by Lee Billings
I asked Kasting to tell me again, in simple English, the crux of what the paper had said.
“It’s pretty simple,” he replied. “It said that when the temperature of Earth goes up, the rate of water evaporation increases, too. That puts more water vapor in the air, which ‘takes up’ more carbonic acid, which falls out in more frequent and intense rains. All that increases silicate weathering, which draws down CO2 and cools the Earth. If the temperature goes down far enough to tip over into runaway glaciation, the buildup of CO2 through decreased weathering provides a way to warm the planet again within tens of millions of years.”
Kasting’s voice had risen, and his hands had leapt from his lap to conduct the carbon-cycle symphony he held in his mind. “What we showed, what Walker showed, was that the carbonate-silicate cycle is just like a big thermostat, a stabilizing feedback that generally keeps an Earth-like planet’s temperature away from dangerous tipping points. That’s the whole key—the answer to Hart’s problem, the abiotic alternative to Lovelock’s Gaia hypothesis, the reason why the habitable zone is wide rather than narrow! Without this kind of stabilizing feedback, habitable planets would probably be as rare as Hart thought they were. With it, I can’t help but think they must be very common.”
After the paper’s 1981 publication in the Journal of Geophysical Research, its core conclusions were quickly adopted by stunned planetary scientists around the globe. A separate trio of researchers—Robert Berner, Antonio Lasaga, and Robert Garrels—independently verified the paper’s findings via a more complex study of the carbonate-silicate cycle, one partially based on measurements of dissolved minerals in rivers around the world. The new data revealed that rivers nearer the warm equator contained more of the carbon-rich minerals, while those in higher, colder latitudes contained less, in proportions consistent with Walker’s derivation of temperature-dependent weathering rates. In the 1990s, geologists discovered the Snowball Earth episodes that occurred during the Proterozoic, further boosting acceptance of carbonate-silicate stabilization. In crust that had formed near the equator billions of years ago, they found layers of crushed rock that had been pulverized and deposited by glaciers. In equatorial beds of fine-grained deepwater marine sediment, they found dropstones—large, heavy rocks that had been plucked up and carried far from shore on the crumbling undersides of spreading glaciers. Glacial transport is the only plausible explanation for Proterozoic dropstones, for in that single-celled era of Earth’s history no living creature existed that could hurl massive stones into the sea. Directly surmounting the ancient glacial deposits, geologists found the smoking-gun evidence for Kasting’s proposed carbonate-silicate thermostat: hundred-meter-thick layers of warm-water carbonate rock, laid down in surges of photosynthetic productivity after an atmosphere saturated with volcanic CO2 had rapidly melted away a shell of glacial ice.
In hindsight, the mechanism that Walker, Hays, and Kasting had discovered seemed as glaringly obvious as its applications. Suddenly, the differing fates of Venus, Earth, and Mars became much less mysterious. All seemed to have started with warm temperatures and liquid surface water, but only Earth had maintained those conditions, because only Earth had kept its carbonate-silicate thermostat. Venus lost its thermostat when it lost its water, since water is required to lubricate the motions of tectonic plates and to draw CO2 from the atmosphere to form carbonate rock. Mars lost its thermostat not because it formed too far away from the Sun, but because it was too tiny. The planet ran out of geothermal heat to sustain the volcanism required to recycle carbonates, and its small size allowed most of the Martian atmosphere to slip away into space. Martian water that had once flowed in rivers and pooled in seas instead froze in the ground. If Mars had been a bit larger, it would have been able to recycle its carbon more easily, and it would probably still be habitable today.
• • •
The air had grown damp and cold in the shade of great oaks and black cherry trees, and only small patches of sunlight hit the road through occasional stands of tall, skinny pine. Ahead, Black Moshannon Lake stretched sinuously through a boggy clearing of sphagnum moss, evergreen sedges, rushes, grasses, and leatherleaf shrubs. Plant tannins tinted the lake’s water the color of strong tea. Not another soul was anywhere in sight. We parked alongside a small man-made beach, and emerged beneath a cloudless blue sky. Kasting joked it was perfect weather for one of his 1-D models. “This place will be swarming with people next weekend,” he said, glancing back to the forest’s edge, ablaze with autumnal shades of crimson and gold. “We’re only a few days ahead of peak color. Only a short while after that, maybe a week or so, the leaves will start to fall.”
