Earth in Human Hands Page 10

  Life on Earth has both withstood and itself induced many catastrophic global changes. Now an entirely new catastrophe is unfolding that is, in some key respects, unlike anything that has ever happened to our planet. In the next two chapters we’ll look at how this new transformation compares with the other major changes Earth has experienced, in an effort to see our time, and our role, more clearly.

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  MONKEY WITH THE WORLD

  Who can stop what must arrive now? Something new is waiting to be born.

  —Robert Hunter

  Something New

  Have you noticed that something strange is happening to Earth? Take a good long look at this world: a dazzling blue orb festooned with spiraling clouds, spinning through star-flecked darkness; dayside glinting in slowly brightening sun; winter-white pulsing between north and south as Earth ambles through its orbit.

  Now imagine you are a very patient alien regarding Earth over the eons. If you’ve been watching carefully for, say, the last several billion years, you’ve seen a lot happen: the brown continents drifting around the oceanic globe, coalescing and breaking apart, animated pieces in a morphing spherical puzzle; the polar caps growing and shrinking, advancing and retreating, as climate rocks between ice age and hothouse. Throughout all these changes, the nightside remains a nearly unbroken black, and the dayside continents are the stark, dull gray of bare rock. After four billion lonely years, a green fringe first edges over the land, and the night starts to sparkle with occasional forest fires. Still, for the longest time, the unlit hemisphere remains as black as the starry space surrounding it, the dark interrupted only by these fleeting fires, and by occasional flash of lightning or splash of aurora—until, very recently, in the last few hundred years, just a twitch in the life of the planet: whoa—what is this?

  Something new! Suddenly the planet lights up, in a peculiar, spidering pattern that seems to reflect an organic process but something else as well. Something… cognitive? Starting in a few isolated river valleys and coastal areas, glowing points appear, abruptly dotting the night, then stitching together and spreading along widening and brightening webs, hugging the shores and eventually growing in loose nodal patterns across the interiors of the lands. [See here of the photo insert.] On the dayside, a mesh of dark lines becomes visible, winding between the locations of those night lights, each swiftly surrounded by a growing verdant grid of novel angular geometry. Soon regular movements of small wave-generating objects start crossing the oceans, and bright linear clouds start streaking the skies. At the same time a host of other accelerating changes are observable in the atmosphere, the land, the oceans, and the ice.

  Finally, just sixty years ago, a blink-and-you-missed-it interval in this fast-forward view, began the curious anti-accretion, with small bits of Earth stuff jumping into space. Little insect-like constructions of refined metal, bristling with sensors, thrusters, and radio antennae, started leaping off-planet, sallying first to the nearest worlds and then to those farther afield, sending pictures and other information home to their inquisitive builders, signaling the arrival on Earth of curiosity and gravity-defying technology. Yes, after billions of years of geology as usual, something new and strange is definitely happening here.

  What is the meaning of these new changes?

  The Evolving Past

  One of the things I do for fun is read out-of-date popular science books, paying attention to what, from our perspective, is right, and what is obviously wrong. It animates the shifting of scientific verities and provokes thoughts about which of our current sureties will soon become antiquated. George Gamow’s 1941 Biography of the Earth provides a terrific snapshot of scientific thought after “modern physics” but before the space age. By then, atomic physics and radioactivity (which Gamow, one of the twentieth century’s great physicists, had helped explain) had allowed us to use Earth’s rocks as clocks, recording the timing of ancient geological events in patterns of isotopes. So Gamow knows that our planet is billions of years old. He has the order of magnitude right, but he states confidently that Earth is two billion years old, less than half its true age.*

  Some ideas in the book seem surprisingly current and sophisticated. Gamow accurately describes the structure of Earth’s deep interior as derived from seismic data. He describes the requirements for life on a planet the way a modern astrobiologist would, in terms of the need for liquid water, energy, and nutrients.

