The city of Boston on Tuesday launches Carbon Free Boston, which officials describe as the city's next step to becoming carbon neutral by 2050.

The initiative aims to address how to fuel a growing Boston while also meeting the city's climate goals.

There are currently 106 buildings, worth $9 billion, under construction in Boston. Over the past five years, the city has approved adding the square footage equal to 45 new Prudential Towers.

"We're very fortunate right now," said Austin Blackmon, the city's chief of energy, environment and open space. "Boston is in one of its largest building booms in its history, and that means there are great opportunities for us to have a lasting impact on the future of our city."

Boston already has more LEED-certified energy efficient buildings than any other city in the nation. And Massachusetts has been named the most energy efficient state seven years in a row.

By 2050 the state is committed to reducing emissions from fossil fuels by 80 percent. Boston is even more ambitious: It wants to be carbon neutral by then.

What it doesn't have is a road map detailing how to get there.

"We need to understand what policies and actions are going to be most cost-efficient and most effective to help us accomplish that carbon neutral goal," Blackmon said.

Boston is setting up a new team of outside consultants, city staff members and researchers from Boston University's Institute for Sustainable Energy to investigate the alternatives and plot a path to a carbon neutral city by 2050.

They'll focus on four sectors to curb climate emissions: transportation, waste, electricity and heat. Heat is the largest sector and toughest place to cut greenhouse gases. Forty percent of energy used to heat homes and companies comes from burning natural gas, and Boston's building boom has fueled the need for more.

"Gas demand continues to be strong in Massachusetts," said John Stavrakas, National Grid's vice president of gas asset management, who oversees the utility's network of gas pipelines in the state. Stavrakas looks five and 10 years out to plan to meet future needs for natural gas.

"People want gas," he said. "We connect 11,000 to 12,000 new customers a year. There's been tremendous growth in the Seaport [District of Boston], so there is a need for additional gas supply to serve that customer base."

Using natural gas instead of coal to generate electricity has dramatically reduced climate emissions in Massachusetts, and demand has declined. And Stavrakas says natural gas can be a bridge fuel that can cut carbon emissions for heating as well.

But critics say more gas is a bridge too far.

Ania Camargo wants to stop the building of new fossil fuel infrastructure in Boston.

She's with Mothers Out Front, a group trying to get leaky gas pipes plugged in the city, and a member of the Boston Clean Energy Coalition, which wants to halt a new gas pipe from being laid under city streets to service a hotel being constructed near the Christian Science Center. (She's also volunteered to co-lead an effort to raise awareness and funding for an environmental reporting initiative at WBUR.)

"We can't be carbon neutral by 2050 if we continue to build with natural gas," she said. "The only way we can get there is if we transition from fossil fuels to renewable energy."

A call for radical change in constructing new buildings comes from perhaps a surprising voice: John Fish, who's president and CEO of Suffolk. The Boston-based firm is the biggest builder in New England and one of the largest in the nation.

Fish said new sustainable construction methods and materials will help reduce the demand for fossil fuels, "and there's no doubt with all the innovation and technology that's going on, we can do a much better job than we're doing now."

But Fish says the marketplace -- developers and buyers -- needs to be educated and make the big changes needed to reduce emissions.

Less than 10 percent of the lifetime costs of a building are in constructing it; the rest is maintenance and utilities. Fish says you pay now, or you pay more later.

"The long-term cost, I would argue, to build more sustainable technologies and buildings is much more competitive than it is constructing buildings the way we build them today," Fish said.

As Boston's skyline rises, the choices we make over the next few years on how we're going to fuel the city's future, and heat the buildings we construct today, will have profound effects, Fish says.

"We have to make this investment now if we're going to be able to preserve ourselves for the future," he said. "If we don't, the price will be tenfold what it could be today. We need to come together as a thoughtful community [and say not], 'What can we do?' but 'What are we going to do?' "

Boston's new committee that will consider how the city will fuel its future is expected to release its report next year.