Through the first decades of this century, the greater Seattle area had a secret weapon for fighting climate change: it let people choose to live closer to each other.

Using a study of neighborhood-level carbon emissions, Sightline estimates that new residents to the Seattle area produced about 5 percent less greenhouse gases from 2000 to 2020. That's simply thanks to their metro area being better than any other in the United States at adding homes to existing neighborhoods.

The region eliminated about 1 million tons of potential greenhouse gas emissions basically for free: no government spending, no tax credits for efficiency, no carbon trading market, no carbon tax. This public benefit came just from reducing red tape and letting construction workers build the kinds of homes that people want to live in.

Building those homes created jobs. Letting people live in them created economic growth, more location choices, and lower energy bills. The metro might have done even better, of course, if it had allowed more homes throughout the region; and it can keep improving in the future.

But for the economy of greater Seattle, this category of climate action has been better than a free lunch. It's been a lunch the region got paid to eat.

In general, it's a simple relationship: when people live close enough together, they use less energy and emit less greenhouse gases. Take it from the United Nations Intergovernmental Panel on Climate Change: "compact and resource-efficient urban growth through co-location of higher residential and job densities, mixed land use, and transit-oriented development (TOD) could reduce GHG emissions between 23 percent and 26 percent by 2050 compared to the business-as-usual scenario."

This effect comes from three main factors: less driving, more energy-efficient buildings, and less displacement of nearby ecosystems by energy-intensive objects like pavement.

Cars and trucks are the leading source of greenhouse gas emissions in most states and provinces. Residents of the most compact neighborhoods—areas of apartments mixed with workplaces and shops, whether in cities or suburbs—drive much less than residents of spread-out neighborhoods of detached houses on large lots. "Infill housing reduces pollution by reducing driving," as UC Berkeley's Terner Center for Housing Innovation puts it.

A Terner Center study finds that San Franciscans emit less than a third as much carbon from transportation as residents of the sprawling Bay Area exurb of Oakley, while making a similar number of trips for the same purposes. Another found that households in California's suburban, single-detached neighborhoods drive more than twice as much every weekday as households in urban neighborhoods with high transit usage. And public transit usage increases with population density (among other factors). In places where few people live close enough to walk to the bus stop, transit can even be less climate-friendly than personal automobiles.

These emissions benefits also accrue to existing residents of a neighborhood that adds people, not just new residents moving in. When there are enough people in your neighborhood to support a grocery store or a new bus line, now you get to walk to the grocery store or take the bus to work instead of driving.

There's no better insulation than another entire climate-controlled home on the other side of your wall. Urban neighborhoods are much more likely to have such wall-sharing multifamily housing, which one study finds uses 35 percent less energy for heating and 21 percent less energy for cooling than equivalent households in single-detached houses. (What's more, multifamily homes generally have smaller floorplans, which further reduce energy consumption from heating and cooling.)

It takes energy to make stuff, and spread-out cities require more stuff. One researcher at Delft University of Technology found that "urban sprawl accounts for a third of all greenhouse gas emissions" via increased raw materi...