Five Billion Years of Solitude (26 page)

Kasting’s thesis leaned heavily on the ideas of James Walker, an eminent atmospheric scientist at the University of Michigan who had taken Kasting under his wing. Consequently, Walker sat on the panel that reviewed Kasting’s thesis. Kasting successfully defended his thesis before the review panel, and afterward the newly minted Dr. Kasting sat down for lunch with his former interrogators. Kasting, Walker, and another panelist, the atmospheric scientist Paul Hays, began discussing Hart’s troubling results and potential solutions to runaway glaciation on the early Earth. Lovelock’s theory seemed plausible but remained frustratingly vague and tautological: the Gaia hypothesis suggested that for a planet to be habitable, it must first be inhabited. Perhaps, Walker offered, something independent of life had acted to circumvent or counteract the runaway glaciation—a dearth of clouds on the early Earth, for instance, could have allowed more ice-melting sunlight to reach the planet’s surface, or volcanic eruptions could have over time covered a glaciated planet in a dark blanket of ash that absorbed more sunlight and melted
the ice. But these explanations felt unsatisfactory—whether or not they would occur seemed more a matter of chance than necessity.

With all the details of the inorganic carbonate-silicate cycle fresh in his mind from his thesis defense, Kasting paused for a moment to think, then said he had a simpler idea. “If the Earth were completely covered in ice, its interior would remain hot and volcanic activity would continue pumping CO
₂
into the atmosphere,” he haltingly began. “But there would be less exposed silicate rock, and the low temperatures would have frozen the water vapor out of the air. . . . So where could the CO
₂
go? Why wouldn’t it just keep building up in the atmosphere until the greenhouse effect melted the ice? Shouldn’t weathering rates depend on temperature? Maybe that’s the way out.” Though they tried for the remainder of lunch, neither Walker nor Hays could find any objections to Kasting’s observation. The next day Kasting left Michigan to begin a stint of postdoctoral research at the National Center for Atmospheric Research in Boulder, Colorado.

After taking us across Bald Eagle Ridge, Route 322 had dumped us in the adjacent valley, where we took another highway northeast toward the town of Philipsburg. Five miles of tree-covered ridgelines, grassy fields, and railway tracks passed by out the window, until Kasting motioned for me to turn left off the highway. As we began a looping ascent up a rough-paved road into rolling, oak-forested hills, he chuckled next to me in the passenger seat and with whimsy in his voice said, “That was probably the single best idea I’ve ever had, but I didn’t even realize it at the time—I was still more interested in the rise of oxygen.”

Ten months after his PhD defense, Kasting was working in Boulder when he received a large package in the mail. Inside was a thick manuscript titled “A Negative Feedback Mechanism for the Long-Term Stabilization of Earth’s Temperature.” Kasting was listed as the paper’s third author, after Walker and Hays.

“Walker had gone off and worked out this whole thing,” Kasting recalled. “Hays, I think, had helped him with some of the math. [Walker] had gathered all the information available about silicate weathering rates,
looking mostly at lab data, and he pretty convincingly demonstrated that, yes, they do depend on temperature and rainfall. From all that data he derived an expression for the weathering rate as a function of CO
₂
partial pressure and the temperature of the planet.”

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 CO
₂
and cools the Earth. If the temperature goes down far enough to tip over into runaway glaciation, the buildup of CO
₂
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 CO
₂
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 CO
₂
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.

•   •   •

T
he 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 CO
₂
. As the planet’s interior slowly cools, volcanism will decrease, pumping less CO
₂
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 CO
₂
. Eventually, atmospheric CO
₂
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 CO
₂
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 CO
₂
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 are C4. So are corn and sugarcane. They can subsist on only ten ppm CO
₂
. Our model showed CO
₂
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 CO
₂
—so in all that time evolution might get even cleverer. But once you get below ten ppm, you’re losing most of CO
₂
’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 CO
₂
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.”