US President Donald Trump is expected to decide soon whether to withdraw from the Paris Agreement on climate change, which is the world’s promise to end the era of fossil fuels and avoid climate catastrophe. Now it’s become clear Trump’s decision could come down to a single sentence. So … let’s unpack that sentence. Because this sentence – and, consequently, the United States’ involvement in the Paris accord – could help determine the very fate of the planet. Here it is, Article 4.11 of the Paris Agreement (Warning, it’s boring. But stay with me): “A Party may at any time adjust its existing nationally determined contribution with a view to enhancing its level of ambition, in accordance with guidance adopted by the Conference of the Parties serving as the meeting of the Parties to this Agreement.” And here is my rough translation into human-speak: Any country that signs onto the Paris Agreement has to make a pledge to reduce pollution, called a “nationally determined contribution.” The United States, for example, pledged to reduce heat-trapping emissions 26% to 28% below 2005 levels by 2025. Anyway, these pledges are the building blocks of this agreement. The overall goal is to eliminate fossil fuel pollution this century, to avoid sinking low-lying islands, flooding cities like New Orleans and creating the sort of runaway warming that scientists say could lead to mass extinction. Hopefully these pollution reduction pledges will get MORE AMBITIOUS over time. But we’re not saying explicitly that they MUST get more ambitious. That last bit – the part about ambition – appears to be the sticking point for Trump. Does this sentence REQUIRE the United States to stick with its current pledge, which was made by the administration of Barack Obama? Or could it revise its pledge to be LESS AMBITIOUS? The former scenario might push Trump out of the agreement. The latter might be easier for this administration to stomach. It’s worth stating clearly that neither of these options is good for the planet. The best thing for the health of Earth and its inhabitants would be for Trump to stay in the Paris agreement and stick to the existing pledge. Policy makers have identified an increase of 2 degrees Celsius (3.6 Fahrenheit) as the red line for warming. Go past that much warming and we are living in a fully climate-changed world (more than we already are, since human fingerprints already are on many extreme weather events, especially heatwaves and temperature records). Existing pledges to the Paris Agreement put the world at about 3.4 degrees Celsius of warming, which is unacceptable. That’s why negotiators encourage countries to boost their pledges over time. Often, climate wonks talk about this issue of ambition as a “ratcheting mechanism.” It’s an apt metaphor. Think of your toolbox. A ratchet – or socket wrench – is the tool that lets you tighten bolts in one direction only. You can’t turn them the other way seamlessly. The same is the intent of the Paris Agreement – that countries acknowledge their current pledges aren’t enough, and they’re going to keep moving toward less pollution, until the world eventually relies upon clean power from wind, solar, hydro, etc. Todd Stern, Obama’s special envoy for climate change, told me this week that while this is the intent of the Paris Agreement, countries are not legally bound to ratchet up their ambitions. In fact, he said, negotiators discussed and then left out language in the all-important sentence that would have required countries not to moderate their pledges. “We obviously didn’t want parties to be going back, but we also thought it would be counterproductive to have a legal bar saying ‘thou shalt not go back,’” Stern told me. Stern would rather see the Trump administration stay in the Paris Agreement while pursuing whatever climate-warming policies it would like, rather than formally withdrawing from the agreement. Both are horrible choices, he acknowledges. But the first would be less of a catastrophe. Think of the Paris Agreement like a big global party, Stern said. He’d rather see Trump’s America “sitting in the corner sulking” than leaving the party entirely. Because then, he said, the other countries would start to think, “all of a sudden it feels like, well, this isn’t a very interesting party,” he said. And then maybe they cancel the party entirely. Will the United States stay or will it go? The decision may come down to a sentence.