Earth in Human Hands Read online

  By the time Sagan was ejected from Harvard’s orbit and captured by Cornell in 1968, he probably knew more than anyone alive what the heat-absorbing properties of CO2 and water vapor could do to climate on an Earth-like planet. He was also becoming more visible—as a popularizer, activist, and public spokesman for science—and concerned about widespread public ignorance of science in a democratic society where, increasingly, scientific literacy was key to understanding important issues. Once the science of climate modeling became good enough (in the 1970s) to indicate that global warming from industrial CO2 was cause for serious concern, Sagan became the first person to speak about the problem effectively to the American public; he brought it up often and with urgency in the 1980s. He inspired Al Gore to focus on the topic, and in this way can be said to have helped plant the seeds for our current noisy cultural debates on the issue.

  The Cold and the Dark

  Shortly after Sagan left Harvard, Jim Pollack took up residence at Ames Research Center, a sprawling NASA facility housed in a former naval air base built on the bay-fill flats jutting into San Francisco Bay, a few miles north of San Jose. Geographically, it would be accurate to describe it as being nestled within Silicon Valley, but Ames was there first, since the founding of NASA in 1958, long before anyone imagined such a thing as a personal computer or an industry built around it.

  Much of our knowledge of planetary climate has come from Ames, and this can be traced largely to the outsize influence of Jim Pollack. Pollack was knowledgeable about seemingly everything in planetary science. He was ubiquitous at conferences, where he always sat in the front row (like his mentor Sagan) and seemed to ask the first question after every single talk.

  Jim Pollack in his office at NASA Ames.

  One of the most thrilling intellectual experiences of my life was, after grad school, going to work at Ames as a postdoc, with Jim Pollack as my adviser. Pollack was a classic nerd in the most loveable way, one of the quirkiest people I’ve ever known. His office looked like it had been decorated in 1970, on the day he moved in, and never altered since. It had the feel of a place where minds ranged freely across the universe, paying scant attention to the here and now. Atop his filing cabinet was a little plastic diorama of an Apollo lunar landing site, complete with astronaut and flag, and tacked on the walls were several large posters, yellowing with age, of cute lions and tigers lounging like pussycats. I don’t know if Jim identified with these big, mellow felines, but I thought he was one of them: he was at the top of the scientific food chain, but I never saw him bare his claws. Once, Carol Stoker, another planetary scientist at Ames, told me, in shock, that Jim had complimented her on something she was wearing. “I always thought that I could walk in there completely naked,” she explained, “and he wouldn’t notice.” Jim had a habit of absentmindedly tearing off pieces of Scotch tape and sticking them on his desk as he spoke to a visitor or on the phone. His desktop, showing years of tape accumulation, had a semitranslucent sheen. He spoke so slowly and deliberately—sounding a lot like Cheech or Chong without the cannabis haze—that you were always tempted to finish his sentences for him. This, you quickly learned, was a mistake, because his response to any interruption was to begin the sentence over again. You had to let him proceed at his own pace, but if you listened, you always learned. I can still picture his diminutive handwriting on the dusty blackboard in his little office as he drew diagrams of cloud layers and wrote equations describing the clouds’ response to radiation.

  Building for decades on his thesis work with Sagan, Jim had become a leading expert in “radiative transfer,” a key tool for calculating the magnitude of the greenhouse effect and other aspects of planetary climate. Radiative transfer is the mathematical treatment for how different kinds of radiation pass through different kinds of atmospheric gases and particles (a word that, in Jim’s quirky pronunciation, rhymed with “sparkles”). His numerous students and mentees spent hours imitating his singular mannerisms and patterns of speech. This was done for amusement but also with great love. Maybe my memory is hazy or rosy, but I think I can say, as much as anyone I have ever known, that everybody loved Jim Pollack. He was also openly gay at a time when that was nearly unheard of in our field.

