Some of the initial resistance to the Alvarez hypothesis was surely due to this training. Yet as we connected the dots between Earth, the heavily cratered surfaces of other planets, and the stray asteroids and comets populating interplanetary space, we realized that this picture was incomplete.
One thing we discovered in exploring the solar system is how very young Earth’s surface is compared to almost everywhere else. Most of the planets have been much more inactive than our oddball, hyperkinetic Earth, and their beyond-ancient histories are laid bare, billions of years unburied and raw on their surfaces waiting to be observed and interpreted.
When geology discovered deep time, it seemed to eliminate the need for catastrophic explanations. Then, however, space exploration uncovered deeper time. This expansive view revealed that, on even longer timescales, sudden, catastrophic events actually have played a big role. Of course we missed this before we looked beyond Earth. The typical interval between very large impacts is not only greater than the life expectancy of a geologist, and not only greater than the whole expanse of human civilization, it is also much longer than the few million years during which there have been creatures anything like human beings. There are, we started to realize, important events shaping Earth that come around only every few tens of millions of years. To understand this, we needed to expand our horizons and realize that, geological dogma notwithstanding, the present is not the key to the past—or, at best, it’s an incomplete key. Guided by the flawed uniformitarian doctrine that what we can observe occurring now represents the complete set of geological processes, we had missed a major element of Earth’s story.
Yet the Alvarez hypothesis remained controversial. Many old-school geologists and paleontologists strongly resisted the idea. There was a fair amount of scientific mud wrestling, with some insisting that the mass extinction had been caused by sea-level changes or a sudden outpouring of volcanic lava that flooded the climate with CO2. Eventually, as usually happens, the net effect of this conflict was to move science along. Those defending the Alvarez hypothesis came up with better and better evidence for it. Then, in the 1990s, the hole left by the impact was finally found, a 190-mile-diameter crater called Chicxulub buried along the coast of the Yucatán Peninsula in Mexico. Today there is little doubt that a large impact was the major cause of the end-Cretaceous extinction.2
When I started grad school in 1982, large-impact events were all the rage. The Alvarez hypothesis was the source of much buzz and debate at several of the first conferences I attended. Also around that time, another huge mystery was solved invoking a giant collision. The origin of Earth’s huge moon* was still unexplained. This was an embarrassment—kind of pathetic, really, given our two decades of exploring the solar system, our collections of Moon rocks, and our (we thought) sophisticated understanding of planetary origins. How come we still couldn’t explain our moon? We had theories, but none of them really made sense. Then, in 1984, scientist/artist/Renaissance man Bill Hartmann from the Planetary Science Institute in Tucson proposed that the Moon was formed in the last act of accretion, when proto-Earth suffered a collision with a Mars-size protoplanet. The idea works. It explains why Moon rocks, chemically, are like Earth rocks that have been thoroughly cooked. Sophisticated new computer simulations showed that such a collision would have surrounded Earth with a ring of molten material that, in a matter of months, would have coalesced to form a giant moon. Problem solved.
Also at that time, yet another wild idea involving worlds in collision was causing a stir: meteorites from Mars. About a dozen strange meteorites had turned up in collections around the world. Based on their rock types and ages, some researchers suggested the outlandish hypothesis that these stones came from the Red Planet. The community reacted with appropriate skepticism to what seemed like a marginal idea. Then it was found that some of these rocks had little bubbles of trapped air that did not seem like Earth’s atmosphere but that did exactly fit the atmosphere measured on Mars by the Viking landers. That clinched it. They really did come from Mars! Only, how did they get here? It was proposed that they were blasted off the Martian surface in large impacts, drifted through space, and eventually crashed to Earth. Computer models projecting the orbital path of shrapnel ejected from Martian impacts confirmed that it was not only possible but likely that some scraps of Mars should have ended up here.
