Way back in the 1870s, an Italian geologist named Antonio Stoppani proposed that people had introduced a new era, which he labeled the anthropozoic. Stoppani’s proposal was ignored; other scientists found it unscientific. The Anthropocene, by contrast, struck a chord. Human impacts on the world have become a lot more obvious since Stoppani’s day, in part because the size of the population has roughly quadrupled, to nearly seven billion. “The pattern of human population growth in the twentieth century was more bacterial than primate,” biologist E. O. Wilson has written. Wilson calculates that human biomass is already a hundred times larger than that of any other large animal species that has ever walked the Earth.
In 2002, when Crutzen wrote up the Anthropocene idea in the journal Nature, the concept was immediately picked up by researchers working in a wide range of disciplines. Soon it began to appear regularly in the scientific press. “Global Analysis of River Systems: From Earth System Controls to Anthropocene Syndromes” ran the title of one 2003 paper. “Soils and Sediments in the Anthropocene” was the headline of another, published in 2004.
At first most of the scientists using the new geologic term were not geologists. Zalasiewicz, who is one, found the discussions intriguing. “I noticed that Crutzen’s term was appearing in the serious literature, without quotation marks and without a sense of irony,” he says. In 2007 Zalasiewicz was serving as chairman of the Geological Society of London’s Stratigraphy Commission. At a meeting he decided to ask his fellow stratigraphers what they thought of the Anthropocene. Twenty-one of 22 thought the concept had merit.
The group agreed to look at it as a formal problem in geology. Would the Anthropocene satisfy the criteria used for naming a new epoch? In geologic parlance, epochs are relatively short time spans, though they can extend for tens of millions of years. (Periods, such as the Ordovician and the Cretaceous, last much longer, and eras, like the Mesozoic, longer still.) The boundaries between epochs are defined by changes preserved in sedimentary rocks—the emergence of one type of commonly fossilized organism, say, or the disappearance of another.
The rock record of the present doesn’t exist yet, of course. So the question was: When it does, will human impacts show up as “stratigraphically significant”? The answer, Zalasiewicz’s group decided, is yes—though not necessarily for the reasons you’d expect.
PROBABLY THE MOST OBVIOUS way humans are altering the planet is by building cities, which are essentially vast stretches of man-made materials—steel, glass, concrete, and brick. But it turns out most cities are not good candidates for long-term preservation, for the simple reason that they’re built on land, and on land the forces of erosion tend to win out over those of sedimentation. From a geologic perspective, the most plainly visible human effects on the landscape today “may in some ways be the most transient,” Zalasiewicz has observed.
Humans have also transformed the world through farming; something like 38 percent of the planet’s ice-free land is now devoted to agriculture. Here again, some of the effects that seem most significant today will leave behind only subtle traces at best.
Fertilizer factories, for example, now fix more nitrogen from the air, converting it to a biologically usable form, than all the plants and microbes on land; the runoff from fertilized fields is triggering life-throttling blooms of algae at river mouths all over the world. But this global perturbation of the nitrogen cycle will be hard to detect, because synthesized nitrogen is just like its natural equivalent. Future geologists are more likely to grasp the scale of 21st-century industrial agriculture from the pollen record—from the monochrome stretches of corn, wheat, and soy pollen that will have replaced the varied record left behind by rain forests or prairies.
The leveling of the world’s forests will send at least two coded signals to future stratigraphers, though deciphering the first may be tricky. Massive amounts of soil eroding off denuded land are increasing sedimentation in some parts of the world—but at the same time the dams we’ve built on most of the world’s major rivers are holding back sediment that would otherwise be washed to sea. The second signal of deforestation should come through clearer. Loss of forest habitat is a major cause of extinctions, which are now happening at a rate hundreds or even thousands of times higher than during most of the past half billion years. If current trends continue, the rate may soon be tens of thousands of times higher.
