COVER STORY
Swansong Earth Earth won’t always be a blue marble – give it a billion years or so and things will start getting decidedly nasty, says Andy Ridgway
sam chivers
W
E 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. 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 28 | NewScientist | 7 May 2016
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. 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
“ What will be the last animals to survive as the planet fries and where will they hide?” 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, page 30). “It’s a little like following the evolutionary tree of life in reverse,” O’Malley-James told New Scientist. “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 >