President Barack Obama leaves office three weeks from now having done more than any other leader in our time to defend clean air and water and preserve public oceans and lands.

He quadrupled the Pacific waters protected by the magnificent Papahanau- mokuakea Marine National Monument surrounding the Hawaiian archipelago.

He’s put us on track to cut the dangerous carbon pollution that’s driving global climate change, by cleaning up our cars, trucks and dirty power plants. And he’s helped lead the world to a global shift away from the dirty fossil fuels and toward cleaner, smarter ways to power our future.

We’re all the better for Obama’s measured yet determined environmental leadership — and so are our children.

President-elect Donald Trump and his Republican allies in Congress have a golden opportunity to build on this progress for the good of the country.

Instead, they’ve vowed to roll back these vital gains and dismantle the common-sense policies behind them.

With the help of the billionaires and fossil fuel backers he’s named to his Cabinet, Trump and congressional Republicans are poised to launch what’s shaping up to be the worst executive and legislative assault in history against our environment and health.

If we care about leaving our children a livable world, it’s time to stand up and fight this extremist agenda to put polluter profits ahead of public benefits.

Over the past eight years, Obama has protected nearly 6 million acres of public lands, from the San Juan Islands of the Pacific Northwest to the Katahdin woods and waters of Maine.

These lands help tell the story of who we are as American people. Obama has made that story more faithful to our national experience, by preserving sites important to Native Hawaiians, American Indians, Latinos, African-Americans and others.

He’s also protected more than 450 million acres of Pacific, Arctic and Atlantic waters from industrial fishing, mining and drilling. And he’s placed another 118 million acres of Arctic and Atlantic waters off limits to oil and gas drilling, making the United States the world’s largest steward of protected ocean waters.

The heart of Obama’s oceans protection effort was to add 283 million acres to Papahanaumokuakea, the unique marine monument originally designated by President George W. Bush.

A blue-water sanctuary for endangered whales, sea turtles and more than 7,000 other species, Papahanau- mokuakea preserves places sacred to Native Hawaiians and is world’s largest protected ocean area.

Adding to the public lands and waters set aside for future generations is a proud tradition carried out by presidents from both parties dating back more than a century to Republican Teddy Roosevelt. Trump, though, has said he’ll hand over much of our public waters and lands to industrial mining, drilling and logging, putting our natural inheritance at risk for the sake of corporate profits.

That’s the wrong direction for the country – and so is Trump’s plan to eviscerate the essential safeguards Obama has put in place to protect wetlands and streams that feed the sources of drinking water for one in every three Americans; to defend coal communities against reckless mining practices that can pollute groundwater and streams; and to clean up the dirty power plants that account for 40 percent of the nation’s carbon footprint.

A reckless assault on responsible public oversight won’t help the Americans who are counting on Trump to improve their lives. It will put our health and future at risk.

I hope you’ll join me in letting the president–elect know we won’t turn our backs on eight years of environmental achievement and leave our children to pay the price.

Rhea Suh is president of the Natural Resources Defense Council, an environmental advocacy group.