In short, the biggest difference between electing Trump in 2016 and reelecting Trump in 2020 would be irreversibility. Climate policy is now the most obvious example. For a long time, even many of the people who acknowledged the reality of climate change thought of it as a slow process that did not demand immediate action. But today, amid extreme weather events and worsening scientific forecasts, the costs of our delay are clearly mounting, as are the associated dangers. To have a chance at keeping global warming below 1.5 degrees Celsius—the objective of the Paris climate agreement—the Intergovernmental Panel on Climate Change says that by 2030, CO2 emissions must drop some 45 percent from 2010 levels. Instead of declining, however, they are rising…

The 2020 election will also determine whether the U.S. continues on a course that all but guarantees another kind of runaway global change—a stepped-up arms race, and with it a heightened risk of nuclear accidents and nuclear war. Trump’s “America first” doctrine, attacks on America’s alliances, and unilateral withdrawal from arms-control treaties have made the world far more dangerous. After pulling the United States out of the Iran nuclear agreement (in so doing, badly damaging America’s reputation as both an ally and a negotiating partner), Trump failed to secure from North Korea anything approaching the Iran deal’s terms, leaving Kim Jong Un not only unchecked but with increased international standing. Many world leaders are hoping that Trump’s presidency is a blip—that he will lose in 2020, and that his successor will renew America’s commitments to its allies and to the principles of multilateralism and nonproliferation. If he is reelected, however, several countries may opt to pursue nuclear weapons, especially those in regions that have relied on American security guarantees, such as the Middle East and Northeast Asia.