The image on this issue’s cover is a close-up of coral polyps. You can read on page 15 about the specially designed camera that took the picture. What the beautiful image evokes is in the brain of the beholder. In my case, coral polyps brought to mind a hypothesis published in 1842 by Charles Darwin.
One of the scientific goals of the first voyage of HMS Beagle was to explain how coral atolls form. The question was puzzling. Unlike Australia’s Great Barrier Reef or Central America’s Great Maya Reef, which are located in shallow coastal waters, coral atolls are found in the deep ocean. Corals grow upward from the seafloor. If the water is too deep, the photosynthetic algae that live within the polyps and supply their energy can’t get enough light to live.
Darwin theorized that coral atolls grew in a ring-shaped zone around the sloping sides of marine volcanoes that had once risen above the sea. Over time, the seamount subsides and becomes submerged. The still-growing coral, however, continues to reach upward. If the sea level falls or if the seafloor lifts up, the uppermost part of the ring of coral is pushed up above the surface to create an atoll.
One of the ingredients of Darwin’s atoll theory was the idea from James Hutton, Charles Lyell, and other geologists that Earth’s crust was shaped in the past by physical processes that continue to operate in the present. Some of the processes, such as earthquakes and volcanoes, are sudden and violent, whereas others, like the formation of mountain ranges or the transformation of marine sediment into rock, play out gently over eons.
Darwin’s leap to consider how living things might have changed on similarly long time scales is one of the greatest feats of scientific imagination. What is perhaps less well known is that some of Darwin’s successors have tried to predict how Earth’s biosphere will fare billions of years into the future.
In five billion years’ time the Sun will turn into a red giant whose outer envelope will expand to engulf Earth in few-thousand-degree plasma. Before that fiery fate, as helium fusion supplants hydrogen fusion as the star’s principal source of energy, the Sun will shine more brightly and Earth’s surface temperature will rise.
The hotter the Earth becomes, the more vigorously the hydrological cycle will spin—up to a point. When water molecules reach the stratosphere, UV photons will split them, and the hydrogen atoms will escape into space, never to rain down on Earth again. At that point, Earth will have lost its surface water and our planet’s sterilization will be complete.
But life could end before then. In a paper published in 1982, James Lovelock and Michael Whitfield recognized that on a warmer Earth the weathering of silicate rocks would accelerate.1 Metallic cations released into the environment would bind to atmospheric carbon dioxide to form carbonates. The CO2 that plants need to survive would be sequestered out of their reach. Earth’s biosphere, Lovelock and Whitfield estimated, would collapse within 100 million years.
Ten years later Ken Caldeira and James Kasting published a more elaborate derivation2 of the future atmospheric concentration of CO2. Besides certain feedbacks, they also took into consideration the fact that some species of plant, including maize, millet, and other food crops, use a carbon fixation pathway, C4, that is more resistant to heat than the C3 pathway used by trees and considered by Lovelock and Whitfield. According to Caldeira and Kasting, Earth’s biosphere will be dead within 1.5 billion years at the latest.
The prospect that Earth’s atmospheric CO2 will dwindle to the point that all the planet’s plants die off might be taken as justification for doing nothing to curb humankind’s emission of greenhouse gases. Some climate skeptics might argue that burning more fossil fuels would even forestall that fate. It can’t. Earth has far more silicate rocks than coal, oil, and natural gas.
Despite their grim prognostication, Caldeira and Kasting ended on an optimistic note. It should be encouraging to humans, they wrote, “because it implies that we could survive for geologically long time periods if we can manage to cope with our other societal problems.”