  One of Jim’s intellectual passions was figuring out general principles of climate evolution on Earth-like planets. Fortunately for me and many others, he was an enthusiastic and patient mentor. There was usually a line of acolytes outside his door, as his assistants, students, and postdocs on the many projects he juggled queued up for an audience with the master. Once you made it inside the room, there was no hurry, though. Learning radiative transfer from Jim felt like training with a Jedi master. Together we worked on a number of problems involving the influence of clouds, dust, and impact events on climate and planetary evolution. Jim was a “great attractor” of planetary climate. Through him I met many of the people at the interface of planetary exploration and climate studies and saw firsthand the cross-fertilization between earth science and planetary climate science that has helped us to expand our ideas, gain confidence that we haven’t missed anything important, and sharpen our tools as we continue to study the phenomena that all these worlds have in common.

  At Ames for three decades, until his untimely death to spinal cancer at the age of fifty-five in 1994, Jim led, trained, inspired, and guided a small army of researchers at the forward edge of interplanetary studies. His mentorship touched an impressive number of careers. Twenty years after his death, many leading voices in Earth and planetary climate studies count him as a major influence.

  During the 1970s, Pollack and his colleagues integrated the results coming back from the first wave of interplanetary spacecraft into new models of planetary climate. They followed a path of discovery that ultimately led through Martian dust storms, historic volcanic eruptions, dinosaur extinctions, nuclear war, primordial climate history on Earth and Venus, a smoggy moon of Saturn, and current debates about “geoengineering.”

  Comparative planetary studies have increased our ability to model atmospheres in physical condition not found on Earth today. This helps us understand possible past and future environments on Earth. Sometimes what we find on other planets provokes us to ask questions about Earth that might not have otherwise occurred to us. Good science often starts with “what if.” What if massive amounts of light-obscuring dust suddenly filled a planet’s atmosphere?

  In 1971, Jim Pollack arrived at Ames, and Mariner 9 arrived at Mars amid that intense global dust storm. The dust was an annoyance for the geologists who were hungry to study the surface features of Mars. For Pollack, though, it was an irresistible scientific mystery. He wanted to understand the genesis and evolution of Martian global dust storms. Bringing to bear his great insight into radiative transfer, he teamed up with one of Sagan’s first grad students at Cornell, Brian Toon, whose 1975 thesis was on “Climate Change on Mars and Earth.” Toon is a master of microphysics. That’s the part of climate modeling where we consider populations of aerosols, that is, tiny particles suspended in a planet’s atmosphere. A microphysical model simulates all the things that can happen in aerosols’ interactions with one another (colliding and merging, dissolving in raindrops, falling to the ground, etc.) and with radiation, and reveals how, in aggregate, all these minuscule events affect the overall climate. Toon had done a lot of early development work on a model called CARMA (Community Aerosol and Radiation Model for Atmospheres). If you want to include clouds, hazes, or dust in your climate model, you need a microphysics code, and CARMA, which has Toon’s fingerprints all over it, is still one of the most widely used for climate modeling on Earth and other planets.

  Pollack and Toon added large amounts of dust into a Mars climate model to see what would happen. What happened was that, as dust absorbed sunlight and reradiated into the surrounding air, the atmosphere heated up. At first this stirred up winds, kicking up more dust, and feeding back to grow the storms into gargantuan size. Yet, as the dust got thicker and sunlight
was blocked from penetrating into the depths, the lower atmosphere cooled off, muting and then eliminating the greenhouse effect. Eventually, when the global pall got thick enough, this shadowing evened out the surface temperature, which calmed the winds, which allowed the dust to settle, causing the model storm to come to an end in much the same way the actual Martian storm had.

  In elucidating the mysteries of Martian global dust storms, Pollack and Toon learned a lot about the effects of dust on planetary climate, and further developed modeling tools that soon came in handy for some problems closer to home. They used CARMA to study the effects of large volcanic eruptions on Earth’s ancient and modern climates. In the late 1970s they teamed up with their mentor Carl Sagan to study how humans, over the history of civilization, had changed the reflectivity of Earth and thus the climate. Using the techniques, perspective, and language of planetary exploration, Sagan, Toon, and Pollack published a paper in Science in 1979, long before climate change became the issue it is now, entitled, “Anthropogenic Albedo Changes and the Earth’s Climate,” in which they discussed how changing land use practices by human societies (starting with fires set by hunter-gatherers, expanding with the Agricultural Revolution, and accelerating with the Industrial Revolution) had likely been influencing our planet’s climate for a very long time.

