Let’s say Donald Trump wins the election in 2020. There’s no doubt some climate advocates will try to put an optimistic spin on it— just like they did when he first won in 2016. Renewable energy is getting cheaper all the time, they’ll argue. Investors are moving rapidly away from fossil fuels. The European Union and China are stepping up to solve the emergency. And so are U.S. states and cities. Young people are becoming a political force to be reckoned with.

That would mean that virtually all the planet’s coral reefs will bleach a sickly white. The world will lose more than 155,000 square miles of coastal wetlands and drylands. The number of people unable to access reliable drinking water could double . Hundreds of millions won’t have enough nutritious food to eat.

Trump's reelection dramatically increases these risks not only because of the massive additional emissions his fossil fuel-boosting policies could cause—potentially as much as 3.1 gigatons by 2035, which is over four times the entire annual carbon footprint of Canada . A second Trump term could also mean that one by one, countries drop out of the Paris agreement, possibly causing it to unravel completely. In this scenario, keeping temperature rise below 2 degrees would be extremely difficult. “I would say it would make [achieving] it impossible,” said Noah Sachs, a University of Richmond environmental law professor who described in a paper last year what Paris self-destructing could look like. “If Paris fell apart, we would blow right by that target.”

All of this was true four years ago, and will continue to be accurate if Trump begins a second term. But none of it will change a horrifying reality: a re-elected and emboldened Trump would likely destroy the world’s best-case outcome on climate change, and potentially send us hurtling towards a worst-case scenario. His second term could result in global temperatures roaring past 1.5 degrees Celsius, the danger line identified in the devastating United Nations climate report from 2018, beyond which forests, croplands, freshwater sources and other natural systems that support human life could be irreversibly transformed. We would also likely surpass 2 degrees, the target the world’s nations agreed to in 2015 at Paris, raising the odds of catastrophic tipping points like the melting of all Arctic sea ice.

And Trump, who turns 74 this June, likely won’t even be around to see it happen.

The world is on the cusp of climate disaster

Global temperature rise above 2 degrees wouldn't mean the imminent collapse of civilization. Nor would it mean the world should give up on cutting emissions. We’re already facing monumental disasters like the Australian bushfires, and the impacts of climate change will get exponentially more destructive and harder to survive with each increase in global temperature. This will make it worth fighting for carbon reductions, no matter how warm the planet gets.

But above the 2 degrees goal that countries signed up for in the Paris agreement, damage could come at a scale that’s hard to fathom. In a 1.5 degrees scenario the probability of an ice-free Arctic summer is 3 percent in a given year. At 2 degrees that rises to 16 percent. And at 3 degrees it’s 63 percent. Sea-level rise at that scale means cities such as Miami, Osaka, Alexandria and Shanghai could effectively cease to exist. Meanwhile, there could be disasters like 97 percent of wildfire-sensitive regions in Mediterranean Europe burning every single summer. The chance of the American West going up in flames could increase by 400 percent.

The Rhodium Group, a New York-based research firm, calculated in December that for humankind to keep warming below 2 degrees, we need to reduce emissions a colossal one-third from current levels by 2030. They identified five crucial things that could get us globally on track toward closing that gap: the European Union adopting a Green New Deal, Brazil halting its destruction of the Amazon Rainforest, China’s economic growth and the emissions it produces slowing to a more sustainable rate, and demand for electricity in India growing only moderately and being met mainly by renewables. The fifth one is Trump losing in 2020.

Trump has already done a significant amount of damage

A re-elected Trump would keep dismantling environmental rules—he's slashed 95 in total so far—that limit fossil fuel companies or other polluters of the atmosphere. And he would make the damage much harder to reverse by continuing to stack the courts with conservative judges who tend to rule on the side of greenhouse gas-spewing corporations.