That's why dealing with climate change will require getting a handle on urban consumption, said John Barrett, an economist at the University of Leeds who contributed to the report. "We can't ignore demand," he said.
Wealthy cities in the Northern Hemisphere have to take the most dramatic action. Already, the consumption habits of a C40 resident in a rich country like the U.S. or Japan give them a carbon footprint more than four times bigger than her counterpart in a C40 city in Africa or Southeast Asia.
These well-off urban dwellers need to eliminate two-thirds of their consumption-related emissions by 2030 to stay on track to limit global warming to 1.5 degrees Celsius above preindustrial temperatures, the most ambitious goal of the Paris climate accord. (Low-income residents of cities like Nairobi, Kenya, actually have to increase consumption to meet their basic needs.)
It sounds like a tall order, but the report lays out a few key steps that, together, could deliver 20% of cities' necessary emissions cuts. And many are things that individuals can do now.
For instance, eating less meat and dairy and reducing food waste could cut cities' food-related emissions in half. (Local governments could encourage these changes by launching meatless Monday campaigns in schools and creating community gardens, the authors suggested.)
No one has to go vegan, although that couldn't hurt. The report found that cities would be off to a good start if each resident reduces meat consumption to 35 pounds a year (compared with the current U.S. average of 222 pounds) and cuts their annual dairy consumption to 200 pounds (down from nearly 650 pounds).
Such changes would produce significant health benefits too; the authors estimated that shifting toward a plant-based diet would prevent 170,000 deaths due to heart disease, cancer and other ailments in C40 cities each year.
The report also recommended changing shopping habits, like extending the lifetime of computers and other electronic devices and buying fewer clothes (the authors suggest no more than eight new items per person per year as a progressive target and three items as an aggressive goal).
That can be a challenge, since many people want to keep up with trends, said Stephanie Pincetl, director of the California Center for Sustainable Communities at UCLA, who was not involved in the report.
Pincetl recommends buying high-quality items that will last a long time. "It sounds like deprivation, but it's also a way to make your consumption much more thoughtful."