BOSTON (State House News Service) – When Gov. Charlie Baker this summer combed through a clean energy bill he eventually signed into law, one section allowing 10 cities and towns to restrict fossil fuel use in the construction sector gave him “agita.”

And depending on how a tangled web of local decision-making and formal declarations play out in the coming months, Baker, or his successor, might be tasked with deciding whether to allow the largest city in New England to join the pilot program that once turned his stomach.

The Boston City Council plans to vote Wednesday on a home rule petition that aims to rein in greenhouse gas emissions from buildings, the largest source of emissions in the capital city. If the petition is adopted and state officials give the green light, Boston could craft and then impose requirements calling for new buildings and major renovation projects to be free of fossil fuels — an idea that worries real estate industry leaders.

That would achieve a major goal for Boston Mayor Michelle Wu, who made a local Green New Deal a centerpiece of her campaign, and place the city toward the front of a nascent national movement.

Boston Chief of Environment, Energy and Open Space Rev. Mariama White-Hammond told the News Service the effort is “putting our money where our mouth is and really pushing folks to decarbonize as aggressively as possible.”

“There comes a point at which you have to say no more to ways of doing things that you know are not sustainable for the long run,” White-Hammond said in an interview. “Why would we continue to build buildings now that we know are going to lock in carbon dependency and fossil fuel dependency for 50, 70 more years if we’re saying we’re going to be net-zero in 2050?”

In Boston, about 70 percent of greenhouse gas emissions come from the buildings sector, White-Hammond’s office said, making it by far the largest individual contributor as both the city and Massachusetts as a whole move toward a target of net-zero emissions by 2050. Slashing carbon emissions from buildings is “easiest,” White-Hammond said, when they are constructed that way in the first place rather than modified or retrofitted later in their lifespans.

Several other major American cities including New York, Los Angeles and Seattle have kicked off their own efforts to require buildings to deploy some combination of electric-powered heat, appliances and hot water rather than natural gas or other systems powered by fossil fuels.

The route Boston would take toward that sea change, though, remains murky, hinging not only on securing state approval but on the steps taken by smaller Massachusetts cities and towns with similar goals.

A Looming Administration Decision

The pilot program has roots in a disagreement between top Democrats and the Baker administration over a stretch energy code provision in the 2021 climate bill that committed to net-zero emissions by 2050.

Sen. Michael Barrett, who co-chairs the Telecommunications, Utilities and Energy Committee, said lawmakers wanted to allow any municipality “to use that language to go all-electric in new construction.” Baker’s Department of Energy Resources ultimately took a more limited approach in the final code, which did not give cities and towns the ability to mandate all-electric heating in new buildings, as his team raised concerns that banning fossil fuel infrastructure could drive up costs amid a potent housing crunch.

Legislators took another pass at the idea of local fossil fuel bans in the latest clean energy legislation, including a 10-municipality pilot program that would give policymakers a glimpse into both the climate and economic impacts of those restrictions.

Ten municipalities are already in line to join the pilot program, having submitted home rule petitions over the course of the 2021-2022 legislative session. Barrett said some of those measures had been filed in 2021, while others landed on Beacon Hill in the period between Senate passage of the bill in April and Baker’s signature on a final version in August.

However, three of those cities and towns — Arlington, Newton and West Tisbury — are at risk of failing to meet affordable housing requirements added to the legislation late in the lawmaking process, and West Tisbury officials openly acknowledge they do not expect to qualify for the pilot program.

“It’s absolutely out of the question,” Kate Warner, who chairs West Tisbury’s energy committee, said of her town becoming eligible for the program by a February 2024 deadline. “We just don’t do things on the scale that would be needed to meet the goal in 18 months.”

Under a section added to the bill as an olive branch toward Baker, a city or town must either achieve at least 10 percent affordable housing stock under the state’s Chapter 40B law or approve a zoning ordinance allowing multi-family housing by right in a “district of reasonable size” in order to be eligible for the building restrictions pilot program.

West Tisbury has an affordable housing stock of just 1.9 percent and will also be unable to achieve the zoning changes alternative, Warner said.

Officials in Arlington and Newton are more optimistic about their chances. The former had an affordable housing share of about 6.5 percent in April, but Town Manager Sandy Pooler told the News Service he intends to pursue a multi-family by-right zoning change at Town Meeting, which would clear a path.

In Newton, city officials wrote in an August memo that they currently have more than 9 percent affordable housing and called it “possible” to reach the 10 percent figure “sometime in the next six months.”

The other seven communities already in the queue — Acton, Aquinnah, Brookline, Cambridge, Concord, Lexington and Lincoln — all surpassed 10 percent affordable housing inventory in December 2020 state data.

The law Baker signed after a back-and-forth with the Legislature effectively created a queue for the 10 spots. Each would be awarded on a first-come, first-serve basis to municipalities that sent Beacon Hill a home rule petition seeking state approval to limit the role of fossil fuels in the building industry, so long as a city or town fulfilled the housing prerequisites.

