Earth in Human Hands Page 11
At the 2012 AGU meeting, I was one of several speakers addressing the Anthropocene from different angles. In quickly scanning through the unwieldy, phone book–size program, I was amused to find, among the jargon-filled titles, one presentation called simply “Is Earth Fucked?”1
It is—let’s face it—a good question. Although we don’t usually, at least in this forum, put it so crudely, this anxiety is largely what drives the new Anthropocene buzz. How can we wrap our heads around the totality of the human influence on Earth? Do we really hold the fate of this planet in our unschooled and unsteady hands? Can we learn to handle that responsibility? My talk, with the comparatively tame title “Global Changes of the Fourth Kind: Assessing the Anthropocene in the Context of Comparative Planetology,” was an attempt at framing an answer.
Today our planet seems to be in crisis, but let’s calm down and look at this in perspective. It’s normal for a planet to be rocked by occasional calamity. Earth wasn’t born yesterday. Clearly the human presence is stressing it out in new ways, but this old world has been through a lot, including a long series of catastrophic changes, several of which might have nearly wiped out all life. Given all that Earth has gone through, what really is so new and so different about now, about us? If the world has suffered dramatic and deadly fluctuations before, if climate has been so hot that the polar caps completely melted off and so cold that the whole Earth was frozen over, if (as I’ll explain) other species have also thoroughly polluted the atmosphere and threatened the future of the entire biosphere, then how can we claim that what is happening here now is so unprecedented as to mark a new phase in geological history? We have the sense that there is some essential difference about the current change, but what exactly? Is that just a self-centered impulse? What is really so astoundingly new and different about the transformation of Earth under the influence of humanity?
In my research, I’ve studied the major transformations that planets can suffer, and lately I’ve been thinking about how the change occurring now on Earth looks from a planetary perspective that emphasizes deep time and whole planet transformations. In order to assess how we rate as change agents, I’ve tried to distill all the major types of global catastrophes into a simplified taxonomy of planetary traumas. In looking, from an astrobiology perspective, at the revolutions that can rock worlds, I’ve concluded that all planetary catastrophes can be grouped into four major categories, four kinds of planetary change, classified with respect to the role of life and the agency of mind. These four kinds of planetary change are:
1. random change,
2. biological change,
3. inadvertent change, and
4. intentional change.
The first two have profoundly affected life on Earth throughout the history of the biosphere. The third describes a catastrophe that is occurring for the first time now and is rapidly remaking our world. The fourth is currently occurring in very limited ways, but it suggests a potential, a kind of change that may be key to survival of intelligent technological life here and on planets elsewhere in the universe. Indeed, I will argue that the fourth kind of planetary change may suggest the best way to define a certain kind of intelligent life. Planetary changes of the fourth kind will, I suspect, be the key to our survival.
Planetary Changes of the First Kind: Random Catastrophes
The universe can be a dangerous place. Some catastrophes arise from the random acts of nature, those drastic events that “just happen” to a planet, when massive physical change arrives suddenly from within or beyond.
The canonical example is the big comet crash that ended the Cretaceous period. This was the most recent of the Big Five mass extinctions,* one of the largest ever, and is probably the most famous, as it terminated the reign of those paleontological rock stars the dinosaurs. The giant lizards ruled Earth for 135 million years, but their time in the sun ended instantly one day 66 million years ago when a comet crashed down in what is now the Yucatan Peninsula of Mexico. It left a smoking crater one hundred miles across, and a planet shrouded in dust and suddenly wiped clean of its top predators and most other species. Among the few survivors were some tiny early mammals who, in their meekness, inherited the earth and evolved, eventually, into us. [An artist’s conception of one planetary change of the first kind is shown here of the photo insert.]
Planets have bad days, and there’s a lot of stray stuff out there in the solar system. Such collisions will occur, with varying rates depending on the local populations and orbits of asteroids and comets, on every planet in the universe.
