Here’s something we know we have to fix, and we mostly know how to fix it.

I’m talking about the buildings in which we live and work. They play a big role in heating the planet. In some cities, they’re the biggest single source of planet-warming greenhouse gas emissions.

New laws in a handful of cities, including mine, New York, now compel building owners to reduce those emissions or face fines. The European Union recently enacted a law that requires all buildings to be zero-emissions by the middle of the century.

I wanted to learn more about all this. So I reached out to John E. Fernandez, an architect and professor at the Massachusetts Institute of Technology.

Airtight equals efficiency

Not long ago, I went to see an office building under construction in downtown Boston. On what was once a city-owned parking lot, Millennium Partners Boston, a developer, had erected a 21-story, 800,000-square-foot tower called the Winthrop Center. Fernandez was a consultant on the project.

Its 10-foot, floor-to-ceiling windows were triple-pane, oriented to let in sunlight. The walls had a 4-inch layer of insulation. On each floor, a door opened to a balcony, though, as in a lot of offices, none of the windows opened. Any chance of fresh air? Yes, said Brad Mahoney, the company’s director of sustainable development. From the ventilation system.

Winthrop Center was built on Passive House principles, a concept that sprang up in Germany in the 1980s, based on the idea that the cheapest energy is the energy you don’t use. So the emphasis is on creating an airtight envelope for the building. Don’t let unwanted air leak in or out. Insulate. Take advantage of the heat produced inside by circulating it. Our bodies and our computers generate a lot of heat, it turns out.

Upfront construction costs are higher than for a conventional building. In the case of this building, Mahoney estimated, the cost was about 2 percent to 3 percent higher.

(Luxury condominiums sit on top of the office tower, but they’re not, strictly speaking, built to Passive House standards.)

The heating source is key

The Winthrop Center is still heated by a gas boiler. Because of the energy efficiency hacks, Mahoney was quick to point out, it will use less gas than a conventional building of the same size. He said that when construction began, in 2017, electric heat pumps weren’t “commercially viable.”

There was no incentive, either. Now, there is. Boston in 2021 passed an ordinance requiring large buildings to neutralize their greenhouse gas emissions by 2050. Between now and then, building owners are required to report their emissions numbers. In March, Mayor Michelle Wu proposed that new constructions be fully electric; the City Council has yet to vote on the proposal.

Nearly 70 percent of Boston’s emissions come from buildings.

Building materials matter, too

Passive House principles don’t take into account the stuff buildings are made of, like concrete and steel, which are huge polluters. How can that be reduced?

Fernandez said steel would become less carbon-intensive if its production could be fueled by green hydrogen, which is derived from water.

Concrete is more complicated. It can be used to store planet-warming carbon. My colleague Brad Plumer wrote about a building in Manhattan that captures the carbon dioxide emissions produced by its gas heating boilers, converts it to liquid and mixes it with cement, to be socked away for a long time in concrete blocks.

Likewise, coal ash from power plants can go into cement and then concrete, rather than be released into the atmosphere. But, for the most part, making conventional concrete and steel still produces a lot of pollution, and that’s embodied in every concrete-and-steel building.

The United States is catching up

There are many such airtight buildings in Europe and China, including a hospital in Germany. The Winthrop Center is among the largest such office constructions in the United States, but there are airtight housing developments. Cornell Tech has built a 26-story apartment tower for its students and faculty on Roosevelt Island in New York City. An affordable housing project in Brooklyn is built with Passive House principles in mind, as is a 61-unit housing development for seniors in Allentown, Pa.

It’s easier to build an airtight building in a cold or temperate climate, Fernandez said, than in a hot tropical climate, where you have to worry about humidity.

What about older buildings?

A lot can be done to retrofit existing homes and offices. To Fernandez, they are the quickest, most cost-effective ways to reduce a city’s emissions. Windows can be replaced. Walls can be insulated. Heat can be kept inside in winter. Hot air can be kept out in summer

“The smartest dollar spent is to repair a leaky wall,” he said.

For individual apartment buildings, that’s still a considerable expense. But as Fernandez pointed out, those expenses are small compared with many other things needed to quickly reduce climate pollution, like retooling the electricity grid or building a network of electric vehicle chargers.

“It’s going to be a lot easier to reduce carbon emissions in the built environment than almost everything else,” he said.

Related:

Is your city or state considering building codes designed to slow down emissions from offices and homes — or delaying energy efficiency proposals? Here are a few U.S. examples:

New York City | Boston | Washington State | North Carolina

Essential news from The Times

The dwindling Colorado: After months of fruitless negotiations on sharing what’s left of the river’s water, the federal government has a plan to impose cuts on states.

‘Flash droughts’: Dry spells are becoming more common and developing faster around the world. According to a new study, climate change is a major reason.

How electricity became the future: To tackle climate change, we’ll need to plug in millions of cars, trucks, home heaters, stoves and factories.

An auto revolution: The E.P.A. has laid out rules that would tightly limit pollution emitted by cars and trucks. The goal is to turbocharge sales of electric vehicles.

The future of E.V.s: Batteries made of sodium, which is much less expensive than lithium, could be the industry’s next big advance. China is far ahead in developing the technology.

One country, two winters: The United States West was buried under snow this winter, while much of the East was almost snowless. The jet stream is partly to blame.

Toxic fumes: A fire in a plastics recycling plant in Indiana is expected to burn for days. More than 2,000 residents were ordered to evacuate their homes.

Something to watch

Inspired by Jane Goodall: “Jane,” a new series on Apple TV+, tries to deliver the primatologist’s environmental message through a young protagonist with insatiable curiosity.

From the Opinion section

Green hydrogen: Before we invest billions in this clean fuel, let’s make sure it’s actually clean, writes Leah C. Stokes, a professor of political science at the University of California, Santa Barbara.

From outside The Times

• Bloomberg investigated how banks, pension funds and insurers are profiting from California’s scarce groundwater, leaving small farmers and poor communities behind.

• From Grist: From California to Congo, policymakers have long sacrificed Indigenous people in the name of conservation.

• The Climate Question podcast told the story of the woman who first discovered that higher levels of carbon dioxide would warm the planet. A man took the credit.

• The Los Angeles Times showed what California’s wildflower superblooms look like from space.

• Time named Elizabeth Maruma Mrema one of the 100 most influential people of 2023. She is executive secretary of the U.N. convention on Biological Diversity.

• E&E News reported on new research that showed sea levels along the coastlines of the southern United States have risen three times as fast as the global average.

Before you go: ‘Finding value in old things’

Jamila Norman, an urban farmer and host of the TV series “Homegrown,” has a simple home décor philosophy. “I don’t like buying new stuff,” she said. “I like to find stuff that’s already out there and still useful.” She says her regard for older things is elemental: The floorboards in her house came from a forest. The bricks came from the earth. She considers her home “an extension of nature in a built environment.”

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Manuela Andreoni, Chris Plourde, Claire O’Neill and Douglas Alteen contributed to Climate Forward. Read past editions of the newsletter here.

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