In 1997, geologist Walter Alvarez published the book T. rex and the Crater of Doom. It was reprinted in June 2008 by Princeton University Press, accompanied by a foreword by me in which I put the discover of a dinosaur-killing impact into the context of scientific history.
In 1980, Walter Alvarez, a geologist at the University of California, Berkeley, and his colleagues proposed that the dinosaurs had been exterminated by an asteroid that smashed into the Earth. I was fourteen at the time, and that mix of dinosaurs, asteroids, and apocalyptic explosions was impossible to resist. I can still see the pictures that appeared in magazines and books—paintings of crooked rocks crashing into Earth, sometimes seen from the heavens, sometimes from the point of view of an about-to-become-extinct dinosaur. Suddenly the history of life was more cinematic than any science fiction movie.
By luck rather than foresight, I eventually became a science writer. I had the good fortune to start the job in the early 1990s, as the impact story was still unfolding. Until then, I knew Walter Alvarez only as a name on a page. Now I could call Alvarez and talk to him about new evidence other scientists were finding to support his impact hypothesis—evidence showing not only that the impact did come at the end of the Cretaceous period, but even revealing where it hit: a site called Chixculub, along the coast of the Yucatán Peninsula. “It ties everything together,” Alvarez told me with delight in 1991. I had the wonderful privilege of watching the story continue to develop—as a bull’s-eye ring a hundred miles across came to light under the Gulf of Mexico, as a piece of the asteroid itself was fished from the Pacific.
By 1997, the story had matured enough that Alvarez himself was ready to offer a firsthand account in T. rex and the Crater of Doom. It is an intimately readable look at how great science gets done. Scientists notice odd things that seem out of place, they contemplate ludicrous hypotheses, and then they doggedly test those hypotheses for years. T. rex and the Crater of Doom illustrates an important rule about science: some of the most revealing discoveries come not from deep within a single discipline, but at the borders between disciplines. The impact hypothesis would probably have come to nothing if not for the combined efforts of experts on everything from geochronology to pollen fossils to nuclear explosions.
Yet T. rex and the Crater of Doom told a story that was still relatively young. We science writers sometimes leave a misleading impression with readers that new research abruptly settles even the deepest of mysteries. Cancer Cured. Origin of Life Discovered. But science is not like a can of instant coffee. It’s more like a cask of wine, its ultimate merit emerging only after many years. In some cases, scientific ideas turn sour as evidence mounts against them. Other hypotheses grow richer and subtler over the years. The impact hypothesis continued to mature long after the publication of T. rex and the Crater of Doom. Today it is widely recognized as one of the great discoveries in the modern history of geology and paleontology. But sometimes when wines mature, they take on a new and unexpected flavor. That’s what’s happened to the impact hypothesis since Alvarez first published his book. It now has a different significance than when it first came to light.
In the 1970s, when Walter Alvarez and his father, Luis, first proposed a giant impact at the end of the Cretaceous period, their most formidable enemy was not a person, but a concept. Uniformitarianism holds that the processes we see at work on Earth today were also operating in the past, and over vast stretches of time they produced most of the landscape around us, from the mountains that tower overhead to the canyons that plunge away below. Advocates of uniformitarianism often contrasted it with catastrophism—the once-popular concept that sudden, revolutionary changes, such as floods or volcanic eruptions, had shaped geological features. Uniformitarians instead saw the Earth gradually lifting and subsiding, rocks eroding grain by grain, and growing grain by grain.
Uniformitarianism held a powerful grip on the minds of geologists for many reasons, not least of which was that, as an explanation of the world, it works very well. The world is indeed very old. The plates of the Earth’s crust do in fact creep a few inches each year, and over millions of years they slowly collide. A gentle rain of calcium carbonate falls to the bottom of the sea to build vast expanses of limestone.
The idea that a sudden event, one that we have no experience with today, could mark the entire planet and bring the curtain down on an entire geological period was bound to kick up skepticism. T. rex and the Crater of Doom would be valuable enough simply as a reminder that there was a time when such an idea was controversial. Today, the impact at the end of the Cretaceous period is one of the most strongly supported events in the geological record. In 2007, a team of astronomers even identified what they claim is the ultimate origin of the impact—a collision in the asteroid belt some 190 million years ago that sent debris careening around the solar system. It’s also clear now that the impact at Chixculub was not an utterly unique event. Large asteroids and comets have crashed into Earth many times over its 4.5-billion-year history. Despite the uniformitarian recycling of the Earth’s surface, there are still traces of some of those impacts. The oldest yet found is a Russian lake known as Suavjärvi that dates back 2.4 billion years. The marks of younger impacts are still fresh, and thus more numerous. Within the last 70 million years, there are over sixty known impact sites. After our own species emerged some 200,000 years ago, the pelting continued. Some 50,000 years ago an iron meteorite excavated the mile-wide Meteor Crater in Arizona. Meteorites that broke up in the atmosphere also made their effects known, such as the meteorite that exploded over Tunguska, Siberia, in 1908, flattening thousands of acres of trees.
