“Some say the world will end in fire, some say in ice,” the poet Robert Frost mused in 1920. Frost famously held “with those who favor fire,” and that poetic view surprisingly coincides with mainstream scientific consensus about the end of the world, which states the sun will in some seven billion to eight billion years evolve into a red giant star that will scorch and perhaps even engulf Earth.
Yet when that happens, Earth will already have been dead for billions of years, and will more resemble present-day Venus. As the sun slowly brightens over time on its path to becoming a red giant, it will eventually cross a critical threshold in which its luminosity surpasses our planet’s ability to dissipate absorbed radiation out into space. At that point, somewhere between one billion and three billion years from now, Earth’s surface temperature will steadily rise until the boiling oceans throw a thick blanket of steamy water vapor around the planet. All that water vapor, itself a potent greenhouse gas, will raise temperatures higher still to cook another greenhouse gas, carbon dioxide, out of Earth’s rocks. The end result will be a “runaway greenhouse” in which the planet loses its water to space and bakes beneath a crushing atmosphere of almost pure carbon dioxide.
Earlier this year, for the first time in human history, atmospheric carbon dioxide reached 400 parts per million (ppm), surpassing a preindustrial average of about 280 ppm that has prevailed with slight variations for the past several million years. Pessimistic projections from the United Nations Intergovernmental Panel on Climate Change forecast atmospheric carbon dioxide levels soaring beyond 1,000 ppm later this century. As the world warms not from a brightening sun but from fossil fuel–burning humans, some scientists have wondered just how close our planet might be to tumbling into a runaway state. Studies in the 1980s and ‘90s suggested the present-day Earth was safe against a runaway, but a paper published this week in Nature Geoscience argues that “the runaway greenhouse may be much easier to initiate than previously thought.” Indeed, the study suggests that without the cooling effects of certain types of clouds, modern Earth would already be well on its way to broiling like Venus. (Scientific American is part of Nature Publishing Group.)
According to the study’s lead author, Colin Goldblatt of the University of Victoria in British Columbia, the disturbing result hinges less on carbon dioxide and more on humble water vapor, which recent investigations have shown absorbs solar radiation more efficiently than previously believed. “The old answer was that a runaway on Earth right now was theoretically impossible,” Goldblatt says. “Even if you evaporated a big chunk of ocean it would just rain back out, because the water vapor would radiate away more thermal energy than it absorbed through sunlight. Our new calculations show that a water vapor–rich atmosphere absorbs more sunlight and lets out less heat than previously thought, enough to put the Earth into a runaway from which there would be no return.”
The upside of the new study is that even though a climate runaway may be possible in theory, it remains very difficult to cause in practice through human greenhouse gas emissions. “We’ve estimated how much carbon dioxide would be required to get this steamy atmosphere, and the answer is about 30,000 ppm of atmospheric carbon dioxide, which is actually good news in terms of anthropogenic climate change,” Goldblatt says. Thirty thousand ppm is about 10 times more carbon dioxide than most experts estimate could be released from burning all available fossil fuels, he notes, although such high values could in theory be reached by releasing large amounts of carbon dioxide from the Earth’s vast deposits of limestone and other carbonate rocks.