Cities are playing a noticeable role in cutting U.S. carbon emissions, but their overall performance is mixed.

Since the Trump administration pulled back from national climate leadership, more local actors have taken the initiative. Eight states and several territories have passed laws to eliminate carbon from their power systems, and some have targeted economywide decarbonization. All but a handful of major electric utilities have promised carbon-free or net-zero emissions by midcentury. And scores of cities have adopted climate pledges of their own.

Making a commitment is just the beginning, however. It's the follow-through that really delivers carbon reductions. A new study out Thursday from the Brookings Institution examined climate commitments from the 100 most populous cities to benchmark how effective the "bottom-up" approach has been.

"It’s clearly a mixed story," said Mark Muro, co-author and senior fellow at the Brookings Metropolitan Policy Program. "There is inspiring, rigorous leadership and it’s not just on the coasts and California, but at the same time, there are plenty of plans that don’t have much bite, and more than half of all places don’t have a plan at all."

Only 45 of the 100 most populous U.S. cities have a firm greenhouse gas reduction target and a baseline measurement of their emissions, the study found. Another 22 cities have a general carbon-reduction commitment but haven't measured their emissions baseline or established a specific reduction target.

Even among the cities with firm targets and baselines, there is variance in how frequently they conduct new emissions counts, making it hard to assess progress. Some haven't conducted a single inventory since making their plan. For others, the most recent count is a decade old. And roughly two-thirds of these cities were behind their targets in their most recent emissions count.

The leaderboard for relative emissions reductions shows considerable variety in size and geography — and in how frequently cities count their emissions. (Graphic courtesy of Brookings Institution)

A "serious" part of the solution

Even with the limited number of cities taking measurable action on emissions, their collective efforts add up to a meaningful piece of the national response to climate change.

If the 45 cities deliver on their promises in their target years, they would lower annual emissions by approximately 365 million metric tons of carbon dioxide equivalent, the study found. That equates to making 79 million cars disappear. It's also comparable to the emissions reductions that coal-to-gas fuel switching delivered in the U.S. power sector in 2018, a trend that propelled national emissions decreases.

"Those are serious numbers," Muro said. "Knowing the enormity of the natural gas switch-out, it’s very clear that this not trivial at all."

Still, the collective commitments would only get the U.S. about 7 percent of the way to its original Paris Agreement pledge. That suggests the city-level, grassroots climate action so far is not sufficient to make up for concerted national action.

Then again, that 7 percent is only coming from a subset of the 100 biggest cities, accounting for some 40 million residents. If more large cities make their own plans, and the 22 with indistinct plans set and measure firm progress, the aggregate city-level contribution has plenty of room to grow. And many other smaller cities not counted in this analysis are taking such action.

Best practices for cities

The obvious takeaway for other cities is to form a climate plan, take it seriously and benchmark progress as frequently as possible. Beyond that, prescriptions become more difficult.

"There’s no single playbook because places turn out to be in varied situations," Muro said.

Some cities run their own municipal power utilities. One of the quickest ways to cut carbon is to shut down coal plants within a city and replace them with cleaner sources, Muro noted. Indeed, the top 10 list for absolute emissions reductions when targets are achieved includes Los Angeles, San Antonio, Austin and Sacramento, all of which have municipal utilities.

But plenty of other cities made notable progress without a utility to lean on. They can pursue energy efficiency programs, transportation upgrades and building code updates. The increasing cost-competitiveness of electric buses creates opportunities to clean up transit and school bus fleets. And city governments can use their own buying power to source clean energy and vehicles, where possible.

Then again, cities run up against the limits of their influence. A go-getter city in a region or state with lax land-use policies may be hard-pressed to combat sprawl. Cleaning up the power supply may require appeals to state leaders or utility regulators. And if a city experiences rapid population growth, it's going to be hard to avoid emissions rising commensurately. Tucson, Arizona, for instance, has grown rapidly and increased emissions substantially compared to its 1990 baseline.

But the geographic diversity of the cities leading in reductions shows it's possible, even in states that aren't taking drastic action on climate change. Los Angeles and San Francisco lead in relative emissions reductions from baseline, but the top 10 for that metric includes Durham and Greensboro, North Carolina; Minneapolis, Minnesota; and Cincinnati, Ohio.