The anthropocene

A new name for a new geological era

Welcome to the Anthropocene. It's a new geological era, so take a good look around. A single species is in charge of the planet, altering its features almost at will. And what more natural than to name this new era after that top-of-the-heap anthropoid, ourselves? The term was coined in 2000 by the Nobel Prize—winning Dutch atmospheric scientist Paul Crutzen to describe the past two centuries of our planet's evolution. "I was at a conference where someone said something about the Holocene, the long period of relatively stable climate since the end of the last ice age," he told me later. "I suddenly thought that this was wrong. The world has changed too much. So I said: 'No, we are in the Anthropocene.' I just made up the word on the spur of the moment. Everyone was shocked. But it seems to have stuck."

The word is catching on among a new breed of scientists who study Earth systems—how our planet functions. Not just climate systems, but also related features, such as the carbon cycle on land and at sea, the stratosphere and its ozone layer, ocean circulation, and the ice of the cryosphere. And those scientists are coming to believe that some of these systems are close to breakdown, because of human interference. If that is true, then the gradual global warming predicted by most climate models for the next centuries will be the least of our worries.

The big new discovery is that planet Earth does not generally engage in gradual change. It is far cruder and nastier, says Will Steffen, an Australian expert on climate and carbon cycles who from 1998 to 2004 was director of the International Geosphere Biosphere Programme, a research agency dedicated to investigating Earth systems. A mild-mannered man not given to hyperbole, Steffen nonetheless takes a hard-nosed approach to climate change. "Abrupt change seems to be the norm, not the exception," he says. We have been lured into a false sense of security by the relatively quiet climatic era during which our modern complex civilizations have grown and flourished. It may also have left us unexpectedly vulnerable as we stumble into a new era of abrupt change.

We have also been blind, he says, to the extent of the damage we are doing to our planetary home. We often see our impact as limited to individual parts of the system: to trashed rainforests, polluted oceans, and even raised air temperatures. We rarely notice that by doing all these things at once, we are undermining the basic planetary systems. Something, Steffen says, is going to give: "The planet may have an Achilles heel. And if it does, we badly need to know about it." Without that knowledge and the will to act, he says, the Anthropocene may well end in tears.

A report from the U.S. National Academy of Sciences in 2002, under the chairmanship of Richard Alley, of Penn State University—a glaciologist with the slightly manic appearance of an ex-hippie, who has become a regular on Capitol Hill for his ability to talk climate science in plain language—sounded a similar warning. "Recent scientific evidence shows that major and widespread climate changes have occurred with startling speed," the report began. "The new paradigm of an abruptly changing climate system has been well established by research over the last decade, but this new thinking is little known and scarcely appreciated in the wider community of natural and social scientists and policymakers." Or, Alley might have added, among the citizens of this threatened planet.

We have already had one lucky break. It happened twenty years ago, when a hole suddenly opened in the ozone layer over Antarctica, stripping away the continent's protective shield against ultraviolet radiation. We were lucky that it happened over Antarctica, and lucky that we spotted it before it spread too far.

Many of the scientists who worked to unravel the cause of the ozone hole—including Crutzen, who won his Nobel Prize in this endeavor—are among the most vehement in issuing the new warnings. They know how close we came to disaster. Glaciologists like Alley are another group who take the perils of the Anthropocene most seriously. In the past decade, they have analyzed ice cores from both Greenland and Antarctica to map the patterns of past natural climate change. The results have been chilling.

It has emerged, for instance, that around 12,000 years ago, as the last ice age waned and ice sheets were in full retreat across Europe and North America, the warming abruptly went into reverse. For a thousand years the world returned to the depths of the ice age, only to emerge again with such speed that, as Alley puts it, "roughly half of the entire warming between the ice ages and the postglacial world took place in only a decade." The world warmed by at least 9 degrees—the IPCC's prediction for the next century or so—within ten years. This beggars belief. But Alley and his coresearchers are adamant that the ice cores show this happened.

Similar switchback temperature changes occurred regularly through the last glaciation, and there were a number of other "flickers" as the planet staggered toward a new postglacial world. Stone Age man, with only the most rudimentary protection from a climatic switchback, must have found that tough. Heaven knows how modern human society would respond to such a change, whereby London would have a North African climate, Mexican temperatures would be visited on New England, and India's billion-plus population would be deprived of the monsoon rains that feed them.