Probably the most significant change, from a geologic perspective, is one that’s invisible to us—the change in the composition of the atmosphere. Carbon dioxide emissions are colorless, odorless, and in an immediate sense, harmless. But their warming effects could easily push global temperatures to levels that have not been seen for millions of years. Some plants and animals are already shifting their ranges toward the Poles, and those shifts will leave traces in the fossil record. Some species will not survive the warming at all. Meanwhile rising temperatures could eventually raise sea levels 20 feet or more.
Long after our cars, cities, and factories have turned to dust, the consequences of burning billions of tons’ worth of coal and oil are likely to be clearly discernible. As carbon dioxide warms the planet, it also seeps into the oceans and acidifies them. Sometime this century they may become acidified to the point that corals can no longer construct reefs, which would register in the geologic record as a “reef gap.” Reef gaps have marked each of the past five major mass extinctions. The most recent one, which is believed to have been caused by the impact of an asteroid, took place 65 million years ago, at the end of the Cretaceous period; it eliminated not just the dinosaurs, but also the plesiosaurs, pterosaurs, and ammonites. The scale of what’s happening now to the oceans is, by many accounts, unmatched since then. To future geologists, Zalasiewicz says, our impact may look as sudden and profound as that of an asteroid.
IF WE HAVE INDEED entered a new epoch, then when exactly did it begin? When did human impacts rise to the level of geologic significance?
William Ruddiman, a paleoclimatologist at the University of Virginia, has proposed that the invention of agriculture some 8,000 years ago, and the deforestation that resulted, led to an increase in atmospheric CO2 just large enough to stave off what otherwise would have been the start of a new ice age; in his view, humans have been the dominant force on the planet practically since the start of the Holocene. Crutzen has suggested that the Anthropocene began in the late 18th century, when, ice cores show, carbon dioxide levels began what has since proved to be an uninterrupted rise. Other scientists put the beginning of the new epoch in the middle of the 20th century, when the rates of both population growth and consumption accelerated rapidly.
Zalasiewicz now heads a working group of the International Commission on Stratigraphy (ICS) that is tasked with officially determining whether the Anthropocene deserves to be incorporated into the geologic timescale. A final decision will require votes by both the ICS and its parent organization, the International Union of Geological Sciences. The process is likely to take years. As it drags on, the decision may well become easier. Some scientists argue that we’ve not yet reached the start of the Anthropocene—not because we haven’t had a dramatic impact on the planet, but because the next several decades are likely to prove even more stratigraphically significant than the past few centuries. “Do we decide the Anthropocene’s here, or do we wait 20 years and things will be even worse?” says Mark Williams, a geologist and colleague of Zalasiewicz’s at the University of Leicester in England.
Crutzen, who started the debate, thinks its real value won’t lie in revisions to geology textbooks. His purpose is broader: He wants to focus our attention on the consequences of our collective action—and on how we might still avert the worst. “What I hope,” he says, “is that the term ‘Anthropocene’ will be a warning to the world.”
Chapter 3: The Acid Sea
BY ELIZABETH KOLBERT
Elizabeth Kolbert wrote last month about the idea that human impacts on the planet will long outlive us. David Liittschwager’s photos
of life in one cubic foot of soil or sea appeared in February 2010.
CASTELLO ARAGONESE IS A TINY ISLAND that rises straight out of the Tyrrhenian Sea like a tower. Seventeen miles west of Naples, it can be reached from the somewhat larger island of Ischia via a long, narrow stone bridge. The tourists who visit Castello Aragonese come to see what life was like in the past. They climb—or better yet, take the elevator—up to a massive castle, which houses a display of medieval torture instruments. The scientists who visit the island, by contrast, come to see what life will be like in the future.
Owing to a quirk of geology, the sea around Castello Aragonese provides a window onto the oceans of 2050 and beyond. Bubbles of CO2 rise from volcanic vents on the seafloor and dissolve to form carbonic acid. Carbonic acid is relatively weak; people drink it all the time in carbonated beverages. But if enough of it forms, it makes seawater corrosive. “When you get to the extremely high CO2, almost nothing can tolerate that,” Jason Hall-Spencer, a marine biologist from Britain’s University of Plymouth, explains. Castello Aragonese offers a natural analogue for an unnatural process: The acidification that has taken place off its shore is occurring more gradually across the world’s oceans, as they absorb more and more of the carbon dioxide that’s coming from tailpipes and smokestacks.
