In withdrawing the United States from the historic 2015 Paris Climate Agreement, President Donald J. Trump fulfilled one of his most prominent campaign pledges. But far from making America “great again,” the decision will endanger U.S. national security and prosperity by sabotaging U.S. global leadership and accelerating a planetary crisis from which not even an isolationist America can escape. Trump’s myopic and backward-looking decision ignores a reality of contemporary global life: The United States cannot advance its national interests or protect itself from transnational threats by pretending to be an island or by building walls, in the vain hope of insulating itself from dangers that refuse to respect borders.

It is hard to overstate the magnitude of the president’s step, taken despite urgent entreaties from America’s closest allies and U.S. business leaders. By withdrawing the United States, Trump is taking a wrecking ball to the most important multilateral agreement of the twenty-first century. After two decades of fruitless negotiations on a successor to the Kyoto Protocol, 195 countries in Paris in December 2015 finally pledged to major reductions in greenhouse gas emissions. At the insistence of the Obama administration, the climate accord was crafted not as a binding treaty—which could have run afoul of domestic U.S. sovereignty concerns—but as a voluntary agreement. Under this so-called “pledge and review” arrangement, each country declared its own “intended nationally determined contribution” (INDC) to reducing greenhouse gas emissions. This allowed the parties to tailor their commitments to their unique national circumstances, rather than adopting a one-size fits all formula.

The results were impressive. After years of dragging their feet, major emerging economies made concrete commitments to reducing emissions. These included China, the world’s biggest emitter, and India, which had long resisted commitments as a drag on its domestic development. The accord also envisions negotiation of a new global monitoring system to verify that countries followed through—and (at least in principle) to name and shame laggards and shirkers.

As impressive as the Paris Accord was, all signatories recognized that it was but a down payment. Even if fully implemented, the agreement would cut only 54 percent of the emissions needed to prevent average global temperatures rising by more than 3.6 degrees Fahrenheit—a threshold beyond which the planet would experience runaway warming. Leaders in Paris recognized that they would need to “ratchet up” their efforts to reduce emissions over the next two decades. Still, for the first time, global leaders had overcome the inertia that had long plagued climate change negotiations—and committed themselves to real action to save the planet.

By renouncing the agreement, President Trump risks throwing this momentum into reverse, at a perilous moment. The defection of the United States, the world’s second largest emitter after China, will embolden others to renounce their own pledges—or at a minimum delay their full implementation. The diplomatic fallout will be harsh, destroying whatever pretense remains of U.S. global leadership—and U.S. credibility—in the Trump era. Beyond squandering international goodwill, the United States will cede any pretense to influence on global environmental issues to China and lose out on many opportunities to dominate the clean technology future.

As for the planet, the implications are dire. On its current trajectory, the world may well reach the critical threshold of 3.6 Fahrenheit by 2036. The catastrophic results will include more extreme and dangerous weather, more frequent and prolonged droughts and famines, rapid melting of glaciers and polar ice, dramatic sea level rise, accelerated ocean acidification, large-scale die-offs of coral reefs, devastating losses of habitats and species, and mass migrations involving tens and perhaps hundreds of millions of people.

Nor will Americans be immune from these impacts. To pick just one dimension: By 2050, some twenty-six U.S. cities from Baltimore to Honolulu are expected to face an “emerging flooding crisis,” with damages from “Superstorm Sandy”-type events running into the hundreds of billions of dollars. Florida—which has some 2.4 million people living and 1.3 million homes located less than four feet above high tide line—most in the Miami-Dade area—may be hardest hit. But it won’t be alone. Further north, the city of Norfolk, Virginia, which hosts the world’s largest naval base, risks becoming uninhabitable thanks to regular flooding. In 2015, the Environmental Protection Agency (albeit under different management than today) predicted that sea level rise and storm surges could cost U.S. coastal communities $5.0 trillion through 2100. Elsewhere in the United States, the expense of climate-change induced drought and accompanying water shortages could top $180 billion by the end of the century.

And for what? Trump and his nationalist sidekicks contend that leaving Paris will allow the United States to avoid crippling environmental regulations and, among other benefits, revive America’s beleaguered coal mines. That stance may play well in Appalachia, but it ignores far more important pressures on that industry, not least the fracking revolution and the low price of natural gas. It also ignores that nearly twice as many people were employed in the renewable energy sector last year than in traditional fossil fuel industries—and that most major U.S. corporations favor the Paris Agreement. A forward-looking administration, truly interested in competitiveness, would be doing as much as possible to advance research and development in the industries of tomorrow. (Ironically, some parts of the Trump administration appear to recognize this: This week, the U.S. International Trade Commission notified the World Trade Organization that it will launch a safeguard investigation on the influx of foreign solar panels, presumably in an effort to protect domestic manufacturers from unfair foreign competition.)

With this U.S. abdication of global leadership, the world must pin its hopes for slowing global warming on the combined efforts of other major emitters, U.S. states and cities, and private corporations. Tomorrow, China and the European Union will recommit themselves to the Paris Agreement, describing it as “imperative and more important than ever.” Governor Jerry Brown, meanwhile, promises that California will fill the vacuum left by the U.S. pullout, by working with other U.S. states and cities to impose new emissions caps and standards. Finally, the planet is counting on the self-interest of the U.S. business community, which is unlikely to make long-term investments in dirty technologies, given the near-certainty that a future administration will overturn Trump’s policies to pursue a low carbon future—and restore America’s stature in the world. That moment cannot come soon enough.