The implications were enticing. The planets were not isolated. They had, over their lifetimes, been occasionally splashing material onto one another. Could this possibly mean that living organisms might have passed between them, seeding life across the void? Especially when you consider that the frequency of impacts was much greater when Earth and Mars were young (something we know from counting craters on different surfaces) and that some microbes and spores are impressively hard to kill, this seemed to change the equation when it came to figuring the odds of life on neighboring planets. Life might be something that could spread, naturally, between worlds!
When I was figuring out what I would do for my thesis work, all this was in the air: giant collisions had caused mass extinctions on Earth, had formed the Moon, had perhaps determined the nature and timing of the origin of life on Earth, and had blasted rocks, maybe even living organisms, between Earth and Mars. It was an exciting time to be studying planetary science, and Tucson felt like a place where important new ideas were being generated and discussed. One day Walter Alvarez stopped by to talk to my PhD adviser, John Lewis, about the atmospheric effects of large impacts, and I got to join in the conversation. The idea of disruptive invaders from outer space had stirred up ideas about planetary evolution, and the dust had not settled. So I pursued my dissertation research on “when bad things happen to good planets”—my actual nerdy title was “Large Impact Events and Atmospheric Evolution on the Terrestrial Planets.” This led me to a career in studying the (often catastrophic) ways that planetary climates and environments can change.
Building a Greenhouse
One of my thesis chapters examined how giant impacts might have changed the very early environment of Earth. I was learning about the evidence that such impacts were much more frequent four billion years ago, when life was first trying to get a cell hold. I wondered how that would have affected climate during that formative chapter, and realized that I could simulate this.
First I had to learn the basics of climate modeling, build a working model of Earth’s early climate, and then see what happened when you disturbed it with multiple impacts that repeatedly filled up the atmosphere with massive clouds of dust.3 So I built “baby’s first climate model.” The starting point was learning the equations representing the passage of different kinds of radiation through atmospheric gases, and translating them into some lines of computer code. My first model was pathetically simple by today’s standards, but there is something very empowering about building a model that works. It was a rush when the numbers came out, I graphed them up, and they made a temperature profile that mimicked the actual atmospheric temperature structure of Earth’s atmosphere. The model atmosphere got colder with increasing altitude, with the same slope as the actual real-world atmosphere, and then leveled off at the right height. When your model fits the data, the satisfaction has its own specific flavor. You feel that you have really figured out some small part of nature, not conquering it, but learning to hear its music, and to sing along. Now, having had that experience, whenever I see data representing temperature structure on any planet, they are forever slightly less mysterious to me. I simulated a physical process that was then obscure but has since become part of everyone’s vocabulary. I produced a simple model of the greenhouse effect, the process by which an envelope of atmospheric gases heats up a planet’s atmosphere.
An airless planet—and by now we’ve been able to study many of these—absorbs energy from visible sunlight, so it heats up until it is emitting the same amount of energy back into space in the form of infrared radiation. If you surround that planet with an atmosphere, it has more trouble
radiating away the same amount of energy, so its temperature will rise by an amount that depends on both the overall thickness of the air and what portion of it is composed of “greenhouse gases” that are relatively opaque to infrared radiation. What makes a good greenhouse gas? Molecules such as CO2, CH4, or H2O that are composed of three or more atoms. These more complex molecular structures have the arms-a-flying, twirling-hippy-dancing-to-Phish kinds of vibrational motions that get excited when hit with infrared light. Diatomic gases (those with just two atoms), such as O2, N2, and H2, don’t dance like that, don’t absorb infrared, and are not good greenhouse gases. If a planet has a sizable atmosphere with a considerable portion of the big, floppy gas molecules, its climate will be warmed by a significant greenhouse effect.
The greenhouse effect is a blessing for us, one of the crucial factors that make life on Earth possible. Without about 90 degrees Fahrenheit of greenhouse warming, provided mostly by traces of carbon dioxide and water vapor, our planet would be permanently frozen, and quite possibly lifeless. The problem, of course, is that we’ve been loading the atmosphere with carbon dioxide (one of those flippy-floppy triatomic molecules) faster than it can be absorbed by the carbon cycling of Earth, and thus increasing the magnitude of the effect, with consequences that we can’t fully predict.