These discoveries raise a puzzling paradox: if the Earth has been pelted so many times, how catastrophic have impacts actually been? On a geological scale, they are no more catastrophic than the ticking of a clock. Indeed, the Earth itself formed from these very chunks of rock and ice. Once our planet emerged from these collisions, major impacts continued for hundreds of millions of years. A miniature planet the size of Mars probably plowed into the early Earth, and out of the rubble kicked up by the impact was born our moon. The young oceans of the Earth may have been boiled away by the energy of subsequent impacts.
Impacts may have not only caused geological trauma, however. They may have seeded our planet. Comets and meteorites carry with them amino acids and other building blocks of life, and some of those raw ingredients may have survived the fall through Earth’s atmosphere. In recent years, some scientists have even proposed that impacts may have delivered living organisms from one planet to another. It was not long ago that this idea, known as panspermia, was dismissed as brusquely as the notion of an impact at the end of the Cretaceous. But today it looks plausible, albeit unconfirmed. Life might have originated on one planet and then spread to others, or impacts may have cross-contaminated worlds.
It took the discovery of the Chixculub impact for many scientists to give serious thought to the role impacts might play in the history of life. Uniformitarianism was a powerful idea in evolution, just as it was in geology. Indeed, it was Charles Darwin’s early education in geological uniformitarianism that helped him develop his theory of evolution. Life had taken on its current diversity, Darwin argued, thanks in large part to the processes we see at work in nature today. Natural selection might only have a tiny effect in a single generation, but over millions of generations it could become a powerful force.
In the mid-1900s, evolutionary biologists merged Darwin’s theory with genetics and other new developments in biology to produce what’s often called the “modern synthesis.” According to the modern synthesis, small mutations allowed populations to gradually adapt to their conditions. The leading advocates of the modern synthesis didn’t have much to say about mass extinctions; they saw extinction as a gradual process, as some lineages outcompeted others to adapt to a slowly changing world. Mammals were better adapted to the gradually cooling climate at the end of the Cretaceous, and so they survived while dinosaurs became extinct.
Now paleontologists recognize that life has experienced overwhelming disasters. The Chixculub impact offered the first good glimpse of such a catastrophe. Many lines of evidence now indicate that it wreaked environmental chaos, spewing carbon dioxide and sulfuric acid into the atmosphere, and casting a pall over the planet. Food webs disintegrated on land and sea, driving half of all species extinct. Mammals may have been better suited to withstand the catastrophe because they were small and could scavenge for food. But that advantage only existed during the geologically short wake after the impact. The Earth returned to a stable, productive state again, but the dinosaurs were not there anymore to take advantage of it. (Strictly speaking, most dinosaurs were not there anymore. One lineage of dinosaurs—the birds—has thrived ever since.)
The staggering effects of the Chixculub impact caused some scientists to wonder if impacts might be a driving force in the history of life—perhaps the driving force. Some saw evidence of 26-million-year cycles of extinctions that could be caused by periodic showers of asteroids or comets. Others turned their attention to major periods of mass extinctions to see if impacts were responsible for them as well. Today scientists generally recognize five major bouts of mass extinctions. The mass extinctions at the end of the Cretaceous Period, as dramatic as they may seem, was far from the biggest. Some 250 million years ago, as the Permian period gave way to the Triassic, about 70 percent of species on land and about 95 percent of species in the ocean became extinct. Along with the “big five,” scientists have identified a number of smaller bursts of extinctions.
Some researchers have claimed to have found impacts that triggered some of these mass extinctions, but so far none has been accepted yet by the scientific community. In fact, there’s a lot of evidence to suggest that impacts have almost never affected the rate of extinctions. In 2003, for example, John Alroy of the National Center for Ecological Analysis and Synthesis surveyed all the impacts that occurred after the end of the Cretaceous. He then compared them to the well-documented fossil record of mammals in North America. He found no link whatsoever between impacts and changes in the extinction rate. While some of the smaller impacts Alroy studied may have had little effect on the planet, several left craters fifty miles wide or more. Yet Alroy could pin no devastation to those major catastrophes. Likewise, no clear evidence of an impact has emerged around the big-five mass extinctions except for the ones at the end of the Cretaceous.
Chixculub, at least for the time being, stands alone. Why was it so devastating, while other impacts were not? It was particularly big, for one thing, forming the third-largest crater yet found on Earth. It also may have happened to hit a particularly catastrophic spot on the Earth: a shallow Gulf lined with limestone that could generate particularly devastating pollution.
Even if impacts did not have an overriding effect on the history of life, however, paleontologists still see a vital lesson in the discovery of the Chixculub impact. It directed their attention to the potential importance of mass extinctions in the history of life. In his 2007 book, Under a Green Sky, the paleontologist Peter Ward recounts how the work of Alvarez and his colleagues inspired him to make a career out of studying mass extinctions. A quarter of a century of research has led Ward to conclude that impacts may not have been a very common cause of mass extinctions, but other sorts of sudden catastrophes were. Those catastrophes did not come from space, however. Earth generated them from within.