But there’s another wrinkle: as drafted, the ordering effect evaporated after the first 10 cities and towns got in line. If a municipality drops out, such as West Tisbury, DOER — which is managed currently by Baker — gains full authority to pick a replacement participant from the full universe of municipalities who want to be in the mix, regardless of whether they occupy the 11th place in line or the 51st.

The Somerville City Council late last month approved its own home rule petition to get onto the waitlist pool, according to meeting minutes, and the Boston City Council will consider a similar step this week. Boston had about 20.6 percent affordable housing stock in December 2020, while Somerville was at 9.7 percent, below but close to the required threshold, state data show.

“If two cities submit home rule bills in successive weeks, let’s say Boston and Somerville, both are to be considered if they have at least 10 percent affordable housing and have filed the home rule bill, but neither is assured a place,” Barrett said. “DOER gets to choose.”

It’s a policymaking butterfly effect. Whether West Tisbury, a tiny town of about 3,500 people on Martha’s Vineyard, informs the state that it will not meet the affordable housing target could unspool a chance for Boston to impose sweeping changes on construction in the city of more than 675,000, and when the process unfolds could determine whether Baker or his successor makes the final call.

A DOER spokesperson who would communicate only on background said the department is still reviewing the new law, which permits but does not require it to promulgate regulations by July 1, 2023 to govern the pilot program. Officials expect to craft a formal process to implement the effort, which would include a schedule for cities and towns to apply, the spokesperson said.

“We Want Speed and We Want Scale”

In Barrett’s assessment, Boston has three primary routes it could take to secure a position in the pilot program. City leaders could try to get their home rule petition approved as a standalone bill without changing the clean energy law itself, which Barrett said would effectively make Boston an 11th community pursuing the same goals as the 10 in the pilot program without formally participating. The city’s legislative delegation could file a bill to lift the program cap beyond 10 municipalities.

Those options both face obstacles with the Legislature set to operate in only informal sessions, where all matters must advance with unanimous consent and a single objection can stall a bill, until the new term begins in January.

The third pathway would be for West Tisbury to make its withdrawal formal, which would open up a slot for a replacement community. There’s no guarantee Boston would take that position since DOER would choose a replacement.

“All three of the underperforming communities have an interesting choice to make around timing,” Barrett said. “Do they occupy a slot for the full 18 months, only to risk vacating it at the very end, or do they do some internal calculations and internal discussion and make a decision about how likely achieving 10 percent (affordable housing) for them is?”

Barrett said he communicated those three options to city leaders. He also said he approached Boston in June, when some communities were already in the queue but not all spots were taken, and urged the city to move quickly. He called it a “missed opportunity” that Boston did not act at that time.

Asked about an invitation extended to Boston, White-Hammond said she “wouldn’t dispute that” but that she herself did not speak directly with Barrett.

While Boston seeks admittance to the pilot program, its efforts have drawn the ire of developers and real estate industry leaders.

Greg Vasil, CEO of the Greater Boston Real Estate Board, said his group is “deeply concerned” about potential impacts on housing production, even as he praised Wu’s “dedication to combatting climate change.”

“Construction costs are already too high due to inflation and national supply chain challenges. Banning fossil fuels in new developments will only increase costs further,” Vasil said in a statement last month. “This ban would be especially problematic in a city like Boston, which produces huge levels of housing and is an economic engine for all development. Housing production is key to overcoming our state’s housing crisis. Instead of taking part in the state’s pilot program to ban fossil fuels in new developments, we believe the city and state should await the results of the pilot program before considering if and how Boston may implement this ban.”

Elected officials and activists, meanwhile, said Boston’s inclusion would be a boon for the pilot program because of its size, which would amplify both the environmental impact of a fossil fuel restriction on construction and generate more detailed data about what that does to costs.

“This is about decarbonizing and climate change,” said Nathan Phillips, a Boston University earth and environment professor and a vocal climate activist in the region. “We want speed and we want scale, and Boston gives the speed, the scale and the environmental and climate justice that I think none of the other 10 communities had or have.”

White-Hammond said she wants Boston’s impact to be one that centers “workers and poor folks,” not just as the largest city in the state.

“I hope that is one of the hugest parts of our contribution to this conversation, and I don’t think it’s been there adequately before,” she said. “That’s something we are really excited to do — not just to set a precedent for development, but to do it in a way that includes folks who have often been left out and will be directly impacted by this transition.”

If the decision on whether to let Boston take an open slot falls to Baker, it’s still not clear how he would respond.

Baker said before signing the bill he was wary the pilot program could exacerbate housing affordability problems. Once Boston sought to get in the mix, he warned that Massachusetts may be “already heading down the road that the Legislature promised me they weren’t going to do, which is just open this door up and basically say anybody who wants to come in through this door can.”

But he stopped short of slamming the door in Wu’s face.

“If she actually can meet the test and all the rest, I know why she wants to get into this,” Baker told GBH’s Boston Public Radio on Aug. 18. “It’s one of the most expensive housing markets among urban communities in the country, so I do worry a little bit about that, and I have to see what it says. But I understand where she’s coming from and would want to try to be helpful on this.”