Donald Trump’s stunning victory has left millions in dread, and moved thousands into the streets to protest, as the media speculate about what he might do next. Fear is spreading among immigrants, Muslims, and other minorities. The 20 million people who have received health

insurance under Obamacare worry about Trump’s vow to repeal it. Will he really tear up the Iran nuclear deal or order the CIA to start torturing people again? All of these are serious concerns, but it is Trump’s denial of catastrophic climate change—he has repeatedly said he considers it a “hoax”—and his vow to reverse all of the progress made under President Obama to address it that poses one of the most chilling and potentially irreversible threats.

Voters heard little about climate change during the endless election campaign. The contrast between the two candidates on the issue was like night and day, with Hillary Clinton promising to expand on Obama’s climate initiatives and Trump vowing to repeal them. Yet not one question about climate change was posed in any of the presidential debates. The media gave more airtime to the size of Trump’s hands than to the scope of his climate delusion.

The stakes are enormous. Climate change isn’t some distant concern; it is a very real and present danger. Trump may not accept that, but the generals in the Pentagon have no doubt. The Defense Department reports that climate change is an “urgent and growing threat to our national security,” contributing to “increased natural disasters, refugee flows and conflicts over basic resources.… These impacts are already occurring and the scope, scale and intensity…are projected to increase.” In January, the Pentagon ordered its officials to “incorporate climate change impacts into plans…across the full range of military operations,” from weapons testing to preparing troops for war.

The fact is, climate change is already here. Most of the summer sea ice in the Arctic has melted, causing a deluge of warm water to sweep across the South Pacific this past spring, killing a massive amount of coral and destroying a shocking percentage of the Great Barrier Reef off the Australian coast. The warming has caused steady increases in droughts and record flooding in wet areas, and it’s already begun to raise sea levels. Scientists now suggest that, even with last December’s Paris Agreement on climate change, we are on a trajectory to increase the earth’s temperature by 3.5 degrees Celsius or more by 2100. If that happens, great American cities—including Miami, New Orleans, and New York—will be underwater. Our food-producing plains will burn up. In many areas of the planet, it will be too hot to work outside for much of the year.

Global warming and the melting of the polar ice caps are now occurring at a much faster rate than scientists previously predicted. The World Meteorological Organization reports that “dramatic and unprecedented warming in the Arctic is driving sea level rise, affecting weather patterns around the world, and may trigger even more changes in the climate system.”

Under President Obama, the United States played a leading role in getting China and eventually India to join the Paris Agreement, with countries pledging to meet the carbon-emission reductions needed to keep the world from warming more than 2 degrees Celsius. But even meeting that goal—which many scientists believe is not enough—will require dramatic and immediate action. Now 110 countries, collectively responsible for nearly 80 percent of greenhouse-gas emissions, have ratified the agreement, enacting it into international law. The United States accounts for about 20 percent of the pledged reductions in world greenhouse-gas emissions. Much of this is slated to come from Obama’s Clean Power Plan, which enforces EPA standards on carbon emissions by calling on states to reduce power-plant emissions by 32 percent by 2030. (The plan is being challenged in a case headed to the Supreme Court as soon as next year.)

During the campaign, Trump announced that he would seek to repeal the Clean Power Plan and withdraw from the Paris Agreement. He’s also pledged to go forward with the Keystone XL and Dakota Access pipelines; to slash investment in climate research and in renewable-energy and efficiency innovations; to open up the leasing of federal lands for oil, gas, and coal production, onshore and off; and to revive the coal industry.

There are limits on what Trump can do. The stock price of bankrupt Peabody Coal rose on news of his election, but coal will be unable to compete in price with natural gas, the production of which Trump plans to expand. More dangerous than his proclamations about reviving coal are reports that he’ll move to loosen the regulation of gas pipelines. Methane leaks from pipelines are a far more potent contributor to global warming than even carbon emissions.

The Trump threats are not simply campaign bombast: He has named Myron Ebell, a notorious climate-change denier and head of a coal-industry-funded think tank, to head his transition team on the EPA and energy. The conservative Republicans who dominate the GOP majority in Congress have railed against Obama’s plans. Oil and coal barons—including the Koch brothers, among the richest in the world—wield big money and influence on the Republican right.

Elections do have consequences—often ones not intended by voters. Americans elected George W. Bush, who called for a more modest US role in the world—and we got a president who drove the disastrous invasion of Iraq. We elected Barack Obama, who promised hope and change—and we got a continuation of the Wall Street bailout and corporate-friendly trade deals.

A minority of Americans voted for Trump, largely because they too wanted change—but few had any idea of the calamitous consequences that will follow if he carries out his know-nothing stance on climate change.

Trump’s efforts must be resisted; the good news is that activists are already mobilized. Two years ago, we saw the People’s Climate March in New York City—the largest-ever demonstration on climate change. Another protest is being planned for Washington next spring. Civil disobedience helped stall the building of the Keystone and Dakota pipelines, and those protests will surely ramp up. The divestment campaign, which has already removed trillions of dollars from the fossil-fuel industry, will continue to spread. Other countries will invest in renewable-energy and efficiency innovations, winning global markets and jobs that the United States will forfeit if Trump carries out his campaign vows. In this country, California, New York, and other states will continue to push toward renewable energy and require higher effi­ciency and mileage standards, forcing manufacturers to respond. Democrats in the Senate and House will fight a determined rearguard action to limit the damage.

But time is not on our side, or on the side of civilization. Climate activist Bill McKibben notes that the real contest isn’t between Democrats and Republicans or Clinton and Trump, but between “human beings and physics”—and physics “is not prone to compromise.” If we don’t move quickly—very quickly—then any progress we make will come too late.

We can’t afford to drift for four, much less eight, years. We can’t afford to lose what little momentum has been created. Trump may think it will be easy to reverse Obama’s climate measures, since they were largely accomplished through executive orders and regulation in the face of Republican obstruction. He is about to find out that millions of concerned citizens here and abroad will use every nonviolent means possible to stop him from accel­erating the worst crisis that humans have ever faced. This isn’t about politics; it’s about survival.