As lawmakers plan to send a new climate bill to the governor, state officials and lawmakers wrangle over the true costs

Climate change activists crowded into the Massachusetts State House in December 2019.Jonathan Wiggs/Globe Staff/file

When Governor Charlie Baker this month vetoed a landmark bill to address climate change, he told lawmakers that his controversial decision was motivated in part by the legislation’s requirement that the state reduce emissions by 50 percent below 1990 levels by the end of the decade.

That target was only slightly more ambitious than a plan his administration released only a few days before, which sought to cut emissions by 45 percent over the same period. But that difference, he said, would cost the state a whopping $6 billion more.

While Baker said he shared the Legislature’s goal of effectively eliminating emissions by 2050, requiring such steep cuts so quickly risked “imposing undue expense and unnecessary economic impact on Massachusetts households and businesses,” he wrote.

Baker never explained how he arrived at such an astonishing figure and many lawmakers and environmental advocates who have been urging the state to curb emissions more aggressively said the cost estimate seemed exaggerated.

State Senator Michael Barrett, one of the bill’s chief sponsors and lead negotiators, called the $6 billion calculation “misleading.”

Questions about Baker’s estimate have only grown in the week since his veto, as lawmakers prepare to send an identical bill back to the governor as soon as this week. Lawmakers passed the bill with veto-proof margins but couldn’t override Baker’s veto because the legislative session had ended.

Now, after mounting questions and growing frustration from environmental advocates, the Baker administration is defending its estimate, which they said prove it would be foolhardy to mandate such aggressive emissions cuts.

While the difference in emission goals may seem relatively small, it would amount to about 5 million fewer metric tons of greenhouse gases — or 25 percent less pollution than cutting emissions 45 percent below 1990 levels, state officials said.

The state will eventually have to make those cuts, officials acknowledged, but they would be made in a more fiscally prudent fashion over a longer period of time. For example, the costs of replacing an oil-based furnace with a more efficient electric heat pump, or a gasoline-powered car with an electric vehicle, would drop if the older machines were near the end of their operational lives.

“If you go too hard, too fast, you’re incrementally increasing the price tag that’s affecting people’s bottom line, and not taking advantage of opportunities to reduce costs,” said Katie Theoharides, the state’s secretary of energy and environmental affairs.

Baker’s plan, she said, requires that 1 million homes be retrofitted with more efficient heating systems, putting 750,000 electric vehicles on the road, and adding 2,000 megawatts of clean energy by 2030.

To reach the 50 percent threshold, the state would need to eliminate the use of oil to heat all homes, put nearly 1.2 million electric vehicles on the road, add twice as much clean power, and persuade other states to set new regional fuel standards that would reduce by nearly a third the amount of carbon in transportation fuels, Theoharides said.

To do that, the state would likely have to increase electric car incentives by 600 percent, costing the state an estimated $2.4 billion more over the next eight years; eliminating oil heating in all homes and businesses in that time would likely cost about $3 billion more; and producing low-carbon fuels could cost at least $400 million over a five-year period, she said.

Those projections don’t include technical advances that would be needed, federal review and permitting challenges for citing new clean energy, and expansion of the capacity of the region’s electrical grid, she said.

“We believe the Legislature should use analysis, and not use arbitrary or aspirational goals,” Theoharides said. “We should be aggressive as we can be, but we also must back up our work and put in place policies that can achieve our goals.”

After reviewing the state’s explanation for the additional costs, Barrett said state officials were purposefully trying to make the 50 percent standard seem more expensive than it likely would be.

He said they weren’t accounting for the wide ranges in their estimates of overall costs and instead “fixated” on data that would “exaggerate their differences with the Senate numbers.”

For example, he noted that the Baker plan suggested that the state could ultimately achieve as much as 48 percent fewer emissions by 2030.

“So, really, we’re down to 2 percent, and they’re arguing 2 percent is reckless? Come on!” Barrett said. “Arguing the sky is going to fall, when all numbers are subject to significant uncertainty and margins of error, is unwarranted.”

He and others also said the state didn’t account for potentially generous federal subsidies from the new Biden administration that could help defray the state’s costs.

State Representative Joan Meschino, a Hull Democrat and another sponsor of the climate bill, said the legislation could be amended to account for new developments.

“The Legislature set an ambitious goal with ambitious interim targets, because goals matter,” she said, adding the bill aimed to spark innovation across the economy and was “designed for frequent revision.”

She urged fellow lawmakers to send the bill back to the governor as soon as possible, as the Senate president and House speaker have promised. “While we need to understand the cost of implementation, more important is the enormous cost of inaction,” Meschino said.

Environmental advocates also raised doubts about the state’s explanation and urged administration officials to release more data to support their assertions.

“The truth is we can implement smart climate policy while creating jobs and protecting Massachusetts communities, especially those most burdened by pollution and climate disasters,” said Caitlin Peale Sloan, interim director of the Conservation Law Foundation in Massachusetts.

Others cited offsetting benefits to curbing carbon pollution more quickly, such as improvements to public health and the economy. Deborah Donovan, a senior policy advocate at the Acadia Center in Boston, said less pollution would result in fewer sick days, and retrofitting buildings would likely produce more jobs.

“It’s important to look at the full range of costs and benefits when setting our goals and creating a suite of policies that will get us there,” she said.

Even if the higher estimates are accurate, they represent a fraction of the state’s wealth, said Eugenia Gibbons, director of climate policy at Health Care Without Harm in Boston.

The additional $6 billion amounts to just 1 percent of the state’s gross domestic product in 2019, she noted.

More stringent state standards would send a signal to markets that “transformation is the goal” and “climate solutions are the priority,” she said.

“Instead of arguing over the challenges that may present themselves, we need to be working together to fully realize innovative solutions that make ambitious climate action achievable,” she said.