President Donald Trump and Chinese President Xi Jinping are meeting at Trump's Mar-a-Lago resort in Florida Thursday and Friday to discuss North Korea and trade, but will refrain from discussing another extremely pressing issue: climate change.

China and the U.S. are the world's worst polluters, together producing nearly half of the planet's greenhouse gas emissions—and the leaders' failure to discuss the climate crisis comes as frightening signs show the planet is on the verge of disaster.

Environmental news site E&E News observes that the meeting marks an enormous sea change in U.S.-China relations on the issue:

The shift underscores the dramatic change in political temperature on climate change in Washington. President [Barack] Obama spent a significant part of his second term galvanizing international efforts to reduce greenhouse gas emissions—often in tandem with China. He pressed former President Hu Jintao as well as Xi to modify longtime Chinese resistance to curbing climate pollution and laid the groundwork for the world's two biggest emitters to jointly cut carbon emissions. The U.S.-China relationship became key to the ultimate shape of the 2015 Paris agreement, which both leaders signed.

"I think that is a lost opportunity if [the topic of climate change is] not raised," Barbara Finamore, founder of the Natural Resources Defense Council's China program, told The Independent. "For many years, this issue was one of the brighter spots of the U.S.-China relationship, and it helped to build dialogue."

But under the climate change-denying Trump administration, U.S. participation in international climate talks appears doubtful.

Indeed, Trump once went so far as to describe climate change as a hoax "invented by the Chinese":

The concept of global warming was created by and for the Chinese in order to make U.S. manufacturing non-competitive.

— Donald J. Trump (@realDonaldTrump) November 6, 2012

And after Trump signed an executive order repealing critical Obama-era climate policies last month, politicians in China and other world leaders reaffirmed their commitment to international climate action—with or without the U.S.

In fact, since China's cities have been choked with toxic smog as a result of climate change, the resulting outcry has pushed Chinese leaders to take more urgent action to tackle the crisis and move away from fossil fuels. Already, the country's rapidly declining CO2 output shows it may reach its 2030 Paris target a decade early, according to a Greenpeace analysis cited by US News & World Report.

Trump appears happy to let China leave the U.S. in the dust when it comes to climate action and renewable energy.

Meanwhile, observers note that Trump's refusal to discuss climate change is a bit ironic given the location of the meeting: Trump's Mar-a-Lago resort is located in Palm Beach, Florida, which is already suffering from frequent flooding as result of rising seas.