The exact cause of the rise and fall of the ice ages still excites disputes. But it seems that the 100,000-year cycles of ice ages and interglacials that have persisted for around a million years have coincided with a minor wobble in Earth's orbit. Its effect on the solar radiation reaching the planet is minute, and it happens only gradually. But somehow Earth's systems amplify its impact, turning a minor cooling into an abrupt freeze or an equally minor warming into a sudden defrost. The amplification certainly involves greenhouse gases, as Arrhenius long ago surmised. The extraordinary way in which temperatures and carbon dioxide levels have moved in lockstep permits no other interpretation. It also probably involves changes to ocean currents and the temperature feedbacks from growing and melting ice.

We will return to this conundrum later. What matters here is that a minor change in the planet's heating—much less, indeed, than we are currently inflicting through greenhouse gases—could cause such massive changes worldwide. The planet seems primed to leap into and out of glaciations and, perhaps, other states too.

Some see this hair trigger as rather precisely organized. Will Steffen says that for a couple of million years, Earth's climate seems co have had just two "stable states": glacial and interglacial. There was no smooth transition between them. The planet simply jumped, at a signal from the orbital wobble, from the glacial to the interglacial state, and made the jump back again with a little, but not much, more decorum. "The planet jumps straight into the frying pan and makes a bumpy and erratic slide into the freezer," Steffen says. The glacial state seems to have been anchored at carbon dioxide levels of around 190 ppm, while the interglacial state, which the modern world occupied until the Industrial Revolution, was anchored at about 280 ppm. The rapid flip between the two states must have involved a reallocation of about 220 billion tons of carbon between the oceans, land, and the atmosphere. Carbon was buried in the oceans during the glaciations and reappeared afterward. Nobody knows quite how or why. But the operation of the hair-trigger jump to a much warmer state raises critical questions for the Anthropocene.

In the past two centuries, humanity has injected about another 220 billion tons of carbon into the atmosphere, pushing carbon dioxide levels up by a third, from the stable interglacial level of 280 ppm to the present 380 ppm. The figure continues to rise by about 20 ppm a decade. So the big question is how Earth will respond. Conventional thinking among climate scientists from Arrhenius on predicts that rising emissions of carbon dioxide will produce a steady rise in atmospheric concentrations and an equally steady rise in temperatures. That's still the IPCC story. But Steffen takes a different view: "If the ice age seemed to gravitate between two steady states, maybe in future we will gravitate to a third steady state." Nature might, he concedes, fulfill the expectations of climate skeptics and push back down toward 280 ppm; but if it was going to do that, we would already see evidence of it. And we don't.

Other scientists, including Alley, are not convinced by Steffen's sense of order in the system. Sitting in his departmental office, Alley likens the climate system to "a drunk—generally quiet when left alone, but unpredictable when roused." When he is writing scientific papers or committee reports, his language is not so vivid. He talks of a "chaotic system" vulnerable to "forcings" from changes in solar radiation or greenhouse gases. "Abrupt climate change always could occur," he says. But "the existence of forcings greatly increases the number of possible mechanisms [for] abrupt change"; and "the more rapid the forcings, the more likely it is that the resulting change will be abrupt on the timescale of human economies or global ecosystems." Drunks, in other words, may be unpredictable, but if you shout at them louder or push them harder, they will react more vehemently. Right now, moreover, we are offering our drunk one more for the road.

The past 10,000 years, since the end of the last ice age, have not been without climate change. The Asian monsoon has switched on and off; deserts have come and gone; Europe and North America have flipped from medieval warm period to little ice age. None of these events has been as dramatic as the waxing and waning of the ice ages themselves. But most were equally abrupt, and civilizations have come and gone in their wake. Even so, human society in general has prospered, learning to plant crops, domesticate animals, tame rivers, create cities, develop science, and ultimately industrialize the planet.

But in the Anthropocene, the rules of the game have changed. Alley and Steffen agree that humanity is today pushing planetary life-support systems toward their limits. The stakes are higher, because what is happening is global. "Before, if we screwed up, we could move on," says Steffen. "But now we don't have an exit option. We don't have another planet."

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