Hall-Spencer has been studying the sea around the island for the past eight years, carefully measuring the properties of the water and tracking the fish and corals and mollusks that live and, in some cases, dissolve there. On a chilly winter’s day I went swimming with him and with Maria Cristina Buia, a scientist at Italy’s Anton Dohrn Zoological Station, to see the effects of acidification up close. We anchored our boat about 50 yards from the southern shore of Castello Aragonese. Even before we got into the water, some impacts were evident. Clumps of barnacles formed a whitish band at the base of the island’s wave-battered cliffs. “Barnacles are really tough,” Hall-Spencer observed. In the areas where the water was most acidified, though, they were missing.
We all dived in. Buia was carrying a knife. She pried some unlucky limpets from a rock. Searching for food, they had wandered into water that was too caustic for them. Their shells were so thin they were almost transparent. Bubbles of carbon dioxide streamed up from the seafloor like beads of quicksilver. We swam on. Beds of sea grass waved beneath us. The grass was a vivid green; the tiny organisms that usually coat the blades, dulling their color, were all missing. Sea urchins, commonplace away from the vents, were also absent; they can’t tolerate even moderately acidified water. Swarms of nearly transparent jellyfish floated by. “Watch out,” Hall-Spencer warned. “They sting.”
Jellyfish, sea grass, and algae—not much else lives near the densest concentration of vents at Castello Aragonese. Even a few hundred yards away, many native species can’t survive. The water there is about as acidified as the oceans as a whole are forecast to be by 2100. “Normally in a polluted harbor you’ve got just a few species that are weedlike and able to cope with widely fluctuating conditions,” Hall-Spencer said once we were back on the boat. “Well, it’s like that when you ramp up CO2.”
SINCE THE START of the industrial revolution, enough fossil fuels—coal, oil, and natural gas—have been burned and enough forests cut down to emit more than 500 billion tons of CO2. As is well known, the atmosphere has a higher concentration of CO2 today than at any point in the past 800,000 years and probably a lot longer.
What is less well known is how carbon emissions are changing the oceans too. The air and the water constantly exchange gases, so a portion of anything emitted into the atmosphere eventually ends up in the sea. Winds quickly mix it into the top few hundred feet, and over centuries currents spread it through the ocean depths. In the 1990s an international team of scientists undertook a massive research project that involved collecting and analyzing more than 77,000 seawater samples from different depths and locations around the world. The work took 15 years. It showed that the oceans have absorbed 30 percent of the CO2 released by humans over the past two centuries. They continue to absorb roughly a million tons every hour.
For life on land this process is a boon; every ton of CO2 the oceans remove from the atmosphere is a ton that’s not contributing to global warming. But for life in the sea the picture looks different. The head of the National Oceanic and Atmospheric Administration, Jane Lubchenco, a marine ecologist, has called ocean acidification global warming’s “equally evil twin.”
The pH scale, which measures acidity in terms of the concentration of hydrogen ions, runs from zero to 14. At the low end of the scale are strong acids, such as hydrochloric acid, that release hydrogen readily (more readily than carbonic acid does). At the high end are strong bases such as lye. Pure, distilled water has a pH of 7, which is neutral. Seawater should be slightly basic, with a pH around 8.2 near the sea surface. So far CO2 emissions have reduced the pH there by about 0.1. Like the Richter scale, the pH scale is logarithmic, so even small numerical changes represent large effects. A pH drop of 0.1 means the water has become 30 percent more acidic. If present trends continue, surface pH will drop to around 7.8 by 2100. At that point the water will be 150 percent more acidic than it was in 1800.
