WE ALL know the ending: everybody dies. Since our sun’s birth 4.6 billion years ago, its core has been getting ever denser and hotter. It is now 30 per cent more luminous than at birth and it’s only going to get brighter. Ultimately, life’s fate is sealed – it will be fried by the sun’s intense energy. Earth will once more be a dead rock.
But let’s not jump ahead quite that far. Take a few minutes to think about Earth’s final chapter. What will be the last organisms to survive as the planet fries and where will they hide? What will our blue marble look like in its swansong millennia? Humanity will have vanished long before the final act, so we will never truly know – but that hasn’t stopped researchers from making educated guesses about how it might unfold.
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The end of life is not going to be a simple decline into nothing. There will be periods of resurgence when new, bizarre life forms are spawned. Mountains will stop growing. When that happens will determine what lives and what does not. And there’s the question of what role we humans will play in Earth’s future – in particular whether we’re able to give ourselves a temporary stay of extinction.
It was James Lovelock, best known for the Gaia hypothesis, who first considered the effect of the sun’s brightening on Earth. In a 1982 paper written with Michael Whitfield, the pair pointed to a known chemical reaction: carbon dioxide in raindrops reacts with silicate rocks, producing solid carbonates. The process, called “weathering”, takes CO2 out of the atmosphere – and the hotter the temperatures, the more it rains and the faster this happens. Whitfield and Lovelock wrote that as Earth warms, weathering should increase, eventually reducing CO2 levels to such a degree that photosynthesis must cease. Sure, taking CO2 out of the atmosphere would dampen the greenhouse effect and put the brakes on rising temperatures, but that’s only in the short term. Over time this would be overwhelmed by the warming of the sun.
“What will be the last animals to survive as the planet fries and where will they hide?“
No photosynthesis means no plant life and no plant life is never good news for animals. Lovelock and Whitfield suggested the complete extinction of life on Earth could unfold in just 100 million years, a mere blink of an eye on geological time scales. While the basic idea has stuck, the current thinking is that it will actually take 600 to 900 million years for CO2 concentrations to fall below the 10 parts per million necessary for photosynthesis.
Astrobiologist Jack O’Malley-James of Cornell University recently sketched out a possible fate for Earth’s swansong biosphere. He teamed up with other astrobiologists and plant biologists, and used what we know about animal, plant and microbial energy needs, as well as other factors like species’ ability to move to new habitats or migrate, in order to build a rough sequence of extinctions over the next 4 billion years (see timeline).
Take a trip on our timeline – from the first spark of life on Earth to the final microbes huddling on a barren, molten rock“It’s a little like following the evolutionary tree of life in reverse,” O’Malley-James told Â鶹´«Ă˝. “Animals get smaller and simpler.” Along the way, some species are expected to fare better than others. Migratory birds, for instance, are able to seek out cooler, higher regions as Earth warms up. And life in the sea should cope slightly better, since water takes longer to warm up than air.
In the simplest scheme, once large and small vertebrates have died on land and in the sea, only marine invertebrates would remain, with microbes to keep them company. O’Malley-James proposes that the last non-microscopic animals will be tube worms living around deep-sea hydrothermal vents.
Things will go from bad to worse 1 billion years from now. By then, average global temperatures are expected to reach 47 °C. The oceans will rapidly evaporate. The additional water vapour in the atmosphere will trigger a runaway greenhouse effect. Microbial life will cling on in shrinking pockets of water. They will be snuffed out first in the tropics, then at the poles. For a time, mountaintops and underground ice caves will provide shelter from the baking heat. But life’s final bolthole will be the deep subsurface, where microbes will continue to eke out a living until – under the most optimistic estimate – they finally disappear 3 billion years from now.
That’s the idea in basic form. The reality is of course far more complicated and several factors could throw huge spanners in the works. For starters, recent studies suggest our understanding of rock weathering could be flawed, which would mess up O’Malley-James’ timing estimates. “While there is a link between warmer temperatures and increased CO2 draw-down,” he says, “other factors, such as relief, rock type and acidity, could be more important.” The net result is that CO2 may not react as quickly as had been thought, which would mean plants carry on for longer than predicted, stalling the biosphere’s collapse by hundreds of millions of years, or longer.
Plate tectonics could also transform the fate of the biosphere. The process is driven by geothermal heat, which comes from radioactive decay of isotopes deep within Earth. But there is a finite amount of material, so the amount of energy released will slowly drop. When eventually plate motion grinds to a halt, mountains will stop rising and, over millions of years, erosion will level the land. “That could happen any time between half a billion and 2 billion years from now,” says David Catling, an astrobiologist at the University of Washington, Seattle. The exact timing will govern life’s final stages, and whether Earth becomes a water world before drying up completely (see Water world scenario).
