Christopher Lloyd, co-star of the beloved trilogy Back to the Future, says that the next installment in the film series should be about climate change.

Lloyd, who played the madcap Doc Brown in the films, has ruminated about a fourth installment of the hit 1980s series for several years. But now he insists that the next film should “convey a message” about climate change.

“I think somehow it needs to kind of convey a message about something that’s important to everyone, universally, like climate change. Some way of incorporating whatever fever is going on at the moment into the film and keep the feeling of one, two and three,” Lloyd said at the recent Niagara Falls Comic Con, according to ComicBook.com. “That’s a tricky, tricky deal. Because you don’t want to do another one and disappoint. So I don’t know. I’d be happy to, for myself. But we’ll see.”

Lloyd also praised his co-star, Michael J. Fox, for his strength while living and working with Parkinson’s disease.

“A couple months ago there was like a bunch of us, Back to the Future [cast], Tom Wilson and Lea Thompson … we had a panel with all of us, and Michael just goes forth and talks, breaks people up, so it’s very cool,” Lloyd said.

Despite Lloyd’s oft-expressed enthusiasm for a Back to the Future 4, there are more reasons than making the film about climate change that dooms another installment. Series creator, Robert Zemeckis, has been just as adamant that there will never be another installment to the series while he is still alive.

Zemeckis even hinted that he would hope that his estate would fight against a revival after his death. In 2015 he told the Telegraph that there may be an attempt to make a sequel after he is long gone. “I’m sure they’ll do it unless there’s a way our estates can stop it,” he added.

Just last year, Zemeckis also said that another installment “can’t be done.”

Fox has also said that if another installment of the series were to happen, he could not be part of it because of his health.

Finally, the film’s co-creator, screenwriter, and producer, Bob Gale, said that Fox’s health makes another B2TF impossible.

In 2008 Gale said, “As I’m sure you all know, Michael J. Fox is not in the best of shape with his Parkinson’s. The idea of making another Back to the Future movie without Michael J. Fox — you know, that’s like saying ‘I’m going to cook you a steak dinner and I’m going to hold the beef.’ You can’t do that.”

Follow Warner Todd Huston on Twitter @warnerthuston.