In his last address to the nation while in office, President Barack Obama warned Americans about several threats facing democracy, including the threat partisan polarization poses to science and reason.

Speaking to thousands of supporters in his hometown of Chicago, the president cautioned that without agreement on a “common baseline of facts” like the overwhelming evidence in support of man-made climate change, “we’ll keep talking past each other, making common ground and compromise impossible.”

“In just eight years, we’ve halved our dependence on foreign oil, doubled our renewable energy and led the world to an agreement that has the promise to save this planet,” Obama said. “But without bolder action, our children won’t have time to debate the existence of climate change. They’ll be busy dealing with its effects ― environmental disasters, economic disruptions and waves of climate refugees seeking sanctuary.”

Obama added that Americans can and should debate the best way to fight issues like climate change.

“But to simply deny the problem not only betrays future generations,” he said. “It betrays the essential spirit of innovation and practical problem-solving that guided our founders.”

Many Republican lawmakers in and out of Washington deny the link between human activity and climate change, despite overwhelming scientific evidence. Obama’s successor, President-elect Donald Trump, for example, has called climate change a hoax perpetrated by China.