Prior to the revelation of the carbonate-silicate thermostat, astronomers had generally pegged the end of the world occurring some five billion years in the future, when the Sun will balloon to become a red giant that reduces Earth to a cinder. Planetary scientists had reasoned that, while the planet would still indeed exist by that far-future time, without oceans it would already be long dead—and so the end of the world could be set somewhere between one and two billion years hence, when the oceans boiled off into space beneath the light of a brighter Sun. The carbonate-silicate thermostat laid bare a new, more rapid route to the biosphere’s demise: the gradual geologic drawdown of atmospheric CO2. As the planet’s interior slowly cools, volcanism will decrease, pumping less CO2 into the atmosphere. Simultaneously, the steadily brightening Sun will be gradually raising temperatures, pumping more water vapor into the air to weather rock and draw down more CO2. Eventually, atmospheric CO2 levels will drop beyond the point where photosynthesis can occur, the base of the food chain will collapse, atmospheric oxygen levels will plummet, and the vast majority of life on Earth will die. Walker had realized this from the beginning, writing as the last sentence of the 1981 paper that “the terrestrial biota may, over the long term, have to adjust to the steady disappearance of carbon dioxide as well as the steady increase of average surface temperature.”
In 1982, Lovelock and a colleague, Michael Whitfield, created an elaborate model of the carbonate-silicate thermostat to determine just how much time Earth’s biosphere had left. Their results, published in Nature, estimated that doomsday would come in a mere hundred million years—a very short amount of time for a 4.5-billion-year-old planet. Translated into human terms, Lovelock’s and Whitfield’s prediction was equivalent to telling a forty-five-year-old woman that she has only one year left to live. Astronomers, planetary scientists, and geologists were shocked, but outside of these rarefied fields the news of the world’s imminent demise largely fell on deaf ears—a hundred million years might as well be forever. Typical of dwellers in deep time, it would be another decade before scientists revisited the looming end of the world. In 1992, Kasting and his postdoctoral student Ken Caldeira performed a more nuanced calculation of Earth’s photosynthetic decline, one that gave the biosphere a slight reprieve.
“Plants photosynthesize, but they also respire—they ‘breathe’ oxygen to help fix carbon in their bodies,” Kasting explained as we approached the lake and began to walk along its shore. “Ninety-five percent of all the plant species on Earth, all the trees and most crops, almost everything, they rely on what’s called ‘C3’ photosynthesis. The first step of their photosynthetic pathway makes a chain of organic carbon with three carbon molecules. If you get down below 150 parts per million, C3 plants have to respire faster than they can photosynthesize, so they die. In Lovelock’s and Whitfield’s model, atmospheric CO2 hits 150 ppm in a hundred million years. Ken used my climate model, which is arguably better, and factored in the decay of organic matter and respiration of plant roots, which can pump up CO2 levels twenty or thirty times greater in the soil than in the atmosphere. If you add that in, C3 plants can probably last five hundred million years.”
Kasting bent down and ripped up a few green blades of grass from the soggy turf. “Lovelock and Whitfield also left out C4 photosynthesizers, which are more efficient with carbon. Grasses a
re C4. So are corn and sugarcane. They can subsist on only ten ppm CO2. Our model showed CO2 staying above ten ppm until about nine hundred million years from now, so maybe you’d lose trees and forests, but for another four hundred million years you’d have grasslands and cornfields. You’d have most of what’s around this lake. C4 is a recent adaptation—maybe because of declining CO2—so in all that time evolution might get even cleverer. But once you get below ten ppm, you’re losing most of CO2’s greenhouse effect, so water vapor’s positive feedback takes over, things heat up, the stratosphere becomes moist, and you lose all your water. All the CO2 eventually comes back out of the rocks, but only after rising temperatures have cooked whatever’s left of the biosphere. I wouldn’t say our result is definitive, but it’s using better assumptions, and it gives life on Earth another billion years instead of a hundred million.”
“So Earth’s biosphere is in its autumn years, in its decline,” I prodded.