There are other possible sources of mass death from the sky that must sometimes cause mass extinctions on planets with life, and may have on Earth, although none has yet been definitively implicated. The most fearsome of these are gamma ray bursts, unbelievably monstrous explosions caused (we think) by the violent collapse of a star into a black hole that then glows briefly with the light of a million trillion suns. We’ve observed these occurring only in distant galaxies, which is a good thing. We’re glad they don’t happen very often in our own galaxy, as they would definitely leave a swath of mass extinction on nearby planets. These and the more frequent supernova explosions, when massive stars blow their guts into space, are possible causes of mass extinctions that would also go in the category of planetary changes of the first kind, or random acts of nature.
Several other mass extinctions were brought about by random changes that came not from deep space but from deep within Earth. As the continents slowly drift around the globe, riding the churning mantle, their shifting arrangement deflects and rearranges prevailing winds and ocean currents, sometimes altering global circulation patterns and triggering big climate changes. A few times, all the world’s land surface has clumped together into one giant supercontinent, surrounded by one global ocean. Even supercontinents drift, and 450 million years ago the planet was put out of sorts when the supercontinent Gondwana drifted down over the South Pole. As this one and only huge continent completely froze over, the ice and snow piled up, miles thick. So much of the world’s water became locked into these colossal drifts that sea level plummeted. During several million years in this strange configuration, there were multiple glaciations punctuated with more mild interglacials. Both sea level and ocean chemistry fluctuated sharply, dooming many species dependent upon the disappearing and changing coastal environments. This caused the Ordovician–Silurian extinction, probably the second-most extreme mass extinction in Earth history. It seems that the perpetrator was just a “series of unfortunate events,” a random convergence between global cycles of plate tectonics, hydrology, atmospheric circulation, and orbital oscillations to produce a rare climate condition that threw Gaia’s homeostasis out of whack long enough to give the tree of life a severe pruning.*
Another kind of threat from Earth’s insides is the occasional rare but intense pulse of volcanic activity capable of causing major climate change, poisoning the oceans, and radically disrupting the biosphere. Earth’s internal heat engine generally runs smoothly, with subducting slabs balancing rising plumes, but occasionally it falters, stutters, or revs up and lurches forward. Sometimes a superheated plume of material rises to the surface from deep within Earth’s mantle and unleashes a mighty gusher of volcanic magma that floods vast areas with smooth basaltic magma and pumps the air full of greenhouse gases. Earth’s surface contains several huge areas of volcanic rock, extending across millions of square miles, created in such outpourings. These include the Ontong Java Plateau, spread along the ocean floor in the South Pacific; the Siberian Traps; the Deccan Traps, in India; and the Columbia River Flood Basalts in the northwestern United States. Several of these outsize outbursts of lava and volcanic gas have been precisely dated to times in Earth’s history when the climate surged and species died en masse.
Among the largest is the Central Atlantic Magmatic Province. Its remains today are found in vast deposits of basaltic rock spanning the Atlantic from Brazil through eastern North America to Afr
ica to northwest France. Its formation 201 million years ago, one of the most massive floods of volcanic rock in the history of the planet, caused the Triassic–Jurassic mass extinction, one of the Big Five. The volcanism produced a great surge of CO2 emission, and as a result Earth was wracked by climate change, sea level fluctuations, and ocean acidification—sound familiar?—causing a sudden decrease in biodiversity. As with all mass extinctions, there were ultimately winners as well as losers. The Triassic–Jurassic extinction cleared the way for dinosaurs to dominate the continents for the next 136 million years, until the day that asteroid came along.
A similar outpouring of magma, forming another of Earth’s largest volcanic areas, has recently been convincingly implicated in the most severe mass extinction Earth has ever experienced.
