There's no denying we have a cultural obsession with the end days—we dedicate countless hours of creative energy envisioning the destruction of the world. But let's imagine, for once, that humans manage to survive the zombie apocalypse or the spread of a super virus or catastrophic climate change without wiping out the planet's biodiversity. What would the End look like if we don't cause it?
Science, it so happens, leaves little room for imagination. Most of us know that billions of years from now, our little blue planet is going to go up in flames, when our dying sun splatters fiery bursts of plasma all over the solar system. But this dramatic finale is really just life's epilogue. Well before the sun scorches its surface, life on Earth will have slowly slipped away, over the course of several billion years.
"The end of life on Earth will look a lot like early evolution of our planet, but in reverse," said Jack O'Malley-James, a recent graduate of the University of St. Andrews, Scotland, who could aptly be described as having a PhD in The End of the World.
In research published last July in the International Journal of Astrobiology, and in an earlier study released in 2013, O'Malley-James used computer models to predict how our planet's climate will change over the next several billion years—and what sorts of life will cling on until the bitter end.
In a nutshell, here's how it's going to go down: As our sun ages, its luminosity will increase. We'll begin feeling the effects in about a billion years, when the brightening sun will start steaming heaps of liquid water off our planet's surface. In the atmosphere, this additional water vapor acts as a heat-trapping greenhouse gas. This will kick start a runaway greenhouse effect that signifies the beginning of the end for life on Earth (again, this is assuming we don't kick start the process ourselves).
Rising temperatures and increased rainfall will accelerate the weathering of silicate rocks, allowing them to suck extra carbon from the atmosphere. On present-day Earth, carbon removed by silicate weathering is replenished through tectonic activity. But, as the oceans start to evaporate, friction will build up between the plates. Eventually, scientists believe, the plates will become locked in place, grinding Earth's geologic carbon cycle to a halt.
This is bad news for plants, which require carbon dioxide for photosynthesis.
"The continual decrease in CO2 levels eventually renders photosynthesis impossible for higher plant species, bringing an end to the age of plants. A corresponding decrease in atmospheric oxygen levels, coupled with the loss of primary food sources, would lead to the concurrent, sequential extinction of animal species, from large vertebrates to smaller ones, with invertebrates having the longest stay of execution," OMalley-James writes.
After the demise of insects, Earth will be returned to the microbes. Roughly 1.1 billion years from now, the tiny creatures that got life started on our little rock will again inherit the Earth.
At this point, it's a race against the clock; life is locked onto a course towards complete annihilation. Earth's surface temperature will continue to climb as oceans worth of water vapor pour into the sky. Some 1.2 to 1.8 billion years from now, when surface temperatures reach about 150 degrees Celsius, planet Earth will be virtually sterilized.
The best chance of getting enough liquid water to sustain life would probably be in areas immediately surrounding volcanic vents. These would be my top candidates for life's final refugia.
But microbes don't give up easily. O'Malley-James predicts some hardy survivors will find refuge in the pockets of Earth where conditions are still tolerable.
"This [future] biosphere would favor unicellular, anaerobic organisms with a tolerance for one or more extreme conditions," O'Malley-James writes.
Where will our distant microbial descendants take refuge? Like H.G. Wells's Morlocks, far future microbes might retreat below ground. Cold-trap caves—large volumes of space beneath a narrow entrance- act as refrigerators, sucking in cooler and denser air while excluding warmer, lighter air. Or microbial life could retreat deeper into the subsurface. Scientists now know that a large fraction of life on Earth inhabits the so-called "deep biosphere," probably using inorganic minerals, methane or hydrogen for energy.
Life may also flee to higher ground. Most of Earth's mountains will have long since crumbled away, there being no tectonic activity with which to replenish them. The last mountains on far future Earth will probably be volcanoes, maintained by convection of molten rock in Earth's mantle.
"The best chance of getting enough liquid water to sustain life would probably be in areas immediately surrounding volcanic vents," O'Malley-James told me. "These would be my top candidates for life's final refugia."
And, my personal favorite: Microbes might take to the sky. Our lower atmosphere today is filled with bacteria; many are transplants from other environments. But on a far future Earth, bugs that can survive and reproduce in cloud water may outlast their Earth-bound counterparts.
For the last billion years, O'Malley-James predicts, microbial life will retreat and hide as the planet continues to cook.
This is all morbidly fascinating, but is there any higher purpose to pondering our biosphere's gradual slide into oblivion? It turns out there is. And it's a purpose that shines hope into an otherwise bleak picture: Finding life on other worlds.
Earth is the only life-bearing planet we know, so naturally, in our search for extraterrestrial life, we focus on planets with Earth-like conditions. But for much of Earth's habitable lifetime, conditions would not seem Earth-like at all when compared to the present.
"Understanding what kinds of life could live on future Earth might help us to recognize the signs of life on other planets," O'Malley-James explains.
The notion of a dying Earth as an analog for other life-bearing worlds motivated O'Malley-James to examine what sorts of biosignatures—chemical clues indicating biological processes—future Earth might hold.
"These final traces of life are like the biosphere's swansong before it slides into complete extinction," O'Malley-James says.
Our planet's collapse will evolve over time, decreasing in complexity as the tree of life collapses. For the final billion years, the loudest voice in Earth's swansong will probably be bacteria living near volcanic vents and using carbon dioxide as their sole energy source. The methane gas these bacteria produce may be our biosphere's last, dying note.
Within our lifetimes, it's almost certain we will be able to detect signs of life on other Earth-like planets.
O'Malley-James's work is a step towards answering the question of whether aging planets could produce detectable biosignatures. Sarah Rugheimer, a PhD candidate at Harvard University's Astronomy department, cautions that there's likely to be no silver bullet when it comes to the detection of alien life.
"You'd need to have a combination of multiple biosignatures, along with information on the size of the planet, the distance to its star, and a host of geological factors to say we found life on an extrasolar planet," Rugheimer told me. "But unless it's intelligent life waving at us, we're never going to be able to say with 100% confidence we have found life."
And we're still years out from having the technology to study the atmospheres of Earth-like planets from remote distances. But we're getting ever closer.
"Optimistically, I'd say we'll have the capability to detect biosignatures around a few of our closest exoplanets in ten years," said Rugheimer. "Within our lifetimes, it's almost certain we will be able to detect signs of life on other Earth-like planets."
Hope for the distant future certainly exists, in the possibility of other habitable worlds. And there's a silver lining in mapping the future of the Earth's collapse.
"It's not as depressing as people think," said O'Malley-James. "Looking at the ways that life can survive gives you hope for the biosphere's ability to weather very extreme environmental changes. For me, this work really highlights that, whatever we as humans collectively do to the planet, we are only really hurting our species' chance of survival."
"The biosphere will find a way to go on without us." That is, until the biosphere itself is incinerated by the sun.