Thread by @bradplumer

I think it's fair to say the US probably has to hit net zero emissions by somewhere around 2030 for the world to stay below 1.5°C of global warming. (Whether that's feasible or not is a different question.) /1

It's true the IPCC doesn't specify this. They just say that the world *as a whole* probably needs to get to net zero by somewhere around ~2050 to limit to 1.5°C with little overshoot. And the world would need to hit net zero by ~2070 to stay below 2°C. /2

(There are all sorts of ways to fiddle with these dates — by, for instance, assuming massive and perhaps unrealistic amounts of carbon removal later in the century. But let's stick with this base case.) /3

The IPCC says nothing about individual countries. But I think this paper by @Peters_Glen is fairly clarifying on this question. He looks at staying below 2°C, not 1.5°C, but the intuition is similar in both cases. agupubs.onlinelibrary.wiley.com/doi/10.1029/20… /4

Basically, the world *could* stay below 2°C if emissions in the US/Europe decline steadily to near-zero by 2050. But in that scenario, China/India would have to make even more aggressive reductions, and the rest of the world could basically emit nothing. /5

There's a reasonable argument that this isn't really fair. That the US and Europe should act even more aggressively (say, getting to zero by ~2030 or so) to give poorer nations more space to develop. That's not a scientific mandate—it's a question of ethics and politics. /6

One could counter, of course, that this is technically impossible—that the US can't and won't get to net zero by 2030. Possibly true! But then you either have to argue that other countries have to make even *more* radical cuts, or that we're very unlikely to stay below 1.5°C. /7

Anyway, I've seen a bunch of people say it's not *necessary* for the US to zero out emissions by 2030, that there's no scientific mandate to do so. And that's totally true, there's not! But it's at least worth being clear on what follows from that. /end

A clarification to the above: I should've specified I was only talking about *CO2 emissions.*

The IPCC's 1.5°C scenarios have global CO2 emissions hitting net zero by ~2050. But other greenhouse gases (e.g., methane) decline more slowly, reaching net zero later in the century.

Seems like quibbling, but it turns out to be an important distinction! For instance, UK's new climate goal is billed as "net zero by 2050." But that's for *all* greenhouse gases.

UK actually envisions CO2 hitting net zero by 2040ish: carbonbrief.org/in-depth-the-u…

To make this even more confusing, some countries/states don't always make clear whether their "net zero" climate targets refer to CO2 only or all greenhouse gases. Isn't climate policy fun?