The Worst Thing That Ever Happened
Two hundred fifty-two million years ago, the Permian geological period, a fertile time of vast conifer forests crawling with diverse reptiles and amphibians, was brought to an abrupt and brutal end. Suddenly animals were going extinct at horrifying rates. More than 90 percent of all species living in the seas and more than 70 percent of land animals vanished completely. Among the multitude of victims were all the trilobites, those classic, collectible horseshoe crab-like fossils; almost all the spiraling, segmented ammonites; most fish and reptiles; and the largest insects ever to live on Earth, the Meganeuroposis permiana, or griffinflies, dragonfly-like creatures with wingspans up to twenty-eight inches. At least judging from the number and variety of creatures suddenly wiped away forever, it was the worst thing that has ever happened on Earth. This event, the greatest loss of biodiversity in the history of animal life, is officially called the end-Permian, Permo–Triassic, or just P–T extinction, as it marks the boundary between the Permian and Triassic geological periods. Unofficially, but widely, it is called the Great Dying.
What the hell happened? The geological record shows that many Earth systems went haywire at this time. Ocean temperatures shot up in a way that implies rapid global warming of 8 to 10 degrees Celsius (15 to 18 degrees Fahrenheit). There was some sort of sudden and mysterious disruption of the global carbon cycle, and ocean chemistry went through sweeping changes, with drastic acidification and a big decrease in dissolved oxygen content.
Two theories have long competed. One is (you guessed it) a huge and deadly extraterrestrial impact, much larger even than the dino-killing one at the end of the Cretaceous. Many scientists have advocated for an impact, and over the past few decades several teams have even announced with great fanfare that they had finally found the “smoking gun,” the remnants of the giant crater, buried on the ocean floor or beneath Antarctic ice. In each case, the evidence has been contested and the community has remained unconvinced.* In the last several decades, the end-Cretaceous impact has taught us a lot. We’ve learned that, in addition to a crater, a giant impact leaves all kinds of other evidence. This collateral environmental damage includes ash layers, highly shocked mineral grains, and vast beds composed of tiny spheres of melted rock. If there was a colossal impact at the end of the Permian, we should have found some of these other signs by now. Rather, as the geological record is studied in more detail, an impact explanation for the Great Dying seems less and less likely.
The other leading theory has blamed the Great Dying on an enormous spurt of volcanic activity, with the obvious suspect being the Siberian Traps. We’ve known for years that this mammoth volcanic deposit in the vast northern Asian province of Siberia, consisting of seven hundred thousand cubic miles of basaltic rock, was formed at around the right time. Yet it has been hard to really pin down the timing of both eruptions and extinctions well enough to know if they could be convincingly linked. Without better data, it was like trying to solve a crime where you knew only roughly when it occurred and that your suspect was probably in the area.
It appears that the culprit has now been identified, thanks to recent breakthroughs in the accuracy of this geochronological detective work. Many efforts to decipher the history of the P–T extinction have focused on the Meishan region of China, where a section of rock, exposed in the walls of an old limestone quarry, abounds with fossils from the very end of the Permian and the beginning of the Triassic. These fossil-rich rocks are interlaced with layers of volcanic ash that, in principle, can be dated very precisely. However, these rocks are a quarter of a billion years old, so an error of just 1 percent in your age determination would mean it was off by 2.5 million years. In order to nail down the chronology of extinction well enough to derive cause and effect, you need to say with confidence what happened over a matter of thousands of years. This requires outlandishly precise lab work.
In 2013, MIT geologist Sam Bowring, working with his graduate student Seth Burgess and with Shu-zhong Shen from the Nanjing Institute of Geology and Paleontology, used a new high-resolution dating technique, analyzing atoms of uranium and lead that, counted properly, transformed the tiny zircon crystals in these rocks into incredibly accurate timepieces. Their work showed that the extinctions in China happened quickly and simultaneously with the Siberian eruptions. The die-offs at Meishan were complete within about twenty thousand years, which is more than ten times faster than previous estimates. In a geologic time frame, this is nearly instantaneous—truly catastrophic. Improved dating of volcanic rocks from the Siberian Traps suggested that the rates of volcanism were extreme, among the highest ever in planetary history, with the bulk of the vast igneous province forming in less than a million years. The environmental effects of all that erupting lava and gas were likely peaking at just the right time to cause the Great Dying.