“I would say it’s summertime for life on Earth, because lots of microbes can live at temperatures of 80, 100 degrees Celsius, which is how hot things will be when the planet starts losing water, and anaerobes and chemosynthesizers can persist even longer beneath the surface,” Kasting matter-of-factly replied.
“Yeah, okay, the meek inherit the Earth. But what about for charismatic megafauna like us?”
“Maybe it’s autumn for complex life. Let’s just generously assume that humans or some form of intelligence can persist until C3 plants go extinct, and then we’ll have real problems. That’s five hundred million years out of what would be five billion years of detectable life on Earth. Intelligence could potentially exist here for one-tenth of the history of life on Earth, maybe one-fifth if you stretch things out with C4. The Cambrian Explosion was about five hundred million years ago. So maybe Earth has a billion, one and a half billion years of complex life in total.”
Kasting stopped walking, and stood silently twisting the grass to shreds between his fingers. “I would say this bears on at least one of the terms in the Drake equation, the fraction of planets that develop intelligent life,” he finally said, once again putting one foot in front of the other. “Whether through limitations of biology or in the geophysical evolution of the planetary environment, it took the first half of Earth’s lifetime to develop complex life. Intelligence has only now appeared at the halfway point in the Sun’s ten-billion-year lifetime, and it won’t be easy to hang around longer than another half-billion years. That’s a legitimate reason to think things like us are rare. People have called me an opponent of Lovelock’s Gaia hypothesis because I helped find an abiotic way to stabilize climates, but I’m more of a critic. It’s clear that life does alter its environment and can modulate climate to its benefit. It’s also clear that life can throw the climate out of equilibrium. It’s all a matter of perspective. Life caused the rise of oxygen, which probably caused runaway glaciation. That’s not Gaian. But then again, the rise of oxygen led to us. It’s probably a mistake to say there’s any kind of purpose in this, but if Gaia could be said to have a purpose, the evolution of higher forms of life, of humans, could be it. And that’s because humans in principle could postpone this planet’s demise and extend Gaia’s reach far beyond Earth. Intelligence and technology could prove to be more powerful than the cyanobacteria. You could call me a techno-Gaian. We probably can’t prevent the Sun from getting brighter, but we could still protect the Earth. The Sun will be a problem in hundreds of millions of years, but if we maintain our progress, within only a century or two we should be in a position to counteract a brighter Sun by making some type of solar shield, maybe orbital clouds of little mirrors to block a fraction of the sunlight from hitting the planet. If we don’t destroy ourselves or destroy the planet in other ways, we could protect the Earth for potentially billions of years. Why wouldn’t we? We don’t want to cook.”
“You don’t think we’re destroying ourselves or the planet right now?” I asked. We had reached the lake’s far shore without seeing another human being. With a sudden heavy roar, a white Ford F-150 pickup truck crested over an adjacent sloping gravel road that ran through the lakeside hummocks, its spinning tires pinging pebbles like buckshot into the trees and underbrush. Three startled cottontail rabbits broke from cover and bounded deeper into the woods.
Kasting frowned and tossed the torn grass on the ground. “I lose sleep over what we’re doing as a species right now. It’s not just the climate, either. We’re squandering Earth’s resources. We’re doing terrible things to biodiversity. I have no doubt we’re living in the midst of another major mass extinction of our own making. I take what little comfort I can from knowing we probably can’t drive life itself to extinction or push the planet into a runaway greenhouse. The carbonate-silicate cycle will erase the fossil-fuel pulse in a timescale of a million years, and then the long decline of atmospheric CO2 will continue. If we knew better, we’d hoard all the oil and coal and gas for when the planet really needed it. There’s easily enough fossil fuel for us to raise the planet’s temperature by ten degrees Celsius and make the Earth as hot as it’s ever been in the past hundred million years, maybe longer. We could probably make the Earth the warmest it’s been since the Archean. That would melt the ice caps, and we might lose twenty percent of continental land area to rising seas. Equatorial regions could become essentially uninhabitable, because many agricultural crops there are already quite near their heat-tolerance limits. Half the world’s population could be displaced. Populations would shrink, and move poleward. Billions of human lives would be lost. . . . But technology keeps progressing. Maybe the global economy will recover in twenty, thirty years. Maybe we’ll figure out reasonable ways to reverse or counteract some of the worst effects of climate change. Maybe we will end up building and launching a TPF, and whatever it finds will make us better appreciate our own planet. I think there’s still time.”
