And the second way is this: Some of those very same countries are newly emerging—not just in terms of their economy, but in terms of their attitude towards climate change. Those countries are not so sure they want to go ahead with a climate policy, and they need encouragement. So you’ll see, I think, countries like India, Vietnam, the Philippines, Indonesia, Brazil—all of which have other problems that probably come first—you’ll see a lot of them backing off the seriousness with which their commitments are pursued and implemented.

Now, some of this stuff they may want to do anyway. India, probably, is going to come around to dealing with its air-pollution problem, like China is doing. In the process, it will deal with some of its greenhouse contribution.

But the Trump action can’t actually have any positive effect on any of that. I don’t believe the arguments that the rest of the countries are going to buckle up, redouble their efforts, and compensate for the absence of U.S. leadership and participation. I think the immediate consequence will be a ripple effect, making countries think twice about doing much about climate change. It won’t be China or Europe. It will be a lot of countries whose economies will exert substantial leverage on global emissions over the next 10, 20, 30 years, as they grow—those are the ones that I expect to hesitate. And that hesitation, in and of itself, could be enough to condemn the two-degree target to oblivion.

Meyer: A lot of people would respond to that: Well, Paris was already symbolic, so what difference—

Oppenheimer: That would be wrong. Paris was far more than symbolic. Paris didn’t have a hard ironclad binding requirement, except to submit new plans, and I criticized it for that at the time. Paris did not have hard obligations and penalties, like the Montreal Protocol on ozone depletion did. There are no trade sanctions in Paris. All that made it a relatively soft agreement.

But nonetheless, that’s the furthest countries were willing to go. And within that context, it held promise. I think we would have wound up in a situation where commitments [to reduce carbon emissions] would have been met at probably the lower level, but a heck of a lot more would have been done than if Paris didn’t exist at all.

And then we could also have used the name-and-shame game: the business of having to report your emissions under a transparent system, and say what you did and what you didn’t do. And that technique is effective in some international circumstances, like human rights treaties. It’s not like there was nothing there.

Meyer: You just mentioned a “transparent system.” I’ve heard American diplomats be concerned about transparency now that the U.S. is out of the treaty. What would make Paris a transparent system?

Oppenheimer: Paris doesn’t have a clear set of provisions yet on what’s called “transparency.” Those were critically important, because those are the provisions for monitoring, validating, and reporting emissions, which make the whole thing clear to everyone and make other countries trust that you’re doing what you said you would do. Those are still in the process of development, and we don’t know how those negotiations will turn out. But with the U.S. out, it’s much more likely they’ll turn out badly, because China doesn’t like transparency and the U.S. did. If for no other reason, the Trump administration should have stayed in Paris to make sure that China doesn’t create a set of rules that let countries obfuscate what they’re doing, which is generally China’s tendency.