The acidification that has occurred so far is probably irreversible. Although in theory it’s possible to add chemicals to the sea to counter the effects of the extra CO2, as a practical matter, the volumes involved would be staggering; it would take at least two tons of lime, for example, to offset a single ton of carbon dioxide, and the world now emits more than 30 billion tons of CO2 each year. Meanwhile, natural processes that could counter acidification—such as the weathering of rocks on land—operate far too slowly to make a difference on a human timescale. Even if CO2 emissions were somehow to cease today, it would take tens of thousands of years for ocean chemistry to return to its preindustrial condition.
Acidification has myriad effects. By favoring some marine microbes over others, it is likely to alter the availability of key nutrients like iron and nitrogen. For similar reasons it may let more sunlight penetrate the sea surface. By changing the basic chemistry of seawater, acidification is also expected to reduce the water’s ability to absorb and muffle low-frequency sound by up to 40 percent, making some parts of the ocean noisier. Finally, acidification interferes with reproduction in some species and with the ability of others—the so-called calcifiers—to form shells and stony skeletons of calcium carbonate. These last effects are the best documented ones, but whether they will prove the most significant in the long run is unclear.
In 2008 a group of more than 150 leading researchers issued a declaration stating that they were “deeply concerned by recent, rapid changes in ocean chemistry,” which could within decades “severely affect marine organisms, food webs, biodiversity, and fisheries.” Warm-water coral reefs are the prime worry. But because carbon dioxide dissolves more readily in cold water, the impact may actually show up first closer to the Poles. Scientists have already documented significant effects on pteropods—tiny swimming snails that are an important food for fish, whales, and birds in both the Arctic and the Antarctic. Experiments show that pteropod shells grow more slowly in acidified seawater.
Will organisms be able to adapt to the new ocean chemistry? The evidence from Castello Aragonese is not encouraging. The volcanic vents have been pouring CO2 into the water for at least a thousand years, Hall-Spencer told me when I visited. But the area where the pH is 7.8—the level that may be reached oceanwide by the end of the century—is missing nearly a third of the species that live nearby, outside the vent system. Those species have had “generations on generations to adapt to these conditions,” Hall-Spencer said, “yet they’re not there.
“Because it’s so important, we humans put a lot of energy into making sure that the pH of our blood is constant,” he went on. “But some of these lower organisms, they don’t have the physiology to do that. They’ve just got to tolerate what’s happening outside. And so they get pushed beyond their limits.”
FIFTY MILES OFF THE COAST of Australia and half a world away from Castello Aragonese lies the equally tiny One Tree Island. One Tree, which actually has several hundred trees, is shaped like a boomerang, with two arms that stretch out into the Coral Sea. In the crook of the boomerang there’s a small research station run by the University of Sydney. As it happened, just as I arrived one spectacular summer afternoon, an enormous loggerhead turtle heaved herself up onto the beach in front of the lab buildings. The island’s entire human population—11 people, not including me—gathered around to watch.
One Tree Island is part of the Great Barrier Reef, the world’s largest reef complex, which stretches for more than 1,400 miles. The entire island is composed of bits of coral rubble, ranging from marble to basketball size, that began piling up after a peculiarly violent storm about 4,000 years ago. Even today, the island has nothing that could really be called dirt. The trees seem to rise up directly out of the rubble like flagpoles.
When scientists first started visiting the island in the 1960s, they posed questions like, How do reefs grow? Nowadays the questions are more urgent. “Something like 25 percent of all species in the oceans spend at least part of their life in coral reef systems,” Ken Caldeira, an expert on ocean acidification at the Carnegie Institution, said one evening before heading out to collect water samples on the reef. “Corals build the architecture of the ecosystem, and it’s pretty clear if they go, the whole ecosystem goes.”
Coral reefs are already threatened by a wide array of forces. Rising water temperatures are producing more frequent “bleaching” events, when corals turn a stark white and often die. Overfishing removes grazers that keep reefs from being overgrown with algae. Agricultural runoff fertilizes algae, further upsetting reef ecology. In the Caribbean some formerly abundant coral species have been devastated by an infection that leaves behind a white band of dead tissue. Probably owing to all these factors, coral cover in the Caribbean declined by around 80 percent between 1977 and 2001.
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