An important existential question about human survival is also relevant to the search for alien life: Is it possible for a civilization to survive long enough to explore the stars?

A new paper suggests there are four pathways a civilization could follow as it develops. On three of those paths, civilizations change their environments too much and overuse resources, which causes them to die off.

On one pathway, sustainable existence is figured out.

That's a good reason to take action on climate change, according to the paper's lead author.



It's possible that the human species has already passed the point of no return, and we just don't know it yet.

Or perhaps we're on our way to figuring out how to live sustainably on our planet.

One of the most important questions for the long-term survival of the human species is how civilizations handle the changes they make to their environments as they become more technologically advanced. That question could also help us evaluate the chances that other intelligent life exists in the universe.

A study recently published in the journal Astrobiology suggests there are four different scenarios a civilization can follow as it develops. One of those four pathways leads to sustainable existence. But in the other three, civilizations overuse resources and collapse or die off as a result.

The logical question, then, is: Which path are we on?

"The laws of physics demand that any young population, building an energy-intensive civilization like ours, is going to have feedback on its planet," Adam Frank, a professor of physics and astronomy at the University of Rochester and lead author of the new study, said in a statement.

As humans have developed new technology, our civilization has become more and more energy intensive. In the industrial era, we've mostly burned fossil fuels to meet our energy needs. In doing so, we're transforming our planet and are causing it to heat up — so much so, in fact, that we're now trying to figure out how much climate change is going to impact our world and how to handle that new reality.

"Seeing climate change in this cosmic context may give us better insight into what’s happening to us now and how to deal with it," Frank said.

This perspective on civilizations could also help explain why we have yet to encounter any other intelligent life in the universe.

The four pathways

Frank and his colleagues developed mathematical models of the effects that harvesting energy could have on a planet. Then they modeled how that energy harvesting might affect the continued existence of a technological civilization. As Frank explains in a YouTube video about his paper, the team's models take into account physics, chemistry, and population dynamics.

The researchers also used data from studies of extinct civilizations on Earth, like Easter Island, to inform these models.

Their results suggested four different trajectories for civilization growth. In all of the scenarios, harvesting energy and using resources triggers world-transforming climate chang for any intelligent planetary civilization (like humans on Earth). The question is at what point a civilization realizes that the world is changing, and whether there's time to stave off catastrophe.

Four scenarios for the fate of civilizations and their planets, based on mathematical models developed by Adam Frank and his collaborators. The black line shows the trajectory of the civilization’s population and the red line shows the co-evolving trajectory of the planet’s state (a proxy for temperature). University of Rochester illustration / Michael Osadciw

In the first scenario, the "die-off" model, population growth transforms the planetary environment so much that almost all — about 90% — of the population is unable to survive the changes. Small pockets of people (or other intelligent beings on a different planet) might live on, but probably without any of the modern energy-intensive technology that caused the environmental changes. This scenario is sometimes explored in post-apocalyptic fiction in which humanity has reverted to a pre-energy-harvesting existence.

The second scenario is the sustainability or "soft-landing" model. In this case, Frank said in the video, "the population rises, the planet starts to heat up because of it, but you're able to find a nice stable equilibrium where population comes to a stable level and planet doesn't change anymore."

The third scenario matches that of Easter Island — a cautionary tale. In that case, the population grew far more rapidly than the island's resources could support. When those resources were used up, the population collapsed. We could do the same thing to the entire planet by changing the climate so fast that it becomes inhospitable to human life.

Although that third model sounds scary, the fourth scenario is the one Frank described as most frightening, since it could be where we are now. In that case, a civilization realizes they've triggered rapid environmental change and tries to shift to using resources in a way that won't transform the planet. But that switch comes too late, and collapse happens anyway.

"Even if you did the right thing, if you waited too long, you could still have your population collapse," Frank said in the statement.

A case study of the inhabitants of Easter Island served in part as the basis for Adam Frank's mathematical model showing the ways a technologically advanced population might develop collapse. University of Rochester illustration / Michael Osadciw

The Great Filter

Frank and other scientists like paleontologist Peter Ward have theorized that climate change could be the answer to a famous question known as the Fermi paradox.

The question is simple: Statistically, other intelligent civilizations should almost certainly have evolved in the universe, so why haven't we seen evidence of them?

One possible answer is that environmental transformation (whether that means using up necessary resources or irreversibly changing a climate) is a filter that prevents civilizations from surviving long enough to travel to distant stars. This idea is known as the "Great Filter."

As philosopher Nick Bostrom has explained, this concept suggests that life on an Earth-like planet has to achieve several "evolutionary transitions or steps" before it can communicate with civilizations in other star systems. But an obstacle or barrier makes it impossible for an intelligent species like ours to progress through all those steps before collapsing. That would explain why we haven't heard from or seen any other life.

Bostrom wrote:

"You start with billions and billions of potential germination points for life, and you end up with a sum total of zero extraterrestrial civilizations that we can observe. The Great Filter must therefore be powerful enough — which is to say, the critical steps must be improbable enough — that even with many billions of rolls of the dice, one ends up with nothing: no aliens, no spacecraft, no signals, at least none that we can detect in our neck of the woods."

If that's the reason we've never found evidence of intelligent life, the "Great Filter" idea would suggest that most planetary civilizations can't figure out the sustainable pathway that Frank and his colleagues describe.

But theoretically, it's likely that somewhere in the universe, some civilizations figure out how to become sustainable and avoid collapse or die-off, as Frank explained in the YouTube video.

There are steps we can take to try to achieve that goal here on Earth — but it requires people to get on the same page and act to limit the avoidable effects of climate change. That would require a transition away from fossil fuels and the restoration of certain natural environments. It may also require us to develop ways to efficiently remove excess carbon dioxide from the atmosphere.

"Climate change is not our fault. We didn't trigger climate change because we're greedy," Frank said in the video. Instead, climate change may just be a part of civilizational development. But the way we respond to it going forward matters a great deal.

"Now, however, we know, and if we don't do something about it, it will be our fault," he said.