(Washington Post) 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.

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