In normal politics, the policies adopted by a president and Congress may zig one way, and those of the next president and Congress may zag the other. The contending parties take our system’s rules as a given, and fight over what they understand to be reversible policies and power arrangements. But some situations are not like that; a zig one way makes it hard to zag back.

This is one of those moments. After four years as president, Trump will have made at least two Supreme Court appointments, signed into law tax cuts, and rolled back federal regulation of the environment and the economy. Whatever you think of these actions, many of them can probably be offset or entirely undone in the future. The effects of a full eight years of Trump will be much more difficult, if not impossible, to undo.

Three areas—climate change, the risk of a renewed global arms race, and control of the Supreme Court—illustrate the historic significance of the 2020 election. The first two problems will become much harder to address as time goes on. The third one stands to remake our constitutional democracy and undermine the capacity for future change.

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.

In his first term, Trump has announced plans to cancel existing climate reforms, such as higher fuel-efficiency standards and limits on emissions from new coal-fired power plants, and he has pledged to pull the United States out of the Paris Agreement. His reelection would put off a national commitment to decarbonization until at least the second half of the 2020s, while encouraging other countries to do nothing as well. And change that is delayed becomes more economically and politically difficult. According to the Global Carbon Project, if decarbonization had begun globally in 2000, an emissions reduction of about 2 percent a year would have been sufficient to stay below 2 degrees Celsius of warming. Now it will need to be approximately 5 percent a year. If we wait another decade, it will be about 9 percent. In the United States, the economic disruption and popular resistance sure to arise from such an abrupt transition may be more than our political system can bear. No one knows, moreover, when the world might hit irreversible tipping points such as the collapse of the West Antarctic Ice Sheet, which would likely doom us to a catastrophic sea-level rise.