  This all seems prescient and accurate. But then he describes how Earth, along with the other planets of the solar system, was born when the gravity from a passing star yanked a huge stream of gas out of the Sun. This, he says, explains why the interior of Earth is so hot. Because such close stellar encounters are extremely unlikely, it means that our solar system is a freak of nature and that stars with planets must be incredibly rare. He explains how the Moon split off from Earth, leaving a giant gaping wound: the Pacific “ring of fire.” On the idea of continental drift, he is dismissive. Although he acknowledges the attractiveness of the concept for explaining the puzzle fit of map shapes and fossil findings, he contends that it must be wrong, because Earth’s rigid outer shell is just too stiff and unbreakable to permit such motion. He suggests instead that it is the thermal contraction of Earth’s crust that causes the cracking and shifting that makes mountains.*

  Less than eighty years later, we know all these ideas about origins (of Earth, the planets, the Moon, and mountain ranges) to be completely wrong. We’re certainly not any smarter today than they were then, and almost nobody is as smart as George Gamow was. We just have better data, much of it from space exploration. As we navigate this age of extrasolar planet discovery, supercomputer climate predictions, personalized gene sequencing, and in-depth planetary exploration, I wonder how our confident statements about what we know now will seem to anybody reading them in another fifty years. Scientific knowledge is always a work in progress. Keep this in mind when I (in this chapter) describe Earth history and (in future chapters) discuss human prehistory. Our best geology books are always only rough drafts, though they continue to get better as we get more data against which to check our account. The story has changed significantly in just the last few years, and will undoubtedly continue to do so.

  On the wall in my office at the Library of Congress, where I have been writing much of this book, there’s a poster showing the history of Earth with a multicolored, annotated cartoon of the layers in the geologic timescale. I love the fact that, even over the last year, some parts of this poster have become obsolete. It says that the oldest-known rock is 4 billion years old, but in February 2014 it was confirmed that there are some rocks in Australia that are much older than this, dating to 4.5 billion years, back almost to the origin of Earth itself, a time when, we thought, there were no rocks, just a hellish sea of molten magma. The poster shows a giant question mark for the cause and timing of “the Great Dying” of 250 million years ago. Yet, as I’ll explain shortly, that question mark has recently become somewhat smaller.

  This is our story, and we’re not sticking to it. We’ll keep changing our account as the evidence comes in, usually in a slow trickle of discovery, but sometimes in dramatic revelations or revolutions, like occasional asteroid strikes to our accumulating edifice of knowledge.

  Living Dangerously

  Life on Earth has had a tumultuous history. Extinction is a fact of life, as certain as death. More than 99 percent of species ever to have graced Earth have gone the way of all flesh. The rate of extinction has never been smooth and steady. It tends to come in pulses, when the slate gets wiped clean, making room for new evolutionary creation.

  The geologic record shows that, repeatedly throughout Earth history, the environment has changed suddenly and lethally. At such times, the tree of life has been severely trimmed, beaten back, burned by frost, half-drowned, choked with poison gas, or singed by lava. Yet it always grows back dense with new species that would never have appeared without these brutal prunings. Earth’s biosphere
is robust, or as Lynn Margulis memorably put it, “Gaia is a tough bitch.” Nearly wiped out many times (at least as measured by diversity), life has always bounced back quickly. What has happened to Earth to cause these repeated and disastrous die-offs? The causes are many.

