A group of natural gas pipeline opponents released a report on Thursday suggesting the operator of New England’s power grid used “unreasonable assumptions” when it concluded that rolling brownouts were possible in 2024 without additional gas supplies.

The new report, prepared by Synapse Energy Economics for the Conservation Law Foundation and a handful of other groups, said the power grid operator low-balled renewable energy development, hydro-electricity imports from Quebec, and liquefied natural gas imports to make its case.

David Ismay, a senior staff attorney at the Conservation Law Foundation, said the new study debunks the earlier report issued by ISO New England. “Even in extreme winters far colder than this year’s, New Englanders don’t need to worry about ISO’s ability to keep the lights on as we transition away from fossil fuel electricity,” Ismay said. “ISO’s initial predictions unnecessarily sounded the alarm based on flawed assumptions and unrealistic scenarios that ignore what the states are already doing to increase system reliability.”

Cynthia Arcate, the president and CEO of Power Options, another backer of the Synapse study, said the issues involved are very complicated with lots and lots of variables. Nevertheless, she said, the ISO study went too far. “I would characterize the ISO’s study as very conservative and a bit alarmist,” she said.

Anne George, a vice president at ISO New England, said the conclusions of the two reports center around differing assumptions about the future. She said the “advocates” behind the Synapse report took state projections about renewables and energy imports at face value, while the grid operator didn’t assume energy development would happen as planned.

There were three key differences between the two studies. For its reference case, the ISO assumed 6,600 megawatts of renewable resources available by 2024 instead of the 8,000 megawatts called for under various state renewable energy regulations and laws. ISO also didn’t include in its reference case imports of 1,000 megawatts of hydro-electricity from Quebec, which are expected to come online some time in 2022 or 2023. The ISO analysis also assumed imports of 1 billion cubic feet a day of liquefied natural gas, while the Synapse study said imports could run as high as 1.25 billion cubic feet a day.

George balked at the suggestion that the ISO’s approach is more conservative. “Maybe more realistic,” she said. “We’re trying not to be conservative, but we have to look at what’s coming at us realistically.”

George’s boss, Gordon van Welie, made clear when the ISO study was released in January that he wasn’t advocating for more pipeline capacity. Indeed, he said it was his assumption that new pipelines would not get built because of opposition in Massachusetts and New York.

Nevertheless, the ISO study has become a lightening rod in the growing debate over natural gas pipelines in New England. Steve Dodge, the executive director of the New England Petroleum Council, wrote last month that blackouts are unacceptable in New England, particularly when they are caused “by needless, self-inflicted constraints on our access to abundant natural gas just 300 miles away in Pennsylvania.” The Boston Globe editorial page has also weighed in in support of new pipelines.

One area where both proponents and opponents of new pipelines agree is that an increase in power plant retirements could put the region’s power grid at risk. That point was brought home recently when Exelon announced it planned to shut down its 1,600 megawatt Mystic power plants in Everett in 2022 unless it gets paid more money to keep them operating. The ISO, calling the proposed retirements “an unacceptable fuel security risk to the region,” is seeking approval from the Federal Energy Regulatory Commission to pay Exelon more for at least two years.

The Synapse study carried this ominous warning: “The ISO is anticipating that the Exelon retirement request may be followed by additional retirement requests.”