In the video titled Our House Is On Fire, created by environmental organization Greenpeace, the pair list some of the ways individuals can help the environment.
These include ditching meat and dairy, and joining climate change protests around the globe.
'We cannot let this happen'
"Our Earth is warming up, and our oceans are rising. Extreme weather is wreaking millions of lives," says Eilish in the video, saying that 'we cannot let' this climate emergency 'happen on our watch'.
"Up to 1,000,000 species are becoming extinct because of mankind's actions and time is running out," she adds.
Harrelson says: "History shows us that enough people rise up and demand change, those in power have no choice but to act."
'Take to the streets'
The pair discuss how millions of people are taking to the streets to demand that world leaders listen to the science and act on climate change.
They then list some of the ways to 'take back power' - these include 'voting for leaders who care about the future of our planet', as well as joining environmental movements like Greenpeace or Fridays for Future.
"And doing what we can to live in a greener lifestyle," adds Eilish. "Like cutting out meat and dairy in your diet and reducing plastic use."