I first learned about greenhouse warming as a subset of the knowledge we needed to understand planetary evolution. It’s one of several aspects of climate physics that have (somewhat strangely for us students of planetary science) become quotidian. Not too long ago this was merely an arcane topic tackled by grad students learning the basics of planetary climate, but in recent years it has become familiar to anyone even remotely plugged in to the cultural and political issues of our time. I have marveled at how some of the esoteric concepts I absorbed in grad school, when it seemed nobody cared, have now become commonplace in the cultural lexicon. Terms such as polar vortex (a whirlwind pattern found around the poles of Venus, Mars, Jupiter, Saturn, and Earth), albedo (the total fraction of light reflected by a surface), and cloud forcing (the ability of cloud formation to magnify or dampen a change in climate), terms that seemed then to be part of our secret nerd language, are now bandied about in newspapers and online political forums. Now the greenhouse effect is everywhere. Community activist groups hold teach-ins about it. Schoolkids learn it in their science lessons. Pundits shout about it on television. This is all for the good. Despite the polarization, the noise, and the fury, this diffusion of science concepts from the priesthood to the masses is necessary and long overdue.
Today, climate change is on everyone’s mind and screens, but few realize how our fundamental understanding of climate and its potential for change or stability has been enriched through comparative planetology. As we look around the solar system, we see a range of planetary atmospheres, each with its own version of the greenhouse effect, some more extreme than Earth’s and some less. Historically, our gradual discovery of conditions on the neighboring planets has been intertwined with our increasing ability to understand, reconstruct, and predict climate change on Earth.
The Discovery of Global Warming
The first to predict that the burning of fossil fuels would warm Earth was Svante Arrhenius, a Swedish chemist who also tried to work out climate conditions on Venus and Mars. In 1896 he crudely calculated the magnitude of global warming that would result from human CO2 emissions, and concluded that it would likely be a wonderful thing for life and civilization. After winning the Nobel Prize in chemistry in 1903, Arrhenius felt free to publish his wide-ranging theories about extraterrestrial life and the environments of other planets. His best-selling 1918 book, The Destinies of the Stars, is an erudite romp through mythology, cosmology, astrophysics, climate theory, terrestrial ice ages, environmental history, and comparative planetology, and includes discussions about the likely climates on Mars and Venus, possible life on those worlds, and the future climate and habitability of Earth. He concludes that
a very great part of the surface of Venus is no doubt covered with swamps, corresponding to those on the Earth in which the coal deposits were formed, except that they are about 30°C. warmer… analogous to conditions on the Earth during its hottest periods. The temperature on Venus is not so high as to prevent a luxuriant vegetation… The vegetative processes are greatly accelerated by the high temperature. Therefore, the lifetime of the organisms is probably short. Their dead bodies, decaying rapidly, if lying in the open air, fill it with stifling gases.
Arrhenius’s vivid description of a swampy, tropical, vegetated Venus proved very influential on scientists and science fiction writers throughout much of the twentieth century. It was pretty much the state of the art until better data, from telescopes and then spacecraft, started to give us hints of a much more extreme climate.
The first good clue came in the 1930s, when American astronomer Theodore Dunham Jr. was observing stars with an infrared spectrometer, which can measure the intensity of light at different colors, revealing the chemical composition of the source. One night, out of curiosity, he turned his telescope toward Venus, and was surprised to find that the bright disk was dark at two specific colors corresponding to absorption by carbon dioxide at high pressure. Realizing that Dunham’s discovery would have big implications for climate on Venus, planetary astronomer Rupert Wildt, in 1940, did the first calculations of greenhouse warming in a thick atmosphere loaded with CO2. The results showed that the temperature must be above the boiling point of water. Romantic visions of Venus as a nearby, moist jungle world began to evaporate.
