How do we build climate friendly cities?

It may be the most important number on Earth: 1,000 gigatons. That’s roughly how much carbon dioxide humanity has left to emit, scientists say, in order to have a two-thirds chance of keeping global warming below 2 degrees Celsius above the temperature in pre-industrial times — and thus, staying within what has often been deemed a “safe” climate threshold.

A gigaton is equivalent to a billion metric tons. Last year, 32.3 of them were emitted.

The 1,000 gigaton number has many implications, but we rarely think about what it means for city planning. A new report, though, finds that if we don’t build cities more wisely, using much greener infrastructure, then they could be a crucial factor that tips the planet over the 1,000 gigaton line — and indeed, that they could play this role in just five years time. By 2020.

The research, by the Stockholm Environment Institute and with funding support from the C40 Cities Climate Leadership Group — a consortium of global megacities focused on sustainable development and fighting climate change — is based on the central concept of emissions “lock-in.” We haven’t literally emitted anything close to the remaining 1,000 gigatons yet, but prior research suggests that with all the coal plants and other forms of fossil fuel infrastructure that are already constructed — not to mention all of the buildings and roads and other types of urban and non-urban infrastructure whose very existence implies a future of using lots of energy — about 800 gigatons are already committed to go into the atmosphere.

All of this is simply because when humans build big things, they then overwhelmingly tend use them over their useful lifetime — and that implies a heck of a lot of emissions.

That leaves 200 gigatons, to be carefully — or, not so carefully — apportioned in a world in which cities and megacities are growing at a mind-boggling rate of 1.4 million individuals per week. And that’s what makes city planning, design and infrastructure decisions so critical.

The new report finds that in the next five years, about 14 gigatons of emissions per year will be locked in by simple infrastructure decisions made as we expand cities and megacities — that is, if we do it the way we’ve always done it, featuring big highways, non-green buildings and more.

Ten of those gigatons each year will be attributable to the design of new buildings — their envelopes, their heating and cooling systems — and new appliances and systems  in older buildings: air conditioners, heaters, lights and more. Another four gigatons per year will be the result of transportation infrastructure — the roads that are built in ways that encourage lots of driving, and the new and not always efficient vehicles that will drive on them.

Overall, then, just by 2020, 70 gigatons of the planetary carbon budget will be used up in city design and planning decisions, and the consequences they unleash. “In the next 5 years, cities will contribute to 30 percent of the remaining locked in carbon budget,” says Seth Schultz, the director of research, measurement, and planning at C40.

The rest of the 200 gigaton budget will then be consumed by non-urban buildings and infrastructure, pushing the planet past the “locked in” carbon threshold by 2020.

“Based on a business as usual trajectory, and based on current rates of urbanization, the infrastructure and policy decisions made in the next five years will exceed a cumulative emissions threshold of 1000 gigatons at some point in the future,” says Schultz.

But the new report also finds that a crucial 45 gigatons of emissions could be averted if growing cities and megacities deploy much more energy-efficient buildings, urban structures that don’t encourage as much driving, and more.

Still more could be accomplished if major industrialized cities — the New Yorks and Londons of the world — extensively retrofit their infrastructure to make it greener. Doing so would subtract from the 800 gigatons of emissions that are already committed to.

Therefore, how cities develop — and how they retrofit to become more sustainable — could greatly shape our planet’s future climate. Granted, how non-urban infrastructure, in all of its forms, develops matters even more based on these statistics. But cities are special for the following reason. Along with other “sub-nationals” like states and territories, they have been major pioneers and drivers of change lately, and the new figures are, in that sense, empowering.

But here’s what’s more troubling — the problem doesn’t end at 2020. Even if we save the 45 gigatons, we could give them back in just a few years of additional urban (or non-urban) expansion, unless sustainability’s advance across all parts of the world is extremely rapid.

And there’s another problem. The foregoing discussion is based on a carbon budget that keeps the world below 2 degrees Celsius of warming. But if the real danger zone is entered at 1.5 degrees Celsius of warming — as many scientists, and many countries, contend — then the math becomes tougher.

The bottom line is that with a rapidly warming and urbanizing planet, we may still be able to find a way to grow that doesn’t lock in the worst impacts of climate change.

This article is published in collaboration with The Washington Post. Publication does not imply endorsement of views by the World Economic Forum.

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Author: Chris Mooney writes about energy and the environment at The Washington Post.

Image: A city is shown at sunset. REUTERS.