National governments have been notoriously slow to address global warming. But a lot of mayors — from Seoul to New York to Rio de Janeiro — are far more enthusiastic about tackling the problem. And, as it turns out, they can do a surprising amount by themselves.

A new report from the C40 Cities Climate Leadership Group argued that the world’s biggest cities, on their own, could make roughly 15 percent of the emissions cuts needed to avoid 2°C of global warming. But they’d need to drastically change their housing, transportation, and garbage policies:

Cities can make about 15% of the cuts needed to stay below 2°C of warming

How would this work? Some 3.6 billion people currently live in urban areas worldwide. That’s projected to grow to 6.3 billion in 2050, as the world’s population grows and more people in Asia and Africa leave the farm for the bright lights of the big city.

That explains why cities are responsible for about half the world’s greenhouse-gas emissions. Much of it comes from the buildings that people live in and the cars they drive around. So that’s where urban policies come in.

Every city trying to cut emissions will look to different specific policies — what works in New York City may not work in Lagos. But the C40 report — written with UN Special Envoy for Cities and Climate Change Michael
Bloomberg — lists four broad steps that could reduce emissions significantly:

1) Strict energy-efficiency standards for all new commercial and residential buildings, particularly with regards to heating and lighting. That means requiring better insulation and switching over to LED lighting.

2) Programs to retrofit older buildings to make them more energy-efficient — again, there’s a huge amount of energy wasted in heating systems, so that’s the biggest area for change.

3) Transportation-planning policies that give people more alternatives to driving and curtail sprawl where possible. It’s hard to overstate how big a difference the latter can make. Barcelona and Atlanta have similar numbers of people, but Atlanta sprawls out over 26 times as much land area, and its residents emit 10 times the carbon dioxide, on average.

4) Increasing waste recycling and capture methane from landfills (this is a relatively small one, but it still makes a difference).

If most big cities pursued all of these policies aggressively, they could potentially cut emissions up to 8 gigatons by 2050 — over and above what national governments have already pledged to do. That’s about 15 percent of the cuts deemed necessary to avoid 2°C of warming (which is still the world’s goal).

That’s not just an abstraction — many cities are starting to do exactly this. At the UN climate summit on Tuesday, some 200 cities announced that they were taking voluntary measures to cut emissions by some 454 megatons by 2020. As Zack Colman reports in the Washington Examiner, the mayors of Houston, LA, and Philadelphia are all setting new emissions goals.

One big example: New York City Mayor Bill DeBlasio has declared that his city will retrofit some 3,000 publicly-owned buildings over the next 10 years, in order to reduce the energy needed to heat, power, and cool them. The city is ultimately planning to cut emissions 80 percent by 2050.

Green cities could also make it easier for people to live there.

If anything, the C40 report probably understates what cities could do.

Over at Scientific American, David Biello has an excellent piece titled “Climate Change Will Be Solved In The Cities — Or Not At All” that details additional measures some cities are taking. In China, cities like Shenzhen are starting to set up their own carbon markets. Paris is experimenting with policies to bolster the spread of electric cars.

Meanwhile, the C40 report doesn’t really talk about another crucial policy angle — cities that really want to be green could make it easier for more people to actually live in them. Study after study has found that people who live in dense urban areas have a lower carbon footprint than people who live in, say, the suburbs. Part of that’s the difference in driving. It’s also more efficient to heat a multi-unit apartment building than it is to heat a block of detached homes.

Yet many cities, like San Francisco, restrict construction of new housing or taller buildings and prevent more people from being able to live there, as my colleague Matthew Yglesias has detailed again and again. Letting more people move there might increase an individual cities’ emissions — but it could decrease emissions overall.

That said, there are limits to what cities can do

Now, there’s obviously a limit to how much mayors can do by themselves. Even the C40 report notes that aggressive action by all of the mayors of all of the world’s major cities would only produce about 15 percent of the cuts necessary to avoid 2°C of warming.

That’s because the really hard problem on climate change is where our energy comes from. Right now, about 87 percent of the world’s energy use comes from oil, natural gas, and coal — a fraction that hasn’t budged since 1999:

If we want to start reducing global greenhouse-gas emissions and stop the planet from overheating, that will have to change. And shifting the world’s energy mix will involve big advancements in clean energy as well as policies to deploy those technologies — say, a carbon tax, or renewable-energy mandates.

Cities have some tools they can deploy here — for instance, Los Angeles recently made it easier to install solar panels on rooftops and parking lots. But they’re also often limited in how much they can do. (For a good example, see Austin’s long-standing struggle to end its reliance on coal.) The mayor of Rio de Janeiro, Eduardo Paes, has been calling on national governments to find ways to give cities more autonomy over energy policy — but it’s slow going.

Indeed, cities don’t always even have the tools to change their own transportation policies. In the United States, for instance, transit policy is influenced by a bewildering mix of federal, state, and local governments. Mayors who want to curtail sprawl or promote transit usually need to work with other policymakers on a variety of different levels. No city can go it alone.

Still, the broader point of the C40 report — and Biello’s excellent piece — is that cities have a key role to play in climate policy. In The New York Times recently, Yale law professor Daniel Etsy argued that future climate talks should include cities and state governments as well as national leaders, since the former are often the ones at the forefront of climate policy.