Aberrations of the Light
The sky was hazy and overcast above Cape Canaveral, Florida, on the morning of July 8, 2011. A light breeze from offshore was the only respite against the sticky summer heat for the estimated 750,000 people who lined the beaches and coastal causeways surrounding Kennedy Space Center. They were there to say goodbye, awaiting the launch of NASA’s space shuttle Atlantis into low Earth orbit and into history as it embarked on the final flight of the thirty-year-old space shuttle program.
As the final countdown commenced, the shuttle’s last commander, Navy captain Chris Ferguson, contemplated the program’s end with the mission’s launch director, Mike Leinbach. “The shuttle is always going to be a reflection of what a great nation can do when it dares to be bold and commits to follow-through,” Ferguson radioed from his seat astride the eighteen-story-tall, 4.5-million-pound shuttle stack—the orbiter, side-mounted to a hulking rust-colored external fuel tank and flanked by twin white rocket boosters. “We’re not ending the journey today, Mike, we’re completing a chapter in a journey that will never end.”
Ferguson, like many before him, was echoing the core sentiment of the reclusive philosopher Konstantin Tsiolkovsky, the father of modern rocket science, who, from a remote log cabin in fin de siècle Russia, wrote passionately about space exploration and a human destiny amid the stars. Around the time that Orville and Wilbur Wright were pioneering powered flight at Kitty Hawk, on the other side of the planet Tsiolkovsky was theorizing about launching multistage rockets into orbit, living and working in outer space, and someday escaping the solar system. He famously devised what is now known as the “rocket equation,” a single mathematical formula that encapsulates all the key variables affecting a rocket’s maneuvers. Later luminaries like Germany’s Wernher von Braun and Russia’s Sergei Korolev cited Tsiolkovsky as influential in their pursuit of rocketry to explore and expand into space. In one of his early papers, Tsiolkovsky laid out the visionary impetus behind his work: “The bulk of mankind will probably never perish, but will just keep moving from sun to sun as each becomes extinguished. . . . There is thu
s no end to the life, evolution, and improvement of mankind. Man will progress forever. And if this be so, he must surely achieve immortality.” Often implied but rarely explicitly acknowledged today for fear of cynical ridicule, this vision of an unbounded future beyond Earth remains the purest and most noble purpose behind any human space program.
“I still have dreams in which I fly up to the stars in my machine . . . ,” Tsiolkovsky reminisced a decade before his death. “It should be possible to go into space with such devices, and perhaps to set up living facilities beyond the atmosphere of the Earth. It is likely to be hundreds of years before this is achieved and man spreads out not only over the face of the Earth, but over the whole universe.” Because of the seeming remoteness of his visions from the world in which he lived, Tsiolkovsky considered himself almost a failure, writing when he was sixty-eight that “I have not accomplished much and have not had any notable success.” Tsiolkovsky died in 1935, still believing the conquest of space to be centuries in the future. Had it not been for World War II, he might have been right. But just over two decades after his death, Russian and American satellites were orbiting the Earth, the progeny of military spending on nuclear warheads and ballistic missiles.
Moments after Ferguson channeled Tsiolkovsky at Cape Canaveral, the shuttle’s engines and rocket boosters surged and crackled to life, pushing Atlantis and her crew skyward on a quivering cataract of golden flame and electric-blue shock diamonds. The thunderous roar of its ascent swept over the surrounding scrubby marsh flats for one last time, attenuating over distance so that the farthest onlookers saw the shuttle launch in ghostly silence. Atlantis rose above the launchpad, rolled into an arcing trajectory bound for the International Space Station (ISS), and slipped beyond sight through the low veil of clouds. As the shuttle soared on a final flight into the heavens, it left behind a space program fallen into quiet decay: NASA lacked a ready replacement for the shuttle fleet and was scaling back programs across the entire agency. For years to come it would possess no direct capability to launch humans into space, and its science missions would be in retrenchment.