Yet climate models show that even the great volume of CO2 released through this sudden volcanism should not have caused the amount of climate change seen, unless the effect was somehow amplified. Recent research has identified several ways the climate impact may have been magnified. The area where the Siberian Traps formed was rich in organic sediments, so heat from the rising magma may have cooked huge deposits of coal, carbonates, and other sedimentary rocks, causing them to release vast amounts of methane and sulfur dioxide (both strong greenhouse gases), and may have started coal fires that raged for millennia, releasing even more greenhouse gases. When these additional sources are added into climate models, the Siberian Traps volcanism can cause the observed climate change of 8 degrees Celsius. The magma percolating through organic sediments may also have released large quantities of mercury and other toxic metals, adding another lethal element to the environment, as well as organohalide gases (including CFCs) that could have destroyed the ozone layer, which protects land-based life from DNA-destroying ultraviolet radiation. All the volcanic sulfur would have caused intense acid rain, which, along with the pulse of CO2, would have acidified the oceans. The fossil record is consistent with this: it shows that the dying was especially concentrated among shallow-water species with shells that would have been dissolved in acidic ocean conditions. In summary, a volcanic pulse that sudden and huge can cause all hell to break loose.
The exact kill mechanisms are still being worked out, but the chronology has now convincingly implicated the volcanic culprit. At just the precise geological moment when most species suddenly dropped dead, a hot plume rising from the mantle caused enormous floods of volcanic magma to pour forth from the Siberian ground, warming Earth, acidifying the oceans, and creating a host of other extreme environmental changes. Most of life just couldn’t cope.
Whether death comes from the sky or from deep within Earth, planetary changes of the first kind are characterized by random forces of nature acting in ways where life is simply an innocent victim of circumstance. Not so with the next category of catastrophe.
Planetary Changes of the Second Kind—Biological Catastrophes
Then there are those mass extinctions where life is not so innocent. Biological evolution can also cause catastrophic global transformations. Sometimes life brings disaster upon itself.
Recently some scientists have taken
another look at the cause of the Great Dying, the most extreme crisis Gaia has ever experienced. They’ve found that while there was undoubtedly a volcanic trigger, there may also have been a biological bomb. The improved time stamping I’ve just discussed has made it abundantly clear that the Siberian Traps volcanism is implicated. Yet the climate modeling of this event doesn’t obviously add up. The predicted temperature does not clearly rise to the level indicated in the rocks. The question has lingered as to whether the impressive quantity of gases released would have been sufficient to have caused such an apocalypse. Could the physical consequences of the volcanism have been greatly amplified by a positive biological feedback? The idea arose when Dan Rothman, a geochemist at MIT, was studying the graphs showing changes in the carbon cycle at the time of the Great Dying. He noticed something about the shape of the curves. He saw a pattern of extremely rapid and accelerating atmospheric and oceanic change that did not look like the simple signature of a volcanic injection of gas into the environment. Rather, it looked to him like a “superexponential” pattern, the kind of increase you see in a gas being exhaled from a rapidly multiplying biological population. He and his colleagues did some detailed mathematical analysis of the geological record, and their results confirmed that the pattern of change was more consistent with biological growth.2 They connected the Great Dying with the evolution and rapid growth of a new type of bacteria that produces methane and multiplies rapidly when it is supplied with the metal nickel, a trace element that was delivered in abundance by the Deccan Traps volcanism. They proposed that the climate change had been enhanced by the rapid multiplication of these “methanogenic” bacteria that convert organic food into methane and flood the air with this very strong greenhouse gas.3 Methane is a much more powerful absorber of infrared radiation than CO2, and its sudden rise could explain the precipitous global warming observed at the time of the extinctions.