  In the 1980s, in the aftermath of the Alvarez revelation about the asteroid that brought an end to life in the Cretaceous, University of Chicago paleontologists Jack Sepkoski and David Raup surveyed the data of extinction rates over time and mapped out the history of “mass extinctions,” those times in Earth history when the majority of species vanished suddenly. Their landmark analysis focused attention on the five largest die-offs, and these became known as the Big Five extinctions. In reality, the history of life shows a complex, variable, and always changing pattern of diversity, with a wide spectrum of extinction events of varying severity, usually showing a precipitous decline followed by a more gradual increase. The Big Five extinctions are simply slightly more intense than the sixth- or seventh-largest declines, and you could just as easily discuss the Big Seven or Big Ten, depending on how you sliced up the data. This tally refers only to extinctions since the “Cambrian explosion,” the sudden proliferation of complex animal life 542 million years ago, and it neglects extinction events that happened earlier, during several billion years of evolution dominated by simpler organisms. So, though the changes occurring right now are often referred to as the beginning of a possible “sixth extinction,” take this with a grain of salt. It’s good to focus attention on the dramatic loss of species currently under way and how this fits into the history of extinction events on Earth, but this also reinforces an incomplete picture of Earth’s dynamic history.

  Post-Alvarez, some scientists, swept along in the new outer space catastrophism, wanted to blame all mass extinctions on asteroids and comets. Yet further evidence has not supported this. Some episodes of mass death are clearly associated with more earthly causes, such as massive volcanic floods or changes in sea level, and some have been triggered by life itself, with biological evolution feeding back on the biosphere, as successful new life-forms have altered the world in ways that doomed established life. The causes of many mass extinctions are still being uncovered and debated, but the intellectual tumult prompted by the Alvarez hypothesis has left us with an increased appreciation for the role played by catastrophe in the history of life.

  Here, I am using the word catastrophe in its scientific sense, to mean a sudden and dramatic change in the state of a planet. This doesn’t necessarily imply a value judgment. Catastrophic planetary change is not in itself a bad thing. Without repeated catastrophe we wouldn’t be here; nor would most any kind of life you value, except possibly some microbes. For the species experiencing them, such changes are indeed catastrophes in the more common sense of the word, meaning a terrible and disastrous turn of events. Yet, for the biosphere, catastrophes are also opportunities. The end-Cretaceous extinction event was obviously an awful disaster for the majority of species that did not survive, such as the dinosaurs, and was probably no picnic for the 25 percent or so that did squeak through. All that death and destruction because the planet was simply in the wrong place at the wrong time, in the path of a wandering space rock—but it opened up a multitude of niches for further evolution, allowing, for example, mammals to get a furry little leg up. Certainly human beings would not be here without this random collision. So if you are a fan of the cinema, the symphony, or the space program, you might consider this to have been a good day for Earth. Or if you value any form of art, music, literature, science, architecture, dance, philosophy, or anything else humans produce, or if there is anyone whom you love dearly, you should be glad this happened. I am, but when I say so, I have to acknowledge I’m expressing appreciation for a mass extinction, which is kind of a strange thought to have.

  The changes occurring on Earth now are clearly another catastrophe, in the “rapid change of state” sense, and they certainly seem like a catastrophe in the other sense, too, for all the other species affected, for people in places starting to feel the bite of global change, and perhaps for our immediate descendants. Whatever happens, no matter what path we take from this point on, we have already left our mark and changed the course of evolution. Yet millions of years from now, how will this seem? Tragic? Or like the dinosaur deaths and other extinctions seem today, a necessary prelude to the precious and unique evolutionary creations enabled in their aftermath? I suppose the answer will depend on who, if anyone, is here to judge. Evolutionary history can be written only by survivors, and not just any survivors. To be able to study and write planetary history requires some special skills. Some of those same unique powers that allow us to change the world and unleash a new kind of mass extinction also have given us at least limited ability to decipher and understand our past, to see what’s coming, and to mourn what is being lost. Whether we can gain the power over ourselves required to avert the worst of it is another question—but I’m getting ahead of myself.

  Catastrophe is not only a mixed bag, but also unavoidable, especially on a world such as this one. Although no place in the universe is safe from nature’s weapons of mass destruction, not all planets are equally vulnerable. Earth is a minefield of lethal hazards. Is this simply a universal fact of planetary existence? No. Other solid planets have more stable tectonic plates and relatively quiescent surfaces. So why are we cursed to inhabit a planet with the strange combination of factors that makes life here so risky? Actually, this is the only kind of planet we could come from. A good planet like ours, one that is suitable for life, will always be especially accident-prone.