  Then, in 1980, Walter and Luis Alvarez published their earth-shattering proposal for the end-Cretaceous mass extinction. The evidence was solid that there had been a massive impact at just the right time in Earth’s history to coincide with the extinction event. It stood to reason that such a large impact would have caused calamitous environmental changes, but what exactly would such an object have done to the planet and its life? What was the kill mechanism? One effect of such an impact would have been to throw massive quantities of dust into the atmosphere, which would quickly have spread around the world by winds. We know this happened because the centimeter-thick layer of clay that marks the end of the Cretaceous sequence of rocks (the same layer where the Alvarezes found the extraterrestrial “iridium anomaly”) is the remnant of this pall, the globally distributed fallout left when the dust settled.

  Could the impact-generated dust cloud itself have brought about enough climate change to cause the mass extinction? Pollack and Toon, armed with knowledge and models from Martian dust storms and terrestrial volcanoes, were well positioned to attack this problem. They teamed up with meteorologists Tom Ackerman and Rich Turco, computer code specialist May Liu, and a young Ames postdoc named Chris McKay, who had already, as a grad student in Colorado, made a name for himself as an expert on Mars and an activist for future Mars exploration.

  To start, they needed to know how much dust would actually have been blasted into the atmosphere and spread around by such a large explosion, and how finely ground that dust would have been. The best data came from nuclear test explosions. So Toon and colleagues immersed themselves in studies of bomb-generated dust plumes and figured out how to scale those numbers up for the much larger explosions caused by giant asteroid impacts.

  In 1982, they published their results: impact dust would have drastically reduced sunlight reaching Earth’s surface, dramatically cooling and darkening our planet for several years. In a suddenly dim and wintery world, many organisms would have frozen to death. Most of those left would have starved because photosynthesis would have been shut down. This work established that an impact-generated dust cloud was a likely cause for the mass extinction, and stands today as the seminal work on the climate effects of a large-impact-generated dust cloud.

  All that immersion in nuclear test data and visions of a world suddenly gone dark fed into another project the team had started. Remember, at this time the Cold War was still raging. This conflict had provided the rocketry and impetus for the first wave of planetary missions, which had given us the tools to imagine and model the climates of past and future Earths. The world had become used to the superpower standoff. Yet the hidden machinery of global mass destruction—the underground silos with their thousands of missiles, armed with multiple independently targetable city-incinerating bombs; the nuclear submarines stealthily prowling the deep; the bomber squadrons at the ready, practicing for a day nobody wanted and everyone feared, when they might, through accident, escalation, miscalculation, or madness be pressed into service—still sat and waited, occasionally snapping to heightened attention when the United States and the USSR squabbled or their proxies came to blows.

  Turco, Toon, Ackerman, and Pollack realized that with their studies of Martian dust storms, ancient mega-volcanoes, and Cretaceous impacts, they had developed the tools needed to model the climate effects of a nuclear war. Looking into the problem, they realized that smoke would cause a more severe climate effect than the dust from the bomb explosions. The smoke from burning cities would rise to the stratosphere in giant thermal plumes and quickly spread around the entire Earth. When they included the massive petrochemical fires that would be ignited in such a conflict, they found that the nuclear destruction of as few as one hundred cities would spread enough soot and dust into the stratosphere to plunge the world into a deep freeze, similar to the aftermath of a large asteroid impact. They named this “nuclear winter.”

  This work proved to be problematic. In addition to the technical challenges of ensuring they had done the modeling right for what would clearly be an incendiary result, they realized their results had huge implications for defense policy. They basically showed, scientifically, that current U.S. defense postures were suicidal. They had followed their intellectual muse into political territory that NASA, the hand that fed them, might perceive as a bite. Could they get away with publishing this work and still retain NASA support? Toon and Pollack enlisted Sagan’s help. Not only did their old mentor have a valued big-picture perspective on this type of scientific problem, but he had also become, by that time (post Cosmos), a big shot within NASA and beyond. Sagan added his gloss to the science, but mostly he helped navigate the NASA politics and prepare for the global cloud of controversy they knew would be raised by the publication of this work. They added Sagan’s name to the paper, which subsequently and forever more became known as the TTAPS study (pronounced “Tea Taps”), for Turco, Toon, Ackerman, Pollack, and Sagan. In late 1983 the TTAPS paper was published in Science with the title “Nuclear Winter: Global Consequences of Multiple Nuclear Explosions.”