Consider once more the Permian-Triassic extinctions 250 million years ago. Today several lines of evidence point to a complex chain of events as their cause. The chain began with the welling up of vast amounts of magma in Siberia. These so-called flood basalts brought with them an infusion of gases from underground, including heat-trapping carbon dioxide and methane. The planet’s atmosphere rapidly warmed, as did the surface of the oceans. The heat began to put the world’s ecosystem under great stress, while the carbon dioxide penetrated the oceans and began to acidify them.
But this global warming was actually just the prelude to the full disaster. The normal circulation of the ocean, which delivered oxygen to its depths, slowed down as a result of the warming. Now the conditions in the deep ocean favored strange microbes known as sulfate-reducing bacteria that exhaled hydrogen sulfide—a foul-smelling molecule sometimes known as sewer gas. This poisonous gas rose into the atmosphere, possibly devastating animals and plants alike, and perhaps even helping to destroy the ozone layer. . Harmful ultraviolet radiation from the sun was now able to reach the ground, harming plants and photosynthesizing organisms in the ocean. With them went the enormous food webs they supported. Ward envisions the Earth 250 million years ago as a truly grotesque place—a glassy purple sea releasing poisonous bubbles that rise up to a pale green sky. While the evidence for this chain of events is strongest for the Permian-Triassic extinctions, Ward points to evidence from nine other mass extinctions that is consistent with the same mechanism. If he’s right, then the Earth is surprisingly well-primed to unleash devastation on the life it usually supports.
The significance of Chixculub went beyond the clues it offered to mass extinctions of the past. It also prompted scientists to give serious thought to how the distant past could help us understand threats to our own future. Earth, astronomers now realize, sits in a part of the solar system that’s uncomfortably crowded with asteroids. While few of them may rival the ten-mile-wide rock that struck the planet 65 million years ago, smaller ones might be reason for worry. The impact of one of these near-Earth objects probably wouldn’t cause global extinctions, but it might put a serious dent in human civilization. It might annihilate an entire city or two. By lofting dust and triggering massive forest fires, it might cast a pall over the planet that might cause crops to fail for several years. Today astronomers are trying to pinpoint particularly dangerous asteroids and develop a way to identify ones that are on a collision course with Earth. Exactly what we’d do if we discovered one on its way is not clear, but there have been a number of proposals for deflecting asteroids away from Earth.
Some scientists have also warned that we could create the same kind of planetary darkness without any help from outer space—by waging nuclear war. In the early 1980s, when Walter Alvarez and his colleagues first published their impact hypothesis, a nuclear war between the United States and the Soviet Union seemed like a very real risk. And it was dawning on scientists that such a war might lead to long-term ecological disaster. The soot from forest fires would be lifted high up into the atmosphere and block out the sun, creating a condition that came to be known as nuclear winter. The warning from the Chixculub impact seemed clear—if we created our own Chixculub impact, we might go the way of the dinosaurs.
Today the threat of a major nuclear war between the United States and Russia seems less likely. But other countries are now developing nuclear weapons, raising the chances of regional nuclear war. In March 2007 a team of scientists created a model of a small nuclear war and found that while it might not create a full-fledged nuclear winter, it might still darken the skies enough to trigger major famines.
But there’s another danger that climate scientists now recognize, which comes from our release of heat-trapping carbon dioxide into the atmosphere--11 billion tons of it in 2006 alone. There’s a broad agreement among the world’s scientists that burning fossil fuels and other human activities have significantly warmed the world over the past century. And they threaten to warm the world far more in centuries to come. The effects may not be instantaneous like a nuclear bomb or an impact from space, but as Jonathan Weiner noted in his 1990 book, The Next Hundred Years, on a geological scale, we are setting off a carbon bomb. The rate at which carbon dioxide is building up in the atmosphere today is far faster than any increase documented for millions of years.
Climate scientists have focused most of their attention on the direct effects of all this extra carbon dioxide. They’ve examined how the temperature will rise in different parts of the world. They’ve begun to look at how that warming will change weather patterns, potentially bringing more hurricanes and droughts. They’ve calculated how high sea levels may rise simply from the expansion of the oceans and the increased flow of rivers. Only in recent years have they begun to contemplate the more catastrophic kinds of changes global warming might bring. It’s possible (but by no means certain) that parts of the ice caps in Greenland or Antarctica will fail and abruptly slide into the oceans, creating a devastating rise in sea level. Some scientists have warned that global warming may unlock new sources of greenhouse gases, such as methane trapped in now-frozen peat, which could increase the temperature far higher than current models would suggest. Carbon dioxide is even now penetrating the oceans and may turn them acidic, potentially making it impossible for some shelled animals and corals to grow.
Would global warming trigger a bloom of sulfate-reducing bacteria, producing a deadly cloud of sewer gas? Scientists have much to learn before they can hazard a guess. But the history of life teaches us that we must take extreme risks seriously. Strange things have happened to Earth with which our species has no experience. Or at least no experience yet.
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