  Other than the external threat of asteroid and comet strikes (a common hazard throughout the solar system), all Earth’s natural hazards go hand in hand with its habitability. Indeed, Earth seems to be constructed almost perfectly to create calamities, with an outer shell thick enough to be rigid but thin enough to be breakable. Earthquakes, tsunamis, and extreme volcanic transformations are the price we pay for the life-sustaining gift of plate tectonics.

  Likewise, Earth’s atmosphere is primed to do damage by just the same qualities that make it so nurturing. Sun-animated, water-saturated, and seasonally shifting, our dynamic atmosphere maintains our comfortable climate. Its motions feed and water us, but can also swirl into violent storms that flood villages and splinter cities. Tornadoes, floods, blizzards, and climate shifts are collateral damage from life-enabling flows of energy, water, and chemical elements. All are also symptoms of Earth’s destructive/creative energy flows.*

  On a planet like ours, lively and deadly conditions are two sides of the same coin. For planetary life, dangerously is the only way to live. Space exploration and comparative planetology have shown us that other planets, and especially those most similar to Earth, have histories of catastrophic environmental changes. Yet the majority of other solid planets are more stable, and consequently less fertile. On Mars you would not have to worry about Marsquakes or eruptions. The surface is wind-carved and frost-heaved, but it overlies a dead crust that is far too cold and thick to break into competing plates. As a result, there are no seismic hazards or active volcanoes on Mars, and it’s no coincidence that there are (apparently) no Martians who can worry about such things.

  Inside and out, Earth is a confluence of chaotic subsystems, a perfect recipe for unpredictable, fluctuating conditions and behavior. The most extreme of these vacillations cause severe global changes that lead to mass extinctions. Calamity is built into Earth’s DNA, an unavoidable feature of a living world.

  A Taxonomy of Catastrophe

  Each December, San Francisco hosts a gargantuan gathering of the nerds. The American Geophysical Union (AGU) meeting is the largest annual congregation of international earth and planetary scientists anywhere in the world. An invited AGU talk carries some geek cred, so in 2012, just as I was embarking on this book project, I was psyched to be invited to talk about catastrophic planetary changes at a special session on “Planetary Evolution and the
Fate of Planetary Habitability.” The session organizer, Nathalie Cabrol, an eclectic and adventurous planetary geologist at the SETI Institute, was (I think) expecting me to speak about runaway greenhouses and other natural climate catastrophes on Earth-like planets. I told her I had been taking some time away from my usual research to look at Earth’s current catastrophe from a planetary perspective, and I asked her if I could speak on this topic at her session. Fortunately for me, Nathalie is a broad-minded and good-natured scientist, and she agreed, so I got my first chance to try out these ideas on an international scientific audience.

  For AGU, more than any other conference I’ve attended, you’ve got to gear yourself up for the onslaught, because over the years it’s grown to an overwhelming size. All week long the great glass escalators of the Moscone Center, like nonstop conveyer belts at a brainiac factory, carry an endless stream of nametag-wearing geoscientists up through the vast, airy mezzanine and disgorge them into the intellectual and social scrum. Open spigots of beer or coffee, depending on the time of day, can be found at the densest parts of the nerd swarm. These days, it’s become impossible for anyone, PhD or average Joe, to keep track of all the breaking science about our changing planet and the myriad ways we humans are implicated in disturbing it. Reflecting this insane proliferation of planetary information, there is no way to follow everything going on at AGU. Still, it’s a great place to take the pulse of the geoscience community, to see what everyone is talking about. That year the new word buzzing through the hive of twenty-four thousand researchers gathered for science, networking, and microbrews was Anthropocene, meaning “age of humanity.” Though its status as an official geological time period was the subject of an ongoing debate, it was starting to become common as an informal moniker for our present-day epoch, acknowledging humanity’s dominant influence and placing us in the landscape of deep time.