  As predicted, the work was controversial. Hawkish politicians denounced it as politically motivated, often in combination with personal attacks on Sagan. Some scientists attacked the work on technical grounds, nitpicking various model assumptions (which is a normal and healthy part of the process). Sagan went on the offensive, arguing in lecture halls and TV studios that nuclear winter theory demanded deep rethinking of the strategic postures of both superpowers. He debated Edward Teller in front of Congress and led a delegation to meet with the pope.

  Whatever one thought about the details of their physical models, the TTAPS study and the wider debate it ignited helped drive home the absurdity of nuclear strategies dependent on massive deterrence. The United States and the USSR had created a situation where even a limited nuclear conflict would cause a climate disaster that could quite possibly, among other things, collapse global agriculture, dooming civilization as we know it. With these weapons, there was no destroying your enemy without also destroying yourself. It brought to mind Stanley Kubrick’s brilliant Cold War dark comedy, Dr. Strangelove, in which the Soviets create a “doomsday machine” that will detonate if a nuclear war starts, rendering the entire world uninhabitable. The TTAPS nuclear winter study revealed that we had, unwittingly, built such a machine. These results were widely discussed in the security communities of both superpowers, and are often cited as helping to motivate the partial disarmament that both sides undertook as the Cold War wound down.

  Anti-Greenhouse

  In all these studies, Pollack and his collaborators were discovering variations that can be induced, by changes in quantities of gase
s or suspended particles, in a planetary greenhouse. Then, by extending this work to the outer solar system, they discovered that planets can also have an anti-greenhouse.

  A billion miles from here is another world where cold rain falls through nitrogen skies. Out in the realm of gas giants and ice dwarfs orbits Titan, Saturn’s strangely Earth-like moon. It’s the only other world we know of with a thick atmosphere made mostly of nitrogen. Complex climate feedbacks seem to have played a central role in Titan’s evolution, and understanding all the competing, interacting processes is an irresistible challenge for comparative climatology. While I was at Ames, Pollack’s research group was modeling the hazes of Titan, and as so often happens when we study alien atmospheres, this work shed new light on a phenomenon that is important for understanding the past, present, and possible future climate of Earth.

  The second most abundant gas on Titan is methane (CH4, otherwise known as “natural gas”). It plays the same role there that water plays on Earth. It’s so cold on Titan that methane condenses into liquid and rains out on the surface. There it carves steep river valleys, erodes desert plains with occasional flash floods, pools in great lakes, and evaporates into clouds, forming weather fronts and storm systems that, again, bring rains to the icy plains. On Titan, this “methalogical cycle” (an analogy to the hydrological cycle that defines so much of Earth’s character) shapes surface landforms that seem dreamlike to our earthly senses—strange yet oddly familiar. Until recently, however, when NASA’s Cassini spacecraft showed up with infrared and radar eyes, these details were completely hidden to us. They were obscured by another feature that also results from all that methane in the air. You’ll never see that interesting surface from orbit or through a telescope. When you look at Titan, all you see is an orange-brown, fuzzy, featureless ball, not unlike Mars in the throes of its worst global dust storms. Also, if you lived on Titan, you would never see the stars, or even giant Saturn hanging in the sky. The upper atmosphere is permanently shrouded in a thick brown haze that, we’ve learned, is smog made up of organic molecules. (And you thought mid-twentieth-century Los Angeles was bad!) This organic haze is produced when the ubiquitous methane molecules are ripped apart by ultraviolet sunlight* and the desperately unstable molecular fragments find one another, eagerly recombining to make various organic molecules. Titan’s upper atmosphere is a nonstop factory of complex organics, which both shroud this world in its permanent smoggy haze and snow down on the icy surface. There they gather in vast dune fields that blow around in the nitrogen winds and dissolve in the methane lakes. The presence of all these organics on Titan resembles our picture of the primordial Earth and the conditions that led to the origin of life. It seems to present a freeze-dried portrait of a crucial lost phase in our own biological origin story. This is one reason we astrobiologists are obsessed with Titan. Another is the fascinating and complex climate balance.