Moreover, there are limits to how much a U.S. president matters on global climate policy. We aren’t the world’s largest emitter any more. And technological and market trends operating globally right now are pushing the United States and the world towards burning less coal (and burning more natural gas) and investing more in renewables and electric vehicles. Some U.S. state-level policies are doing the same. In a recent commentary piece here, MIT’s Jessika Trancik makes the case that so much change is already in motion in the energy and transportation sectors that Trump really can’t do all that much harm, even if he successfully reverses many Obama climate policies.

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Still, it is worth trying to examine the consequences of what the world truly fears: That the United States, starting with Trump but perhaps continuing beyond his presidency, might doggedly refuse to participate in global climate action. And recently, analysts with Climate Interactive, a think tank that conducts analysis of our possible future climate pathways, shared an analysis with the Post that at least provides a good starting point for thinking about this problem.

“The way that we broke this down was that there are two impacts,” explained Andrew Jones of Climate Interactive. “One of them is, what is the impact of U.S. emissions on the climate? That is, the biogeochemical question. What is the direct impact?”

“The second is, what is the impact via global engagement, global leadership,” he continued.

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The resulting analysis isn’t so much about the global consequences of Trump’s election but about the potential consequences of a recalcitrant U.S. generally, thumbing its nose at the world not just now but throughout the century. Trump’s election could push things in this direction, but the true impact would very much depend on the rest of the world’s response to his presidency and policies, and most of all upon his long-term legacy, and how long any tension between the U.S. and the rest of the world lasts.

After all, if Trump is just a climate policy speed-bump, a temporary U.S. domestic anomaly, then yeah, he doesn’t matter very much. “We ran scenarios in which U.S. action was delayed four or eight [years] and then resumed reductions,” said Jones. But then the U.S. went on to meet a hugely ambitious goal of reducing emissions 80 percent by the year 2054 or 2058 (rather than 2050, as the Obama administration currently envisions). In this situation, with such strong cuts coming just a bit later, “the effect of the delay was negligible,” Jones said.

But let’s consider what would happen if instead, Trump turns out to be a trendsetter, the beginning of a retreat from international climate cooperation led by the United States.

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How a rogue U.S. could doom global climate policy

Recall that in activating the Paris climate agreement, the globe has committed to keeping the planet’s warming to “well below” 2 degrees Celsius of warming above pre-industrial levels, of which about 1 degree of warming has already happened. So, let’s say that we have about 0.9 degrees to go, and the question is whether we will break through that boundary by the year 2100. Doing so would put the planet within a range of warming widely considered “dangerous” (although when you see the Great Barrier Reef with a 400 mile stretch of mostly dead coral at only 1 degree, it’s fair to wonder whether “dangerous” isn’t already here).

All analyses show that unless emissions patterns change, we’ll blow way past 2 degrees. They also show that if the U.S., China, and every other country honors its current Paris pledges out to the year 2025 or 2030, we still won’t be on the right course. Much tougher cuts would have to happen and they would have to begin relatively quickly. And emissions would have to keep going down, and down, and down all through the century, eventually nearing or even reaching zero.

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This will be exceedingly hard to pull off. Global emissions would have to stop growing and start declining by the year 2035. By 2050, major countries like the U.S. would have to have brought their emissions down radically — in the U.S.’s case, by 80 percent or more below its 2005 levels. That’s a goal the Obama administration recently laid out, despite little hope that a Trump administration would stick with the plan.

In this context, the Climate Interactive team explored a series of thought experiments. For instance, they said, imagine that every other country in the world does its part to get the world to 1.9 degrees Celsius in 2100, but the U.S. totally fails to meet the targets expressed by the Obama administration, and instead follows a kind of worst-case scenario of ever-rising emissions through the entire century.

This isn’t a very likely scenario, with U.S. emissions actually showing signs of decline lately. But again, let’s just use it as a beginning.

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In this case, the analysis shows, the U.S. would indeed tip the planet over the edge into the range of “dangerous” climate change.

“If all countries but the U.S. reduce emissions to stay within Paris limits, but the U.S. follows its reference scenario, that’s one of the worst-case scenarios for the U.S., [then] temperature in 2100 would go from 1.9 to 2.3,” Jones said. Here’s the result, according to Climate Interactive:

But it gets far worse if the United States were not only to completely cancel its climate engagement and just keep on burning fossil fuels, but also cause the world to lessen its commitment, too. Suppose the United States follows this worst-case path and instead of doing everything that they can, other countries correspondingly lower their own ambitions and only do about 50 percent of what they need to do. This would lead to a planetary warming of 2.7 degrees C, the analysis found, by century’s end.

Neither of these scenarios seems very likely, however. With the U.S.’s emissions already trending downward, due to market forces and technological change, it seems reasonable to expect that even under Trump, the country may manage to lower its emissions somewhat, even if it probably won’t meet its Paris goal without stronger effort. Beyond that, emissions could continue to tick downward, even if not as fast as the Obama 2050 goal would envision.

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It also seems reasonable that other countries might fail to meet their own targets out to the middle of the century — after all, hitting them is just plain difficult — whether because they are responding to Trump or simply because they fail for other reasons.

A middling scenario of trying, but just not hard enough

So now, imagine that the United States only achieves 50 percent of what’s needed and the rest of the world only hits 50 percent. That’s a bad news scenario as well.

“If the entire world (including the U.S.) decarbonizes only halfway of what’s required to keep warming within the Paris limit of ‘below two degrees C,’ then expected global temperature would miss the Paris limit of 2.0°C and warm to 2.5°C,” said Jones. Here’s how it looks:

There are a few things to note here.

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First, in none of these scenarios is the world anywhere close to holding warming to 1.5 degrees C, an even safer limit that many scientists believe will soon be out of reach — if it isn’t already.

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Second, it’s clear that what the United States does matters, but it matters most in the context of what other countries do — and that in judging U.S. action, we have to look at that action across the entire century. This is, again, why Trump alone can’t doom the climate, unless he starts some kind of grand wave of climate isolationism or long-term inaction.

Therefore, based on this analysis — and Climate Interactive aren’t the only number crunchers out there — here is what we can say about a Trump presidency and the planet. It’s basically all about legacy and influence. Knocking the United States off of the Obama administration’s trajectory of lowering emissions, and doing so for only four years, is not that big of a deal if the rest of the world races ahead anyway, and if the United States rejoins the action in four or even eight years, muddling along without much emissions growth, or even emissions declines, in the meantime.

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But if there is a longer-term reversal of progress, and if it poisons the international mood that currently favors action, that’s where you start to worry.

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Oh, and one more thing: Let’s remember most of all that staying below 2 degrees is extraordinarily difficult even without Trump. That’s why middling pathways like the one above representing some U.S. and global action, but not enough, sound pretty realistic right now. In these possible worlds, the planet may not totally cook, but its change would still be sweeping.