As a civilization’s population grows, it uses more and more of its planet’s resources. By consuming the planet’s resources, the civilization then changes the conditions of the planet. Thinking about a civilization evolving together with its play is key, therefore, because the fate of our own civilization depends on how we use Earth’s resources.
In order to illustrate the cooperative population-planet system, astrophysicist Adam Frank and his collaborators have, for the first time, developed a mathematical model that shows the possible ways a civilization and its planet can evolve together. Right now the equations are theoretical, but by thinking of civilizations and planets—even alien ones—as a whole, researchers can better decide what we might do as a species to survive.
“The point is to recognize that driving climate change may be something generic,” says Frank, a professor of physics and astronomy at the University of Rochester. “The laws of physics demand that any young population, by 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 on what’s happening to us now and how to deal with it.” Frank and fellow researchers Martina Alberti of the University of Washington and Axel Kleidon of the Max Planck Institute for Biogeochemistry, as well as Jonathan Carroll-Nellenback, a postdoctoral associate at Rochester, published their findings in the journal Astrobiology.
Using the mathematical models, the researchers suggest four scenarios that might occur in a population-planet system:
1. Die-off: The population and the planet’s state (indicated by temperature) rise very quickly. Eventually, the population declines because there are not enough resources. There is still a population, but it is only a portion of what it was at its peak. “Imagine if 7 out of 10 people you knew died quickly,” Frank says.
2. Sustainability: The population and the temperature steadily rise. 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. Both the population and the civilization level off without any catastrophic effects.
3. Collapse without resource change: The population and temperature both rise, so much so that the population collapses and the species becomes extinct.
4. Collapse with resource change: The population and the temperature rise, and the population recognizes it is causing a problem and switches from high-impact resources to low-impact resources. But the response comes too late, and the population collapses anyway.
“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. In the researchers’ models, the carrying capacity depends on the health of the host planet, which is determined by the planet’s temperature. The higher the temperature, the lower the number of people the planet can support, and the lower the carrying capacity.
But carrying capacity is not fixed. It can be higher or lower, depending on the actions of the population. That makes it hard to predict the earth’s carrying capacity, but “estimates are between 10 and 20 billion,” Frank says. “We’re at 7 billion now, and we’re expected to be at 10 billion in about 30 years. So we’re coming close to the projected carrying capacity.”
Right now researchers can’t definitively predict the fate of the earth. The next steps will be to use more detailed models of the way planets 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 another 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.”
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