If we’re not the universe’s first planet-spanning civilization, says physicist Adam Frank, “that means there are likely to be rules for how the fate of a young civilization like our own progresses.” Our Anthropocene civilization is just the current stage of Earth’s ongoing evolutionary experiments.

Our pale blue dot is just one of ten billion trillion planets in the universe, and it’s highly likely that many of those planets hosted technologically advanced alien civilizations. And like the human species, each of those civilizations must have faced the same knife-edge challenge of civilization-driven climate change.

How common is the Anthropocene? asks Frank. “How often do civilizations trigger climate change on their planets? And, most important, how easy is it for a civilization to make it through its Anthropocene bottleneck?”

It’s time to take the existence of aliens—by which we really mean exo-civilizations—seriously. Everything that has been learned in the astrobiological revolutions of the last few decades, writes Frank in Light of the Stars: Alien Worlds and the Fate of the Earth, “now allows us to see just how improbable it is for us to be the only project of civilization in cosmic history. That realization tells us that if we ask the right kinds of questions, the ones backed by the hard numbers of the new exoplanet discoveries, we can begin making out the contours of a science of exo-civilizations that’s relevant to our own crisis on Earth.”

Scientists like to have a universe of more than a thousand data points for whatever they’re studying. With that much data, quantities like averages make sense, states Frank. “So long as nature’s choice for the biotechnical probability is one thousand times greater than the pessimism line, a thousand exo-civilizations will have already lived out their histories across cosmic space and time. Given the already tiny value of the pessimism line,” Frank concludes, “it’s not much of a leap to imagine that a thousand civilizations have already run their course.”

The massive collective project we call civilization began about almost ten thousand years ago, when the last ice age ended and our planet’s climate grew warmer and wetter with the beginning of what geologists call the Holocene, a planetary epoch following the end of the ice ages. But in driving climate change, the human species is now pushing the Earth out of the Holocene into a new era in which human impacts dominate the planet’s long-term behavior.

Does becoming a winner in the game of cosmic evolution mean we hold the Earth in a perpetual version of the Holocene? Frank asks. “Will we never allow another ice age to form? If that’s true, then what about the species that might have emerged in the ice ages we block? Do we have the right to keep them from ever entering the Earth’s drama?”

What we are really concerned with when we talk about the Anthropocene is the habitability of the planet for a particular kind of energy-intensive, globally interdependent, technological civilization that thrives within the present climate epoch—the Holocene. But the once-global oceans of Mars and the five CO2-driven mass extinctions on Earth show how fleeting and temporary life may be with climate driven epochs.

The Earth endured the last ice age for almost a hundred thousand years. Only after the final laggard glaciers retreated, Frank observes, “did the project of human civilization begin. Our history of farming and cities, writing and machine building fits entirely within the Holocene: the current ten-thousand-year-old interglacial period.”

In the face of climate change, deforestation and biodiversity loss, creating a sustainable version of civilization is one of humanity’s most urgent tasks. But when confronting this immense challenge, we rarely ask what may be the most pressing question of all: How do we know if sustainability is even possible? Astronomers have inventoried a sizable share of the universe’s stars, galaxies, comets, and black holes. But are planets with sustainable civilizations also something the universe contains? Or does every civilization that may have arisen in the cosmos last only a few centuries before it falls to the climate change it triggers?

Frank, a professor of physics and astronomy at the University of Rochester, is part of a group of researchers who have taken the first steps to answer these questions. In a new study published in the journal Astrobiology, the group—including Frank, Jonathan Carroll-Nellenback, a senior computational scientist at Rochester, Marina Alberti of the University of Washington, and Axel Kleidon of the Max Planck Institute for Biogeochemistry—addresses these questions from an “astrobiological” perspective.

“Astrobiology is the study of life and its possibilities in a planetary context,” says Frank “That includes ‘exo-civilizations’ or what we usually call aliens.”

Frank and his colleagues point out that discussions about climate change rarely take place in this broader context—one that considers the probability that this is not the first time in cosmic history that a planet and its biosphere have evolved into something like what we’ve created on Earth.

As a civilization’s population grows, it uses more and more of its planet’s resources. By consuming the planet’s resources, the civilization changes the planet’s conditions. In short, civilizations and planets don’t evolve separately from one another; they evolve interdependently, and the fate of our own civilization depends on how we use Earth’s resources.

In order to illustrate how civilization-planet systems co-evolve, Frank and his collaborators developed a mathematical model to show ways in which a technologically advanced population and its planet might develop together. By thinking of civilizations and planets—even alien ones—as a whole, researchers can better predict what might be required for the human project of civilization to survive.

“The point is to recognize that driving climate change may be something generic,” Frank says. “The laws of physics demand that any young population, building an energy-intensive civilization like ours, is going to have feedback on its planet. 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.”

Eco-Civilization Timelines: employing mathematical model, the researchers found four potential scenarios that might occur in a civilization-planet system:

Die-off: The population and the planet’s state (indicated by something like its average temperature) rise very quickly. Eventually, the population peaks and then declines rapidly as the rising planetary temperature makes conditions harder to survive. A steady population level is achieved, but it’s only a fraction of the peak population. “Imagine if 7 out of 10 people you knew died quickly,” Frank says. “It’s not clear a complex technological civilization could survive that kind of change.”

Sustainability: The population and the temperature rise but eventually both come to steady values without any catastrophic effects. This scenario occurs in the models when the population recognizes it is having a negative effect on the planet and switches from using high-impact resources, such as oil, to low-impact resources, such as solar energy.

Collapse without resource change: The population and temperature both rise rapidly until the population reaches a peak and drops precipitously. In these models civilization collapses, though it is not clear if the species itself completely dies outs.

Collapse with resource change: The population and the temperature rise, but the population recognizes it is causing a problem and switches from high-impact resources to low-impact resources. Things appear to level off for a while, but the response turns out to have come too late, and the population collapses anyway.

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)

“The last scenario is the most frightening,” Frank says. “Even if you did the right thing, if you waited too long, you could still have your population collapse.”

The researchers created their models based in part on case studies of extinct civilizations, such as the inhabitants of Easter Island. People began colonizing the island between 400 and 700 AD and grew to a peak population of 10,000 sometime between 1200 and 1500 AD. By the 18th century, however, the inhabitants had depleted their resources and the population dropped drastically to about 2,000 people.

The Easter Island population die-off relates to a concept called carrying capacity, or the maximum number of species an environment can support. The earth’s response to civilization building is what climate change is really all about, Frank says. “If you go through really strong climate change, then your carrying capacity may drop, because, for example, large-scale agriculture might be strongly disrupted. Imagine if climate change caused rain to stop falling in the Midwest. We wouldn’t be able to grow food, and our population would diminish.”

Right now researchers can’t definitively predict the fate of the earth. The next steps will be to use more detailed models of the ways planets might behave when a civilization consumes energy of any form to grow. In the meantime, Frank issues a sober warning.

“If you change the earth’s climate enough, you might not be able to change it back,” he says. “Even if you backed off and started to use solar or other less impactful resources, it could be too late, because the planet has already been changing. These models show we can’t just think about a population evolving on its own. We have to think about our planets and civilizations co-evolving.”

One of the greatest impediments to thinking about exo-civilizations (or our own deeper future, for that matter) is how can we anticipate what kind of technology a civilization that’s a million years older might have at its disposal? Societies that mature might have found entirely new forms of energy that come from thin air. How can our theoretical modeling of exo-civilizations account for unknown sources of energy we haven’t discovered?

The Daily Galaxy via University of Rochester and Adam Frank.com