National Geographic : 2011 Mar
population growth in the twentieth century was more bacterial than primate," biologist E. O. Wilson has written. Wilson calculates that hu- man 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 Anthro- pocene idea in the journal Nature, the concept was immediately picked up by researchers work- ing in a wide range of disciplines. Soon it be- gan to appear regularly in the scienti c press. "Global Analysis of River Systems: From Earth System Controls to Anthropocene Syndromes" ran the title of one 2003 paper. "Soils and Sedi- ments in the Anthropocene" was the headline of another, published in 2004. At rst 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 Zalasie- wicz was serving as chairman of the Geological Society of London's Stratigraphy Commission. At a meeting he decided to ask his fellow stratigra- phers 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 Ordovi- cian and the Cretaceous, last much longer, and eras, like the Mesozoic, longer still.) e bound- aries between epochs are de ned by changes preserved in sedimentary rocks---the emergence of one type of commonly fossilized organism, say, or the disappearance of another. e 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 signi cant"? e 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 ma- terials---steel, glass, concrete, and brick. But it turns out most cities are not good candidates for long-term preservation, for the simple rea- son 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 e ects 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 agri- culture. Here again, some of the e ects that seem most signi cant today will leave behind only subtle traces at best. Fertilizer factories, for example, now x more nitrogen from the air, converting it to a biologi- cally usable form, than all the plants and mi- crobes on land; the runo from fertilized elds 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 le behind by rain forests or prairies. e leveling of the world's forests will send at least two coded signals to future stratigraphers, though deciphering the rst may be tricky. Mas- sive amounts of soil eroding o 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. e second signal of deforesta- tion should come through clearer. Loss of forest habitat is a major cause of extinctions, which Elizabeth Kolbert is the author of Field Notes From a Catastrophe, a book about climate change. e photographers whose work appears here share a passion for documenting human impacts on the planet.