Ice floats in the Arctic near Svalbard, Norway, in 2009. (Dirk Notz/Associated Press)

Donald Trump’s stunning victory has left millions in dread and moved thousands into the streets. Fear has spread among immigrants and Muslims. The 20 million who have received health insurance under Obamacare worry about Trump’s vow to repeal it. The media speculates about what he might do: Will he really tear up the Iran nuclear deal or order the CIA to start torturing people again? 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 pose some of the most chilling and potentially irreversible threats.

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

Yet the stakes are huge. Climate change isn’t a distant concern; it is a clear and present danger. Mr. Trump may not believe that, but the generals in the Pentagon have no doubts. A Pentagon report says that climate change is an “urgent and growing threat to our national security.” In January, Pentagon officials were instructed to include climate change in every decision, from readying troops for battle to testing weapons.

As environmentalist Bill McKibben pointed out last week, climate change is already here. The world’s global warming — and the melting of the Arctic ice caps — are now occurring at a much faster rate than scientists previously predicted. Most of the summer sea ice in the Arctic has melted. That’s enough heat, McKibben reports, that record-warm water swept across the Pacific this spring, killing “vast swaths of coral.” That’s enough heat to already cause steady increases in droughts and shocking downpours and floods in wet areas. It has already begun to raise the levels of the ocean. Scientists now suggest that, even with the Paris climate talks, we are on a trajectory to increase the Earth’s temperature 3.5 degrees Celsius or more. If we do that, many cities around the world will be underwater.

Under Obama, the United States played a leading role in getting China — and eventually India — to join the 2015 Paris agreement, with countries pledging carbon reduction commitments needed to keep the world from warming more than 2 degrees Celsius. Many scientists believe that goal is not enough, but meeting even that level will require dramatic and immediate action. One-hundred-and-nine countries responsible for nearly eighty percent of greenhouse-gas emissions have now ratified the agreement, enacting it into international law. The United States represents about 20 percent of the pledged reductions of world greenhouse-gas emissions. Much of this will come from Obama’s Clean Power Plan, designed to enforce Environmental Protection Agency standards on carbon emissions, calling on the states to reduce power plant emissions by 32 percent by 2030.

In the campaign, Trump announced that he would seek to repeal the Clean Power Plan and withdraw from the Paris agreement. He has also pledged to go forward with the Keystone XL and Dakota Access pipelines, to slash investment in climate research and development in renewable energy and efficiency innovation, to open up leasing of oil, gas and coal on federal lands onshore and offshore and to revive coal. He has named Myron Ebell, a notorious climate-change denier and head of a think tank funded in part by the coal industry, to head his EPA and energy transition team. Conservative Republicans who dominate the majority in Congress have railed against Obama’s plans. Oil and coal barons — think the Koch Brothers, among the richest people in the world — wield big money and deep pockets on the Republican right.

There are limits on what Trump can do. The stock price of bankrupt Peabody Energy coal company rose upon Trump’s election, but coal is virtually dead, unable to compete in price with natural gas, production of which Trump plans to expand. More dangerous are reports that he’ll move to loosen regulation of gas pipelines. Methane leaks from pipelines are a far more potent contributor to global warming than even carbon emissions.

Elections do have consequences — often ones not intended by voters. Americans elected George W. Bush, who called for a more modest U.S. role in the world and got the president who drove the disastrous invasion of Iraq. A minority of Americans voted for Trump, largely because they wanted change — but few had any idea of the calamitous consequences that will follow if he carries through his know- nothing stance on climate change.

Trump’s efforts will be and must be resisted. 2014 saw the People’s Climate March — billed as the largest demonstration on climate ever in New York City. Another protest is being planned for Washington next spring. Civil disobedience helped stall the building of the Keystone XL and Dakota Access pipelines, and those protests will surely ramp up. The divestment campaign has removed trillions from oil industry investments and 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. In this country, California and New York and other states will continue to push toward renewable energy and require higher efficiency and mileage stands, forcing manufacturers to respond. Democrats in the Senate and House will fight a long, rearguard action to limit the damage.

But time is not on our side, or on the side of civilization. McKibben noted 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 will be too late. We can’t afford to drift for four or eight years; we can’t afford to stall what little momentum has been created. Trump may think that it will be easy to reverse Obama’s climate measures. 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 humans have ever faced. That isn’t about politics; it is about survival.

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