In 1952, a remarkable Symposium on Climatic Change was held by the American Academy of Arts and Sciences in Boston, with contributions from a wide range of experts in earth, space, and life sciences. The gathering was organized and chaired by Harvard astronomer Harlow Shapley, a sort of Carl Sagan of the 1950s, who combined rigorous astronomical research with daring cross-disciplinary studies, humanitarian and political activism,* and the popularization of science. Shapley’s greatest scientific contribution was to do for the solar system what Copernicus and Galileo had done for Earth: he showed that the Sun is not at the center of the Milky Way galaxy but is orbiting at its outskirts. He published numerous popular books explaining astronomy and cosmic evolution for the lay reader, speculating on the possibility of extraterrestrial life and waxing poetically about the cosmic meaning of human existence. His popular 1963 book, the Saganesque View from a Distant Star, begins with the phrase “Mankind is made of star stuff.” The impressively multidisciplinary proceedings of his Symposium on Climatic Change reveal that before the space age, some astronomers were already thinking about Earth’s climate evolution as an extension of space science requiring collaboration with other fields. A round table discussion was held on “Climatic Conditions Required for the Origin and Continuance of Life on This and Other Planets.” In Shapley’s own account,
The session was serious, deep, cheerful, and inciting, for on the inner circle of chairs we had representatives of biochemistry, paleontology, astrophysics, meteorology, geophysics, physical chemistry, geology, and astronomy, and they were not amateurs.
In his talk, Shapley discussed what was then known about the climates of Venus and Mars, calculated the likelihood of life-supporting climates on planets around other stars (extremely pessimistic compared to today’s best estimates), discussed the climatic requirements for the origin and sustenance of life on Earth, pondered whether the extinction of the dinosaurs had been caused by climate change, and reviewed the history and theory of ice ages. Finally, he speculated on the causes of the recorded warming of Earth during the first half of the twentieth century. In his conclusion, he stated that
the growth of industry, and the increasing population of fuel-burning inhabitants of the earth, have in the past seventy years put enough additional carbon dioxide into the atmosphere to affect (perhaps quite slightly) the atmospheric control of climate.
The speakers recognized that climate is a result of a pl
anetary radiation balance that is susceptible to change by a variety of provocations. The fact that the species doing all the talking was the same one starting to perturb that balance was mentioned, but only barely.4
In 1963, a decade after Shapley left Harvard, there arrived there a young, charismatic astronomy professor who was also fascinated by the biological implications of climate evolution on Earth and other planets. Carl Sagan was in many ways Shapley’s successor at Harvard, and even taught some of the same courses to a new generation of undergraduates. Sagan was handsome, hyperconfident, and intellectually audacious, to a degree that struck some of his colleagues and mentors as reckless. In addition to his research on planetary science, he pursued collaborations with biologists in search of the origins and cosmic distribution of life, helping to establish what became known as exobiology and later astrobiology. He played a leading role in the fledgling community of SETI (the search for extraterrestrial intelligence) and was politically active, opposing the war in Vietnam and becoming faculty adviser to the leftist Students for a Democratic Society. It was these activities that likely got him booted out of Harvard when he came up for tenure in 1968.
As part of Sagan’s PhD thesis at the University of Chicago, he had revived and extended Rupert Wildt’s theory of climate warming on Venus, adding in water vapor as a second greenhouse gas, one that absorbed additional infrared radiation and produced higher temperatures than CO2 alone. He defended his thesis in 1960, two years before the age of planetary exploration began with the launch of Mariner 2. Sagan arrived at Harvard right after Mariner 2 had arrived at Venus, reporting back that conditions there were hellishly hot. Building on these results, his first graduate student there, James (Jim) Pollack, under Sagan’s guidance, built the most sophisticated Venus greenhouse model to date. Sagan and Pollack would become lifelong collaborators, and during their three years together at Harvard, they produced a string of important papers on the atmosphere, clouds, and climate of Venus.
Earth in Human Hands Page 4
