s power grid.
While carbon emissions in New England have declined by about 25 percent in the past 15 years, state officials have acknowledged the need to do more to meet the requirements of the 2008 law. They have urged lawmakers to pass legislation that would substantially increase the amount of hydropower and other sources of renewable energy.
Without such new sources of clean energy, the state may struggle to cut emissions sufficiently, especially since the Vermont Yankee nuclear power plant closed in 2014 and the Pilgrim Nuclear Power Station is scheduled to close in 2019.
Nuclear power is largely carbon-neutral, and the loss of Vermont Yankee spurred the need for replacement energy, boosting the use of natural gas-generated electricity by 13 percent. Natural gas plants — which produce far more emissions than nuclear plants — last year provided about half of the region’s electricity, according to ISO New England.
In January, state officials released a report that suggested Massachusetts was on course to meet its emissions requirements. Officials estimated they could achieve the required emissions cuts with increased reliance on hydropower, more energy efficiency, and new fuel-economy standards for vehicles.
Environmental advocates have raised doubts about the state’s plan.
In 2014, before Pilgrim’s owners announced 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 just 20 percent below 1990 levels.
A more recent report by the Conservation Law Foundation, which factors in the closing of Pilgrim, estimated the state is more likely to cut its emissions between 16 and 19 percent without major policy changes.
Environmental advocates said Tuesday’s ruling provided clear evidence that the Baker administration needs to “change the way it does business.”
The ruling is “first and foremost a victory for the residents of the Commonwealth, and their desire for clean air,” said Jack Clarke, director of public policy for Mass Audubon. “The targets for reducing heat-trapping air pollution are not ‘aspirational,’ as the Commonwealth claimed, but actual, real, and statutorily required.”