Visionary physicist Stephen Hawking died early Wednesday at the age of 76. An intellectual leader in the study of black holes, quantum mechanics, and physical cosmology, Hawking also found a degree of beloved celebrity that evades most scientists. The best-selling author was a mainstay in the public eye, using his computer-based communication system to explain the wonders of the universe.

In turn, his numerous appearances on television, radio, and the stage gave us an archive of Hawking’s advice for the future. Not one to shy away from the apocalyptic, Hawking was passionate about protecting humanity, which he predicted would face an onslaught of challenges in the years to come.

See also: “Eddie Redmayne Remembers Stephen Hawking in One Surprising Way”

Here’s a sampling of his scientific soothsaying.

Stephen Hawking is headlining the Starmus Festival.
Stephen Hawking died Wednesday at the age of 76.

Hawking Predicted A.I. May Be “The Worst Thing” for Humans

In November, Hawking warned at a technology conference in Lisbon, Portugal, that artificial intelligence could be “the worst thing ever to happen to humanity.” Because what an A.I. can learn is infinite, Hawking reasoned that it could eventually catch up to the limits of the human brain and surpass us.

“Success in creating effective A.I. could be the biggest event in the history of our civilization or the worst,” Hawking said at Web Summit last year. “We cannot know if we will be infinitely helped by A.I. or ignored by it and sidelined or conceivably destroyed by it.”

Hawking also told Wired in November that he feared A.I. would “replace humans altogether,” a concern he had in common with Elon Musk. Accordingly, the two men endorsed a list of 23 principles they feel should steer A.I. development in February 2017.

Hawking Predicted Meeting Aliens Will Be Bad News

It was Hawking’s belief that when humans inevitably meet aliens, we should run. That dread came less from an idea that aliens will be inherently bad, and more from his observations of humans. Much like Christopher Columbus triggered chaos in his coming to the Americas, colonizing aliens would also bring turmoil to our proverbial shores.

“We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet,” Hawking told the Times of London in 2010. “I imagine they might exist in massive ships, having used up all of the resources from their home planet. Such advanced aliens should perhaps become nomads, looking to conquer and colonize whatever planets they can reach.”

But He Also Predicted We Probably Won’t Encounter Aliens Soon

Despite his concerns about a hostile alien civilization, Hawking never said this alien invasion would happen anytime soon. In April 2016, he explained at a conference for the space exploration project Breakthrough Starshot that the next 20 years, at least, will likely be alien free.

“The probability [of finding alien life] is low — probably,” Hawking told the crowd. “But the discoveries from the Kepler mission suggest that there are billions of habitable planets in our galaxy alone. There are at least 100 billion galaxies in the visible universe, so it seems likely that there are others out there.”

Stephen Hawking, Big Bang
Stephen Hawking: not a fan of aliens.

Hawking Predicted Our Time on Earth Would End

During his work with Breakthrough Starshot, Hawking asserted that within the next thousand or 10 thousand years, humans living on interstellar colonies would be absolute certainty. This would be, in Hawking’s opinion, for the best. Earth, he predicted, was in danger of experiencing astronomical events like asteroids and supernovas. To survive as a species, he declared in April 2016, “we must ultimately spread to the stars.”

This wasn’t a one-time prediction from Hawking. At the Starmus Festival in June 2017, he declared that humans needed to prepare for an exodus off this planet sometime within the next 200 to 500 years because of our own damage to Earth.

“We are running out of space, and the only place we can go to are other worlds,” Hawking told a crowd in Trondheim, Norway. “It is time to explore other solar systems. Spreading out may be the only thing that saves us from ourselves.”

Hawking Predicted Climate Change Could Ravage Earth

Hawking joined many scientists in his assertion that climate change could spell out the end for our planet, but it’s on this topic that he struck a (relatively) more hopeful tone. Sure, climate change could kill us all, but that doesn’t necessarily mean it will happen.

“We are close to the tipping point where global warming becomes irreversible,” Hawking told BBC News in July. “Climate change is one of the great dangers we face, and it’s one we can prevent if we act now.”

To move away from this tipping point, Hawking argued, world leaders like President Donald Trump (of whom he was no fan) would need to stick to the rules laid out by the Paris Agreement. According to Hawking, we aren’t at doomsday yet — and it’s up to our actions and ingenuity to keep it that way.

Photos via Lwp Kommunikacio/Flickr (1, 2), Bryan Bedder / Stringer