The moon has a role to play in these events. It is moving away from us by 3.78 centimetres a year. Sometime between 1.5 and 4.5 billion years from now, it will stop stabilising Earth’s tilt. “The poles will start tipping to the line where the equator would have been,” says Lewis Dartnell, an astrobiologist at the University of Kent, UK. Without the moon’s stabilising influence, Earth’s tilt could swing erratically. “That will have extraordinary climatic effects.” If there were still plants and animals, “they probably wouldn’t stick around for much longer”, says O’Malley-James. “The climatic conditions would be constantly changing. If you change things too rapidly, organisms can’t evolve or adapt to the new conditions, and you are likely to get a lot of extinction.”
“New animals could evolve with special organs to shield them from radiation“
There’s another possibility. While the moon is still exerting some control, Earth’s axis could settle somewhere other than its current 23-degree tilt. If it becomes greater, bigger extremes between seasons could keep some regions habitable for longer, says O’Malley-James (see Tilted planet scenario).
Shifts in Earth’s axis and tectonics aside, it is the changes in temperature and CO2 that give us some of the most intriguing possibilities. That’s down to the fact that they are going to rise and fall in a jerky fashion, says Peter Ward, an astrobiologist and palaeontologist at the University of Washington in Seattle. Rock weathering happens 7 to 10 times faster when plants are around, because their roots break up rock and expose more of it to CO2. “But it will get to the point where complex plants die and so you lose roots and the weathering slows down,” says Ward. At the same time, volcanoes will continue belching CO2, so levels will rise for a while. As the sun gets brighter, its luminosity will also become more erratic. A sudden increase in intensity would boost weathering and bring CO2 back down.
Under these conditions, says Ward, Earth’s biosphere will fluctuate. During cooler times, life will get a reprieve, and complex organisms could evolve again – organisms that may be quite different from what we’re familiar with, specially adapted to low oxygen and warm temperatures. Bizarre body plans could evolve.
Ward imagines animals evolving new adaptations, such as shields to protect them from intense radiation – something like a turtle with a shell made of iron-rich minerals. “Or you could almost imagine an animal that has a big bag of water on its back that would protect its inner organs, because water can also serve as a shield.”
And what of us? A look at the fossil record does not paint an optimistic picture. “Mammal species only last about a million years on average,” says Catling. “A species lifespan of 10 million years is very rare.” We’ve been here for 200,000 years so far, so we still have hundreds of thousands of years ahead of us. But odds are we will be long gone before things get really hairy. “I appreciate this is not a popular view,” says Catling. “The conceit that humans are invincible on geologic timescales is widespread and far more popular.”
He cites disease, natural disaster and self-inflicted ecological collapse as possible curtain calls for our species. A rise of just 8 °C would change civilisation as we know it, says Johan Rockström at the Stockholm Resilience Centre in Sweden. Sea levels roughly 60 metres higher than they are today would eradicate most urban centres. Fresh water supplies would shift towards the poles leaving the tropics essentially uninhabitable. “This would most likely mean a concentration of human populations in the southern and northern tips of the hemispheres,” says Rockström.
Let’s be fanciful and imagine Homo sapiens overcame the rather stiff odds and found a way to cope with all this. In that case, we might well evolve to suit our new conditions, says Rockström. What we currently see as the pinnacle of human evolution may turn out to be transient, agrees Catling: “In the optimistic scenario that humans survive, technology will transform any descendants into a post-human species that will be barely recognisable to us. It’s impossible to imagine how exactly advances in modern gene therapy or prosthetic devices will change the human species into another species. But another species will surely be the result.”
And who is to say what that species would be capable of. “In the far future, if humans are still around, or some other intelligent species, they would presumably be doing everything they could to stave off the temperature rises,” says Dartnell. Options are pretty limited. “The only thing that could really delay the planet becoming uninhabitable would be geoengineering on a truly massive scale – basically a planetary sunshade. But by 4 billion years from now, even maintaining a sunshade becomes problematic,” says Catling. There is, however, an even more radical alternative.
As the sun gets brighter, its habitable zone will sweep towards the edge of the solar system, to the point where Earth would no longer fall within it. “So why not move us outwards to a wider and wider orbit so the planet stays within this migrating habitable zone,” says Dartnell. “We could start sending comets or asteroids down towards Earth so they gravitationally slingshot past. If you arrange that encounter, you can transform the orbital energy of the comet into the orbital energy of the Earth and it will migrate outwards.”
But even that would have its limit – no civilisation could possibly withstand the sun’s red giant phase 7.5 billion years from now. So ultimately, the ending is always the same – everybody dies. Unless we’ve moved somewhere entirely different, that is.
This article appeared in print under the headline “Swansong Earth”
