Mayor London Breed is expected to announce Wednesday that the city will set a more aggressive course for reducing its greenhouse gas emissions, including requiring all buildings constructed starting in 2030 to be net-zero carbon emitters.

Shrinking the carbon footprint of the thousands of spaces that make up the city’s built environment is seen as vital to San Francisco’s efforts to become entirely carbon-neutral by 2050.

According to San Francisco’s Department of the Environment, 46 percent of the city’s carbon emissions can be traced to electricity and gas used in homes and office buildings.

“If you look at where our emissions are coming from, they’re coming from the energy use inside buildings associated with water, heating, cooling — the operations of our built environment,” said Debbie Raphael, director of the environment department.

To date, San Francisco’s green-energy policies have helped cut greenhouse gas emissions by 30 percent below 1990 levels — the equivalent of taking around 400,000 cars off the road, according to the Department of the Environment. Notably, those emissions cuts have paralleled a 20 percent population increase and 111 percent growth in the city’s economy.

Broadly speaking, the city’s 2030 emissions goal for new construction will mean making buildings more energy-efficient and powering them with electricity generated from renewable sources. But the details of the city’s action plan around cutting emissions remains a work in progress.

Raphael said her department will publish a plan this year “that will spell out how we go from today to the 2030 milestone and then the 2050 milestone.”

She added that the department is not yet “at a point where we can make recommendations to our elected officials on exactly what needs to happen, when. But we are at a point where it’s not a mystery — we know what needs to change — we’re not yet sure of the levers and mechanisms to make that change happen.”

Reducing the city’s abundant reliance on natural gas is one of the environment department’s biggest targets, Raphael said. Most of what’s considered natural gas is methane, which can trap 86 times more greenhouse gases in the atmosphere than carbon dioxide.

“We call it a super pollutant,” Raphael said. “The notion of tackling natural gas is a fairly urgent one.”

As a practical solution, Raphael’s department recommends replacing hot water heaters that run on natural gas with more efficient electric models.

Breed is expected to roll out the climate announcements Wednesday alongside Raphael and other city leaders at the Moscone Center, where a major international climate summit will be held next week.

The roof of the convention center will soon be the home of the city of San Francisco’s newest solar panel array. On top of the 2030 construction net-zero goal and the new solar array, Breed is also expected to announce that the city will begin issuing more “green bonds” used to finance clean-energy projects.

And to highlight clean-energy transportation, Ford Go Bikes will be free for the public to use on Green Thursday, Sept. 13.

The announcements also follow an ambitious zero-waste goal issued by Breed last month that would send no garbage to landfills, or at least as little as possible.

The three-day Global Climate Action Summit, which begins Sept. 12, will convene environmental experts and elected officials from across the globe to promote the idea of taking steps to address climate change at the local level.

Locally driven climate initiatives have become more important, city officials say, because of the Trump administration’s reluctance to confront global warming or even expressly acknowledge its existence. The drive to take action locally began to crystallize seriously after the president’s controversial decision last year to pull out of the Paris climate accord.

“What’s significant about this summit is that it’s a recognition that the responsibility and the opportunity to protect the planet does not lie solely with national governments,” Raphael said.

Dominic Fracassa is a San Francisco Chronicle staff writer. Email: dfracassa@sfchronicle.com Twitter: @dominicfracassa