I am walking in the Southern Uplands of Scotland. Soon we see a misty waterfall and some jagged rock. The rock has bands that run vertically, like a layer cake that's been tipped on its side.

My guide, Jan Zalasiewicz, a British stratigrapher studying rock layers, points to a wide stripe of gray. "Bad things happened in here," he says. The stripe was laid down some 445 million years ago as sediment, or bits of rock and earth slowly piled up on the bottom of an ancient ocean. In those days most life was underwater. Between one edge of the one meter (three foot) thick gray band and the other, some 80 percent of marine species died out.

The extinction event, known as the end-Ordovician, was one of the five biggest of the past half billion years. It coincided with extreme changes in climate, in global sea levels and in ocean chemistry — all caused, perhaps, by a giant continent drifting over the South Pole.

Stratigraphers like Zalasiewicz are often hard to impress. Their job is to piece together Earth's history from clues in layers of rock millions of years after the fact. They take the extremely long view of events. Only the most violent events are likely to leave behind clear, lasting signals. It's those events that mark the crucial episodes in the planet's 4.5-billion-year story.

It's troubling to learn what many stratigraphers have come to believe: that human beings have so altered the planet in just the past century or two that we've ushered in a new epoch, called the Anthropocene.

I ask Zalasiewicz what he thinks this epoch will look like to the geologists of the distant future. Will the transition be a moderate one, or will it show up as a sharp band in which very bad things happened — like the mass extinction? We're still trying to figure that out, Zalasiewicz says.

The Word "Anthropocene" Gains Popularity

The word Anthropocene was coined by Dutch chemist Paul Crutzen about a decade ago at a science conference. He published a paper with the term in the journal "Nature," and it quickly gained popularity among many scientists.

Way back in the 1870s, an Italian geologist named Antonio Stoppani had proposed a similar idea but was ignored. The word Anthropocene, by contrast, struck a chord. Human impacts on the world have become a lot more obvious since Stoppani's day. The size of the population has roughly quadrupled to nearly seven billion.

Biologist E. O. Wilson calculates that human biomass — the total weight of all people — is already a hundred times larger than that of any other large animal species that has ever walked the Earth. 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. Did the Anthropocene meet the requirements for naming a whole new epoch? In geologic terms, 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.

Each epoch is defined by changes that are tracked in sedimentary rocks — the appearance of one common fossilized creature, say, or the disappearance of another. Of course, the rock record of the present doesn't exist yet. The question is whether human activity has enough of an impact to affect the sedimentary layers of Earth. The answer, Zalasiewicz's group decided, is yes. However, it's not necessarily for reasons you'd expect.

Most Cities Will Disappear

Probably the most obvious way humans are altering the planet is by building cities. These are essentially vast stretches of man-made materials like steel, glass, concrete and brick. It turns out most cities are not guaranteed to last, though. The simple reason is that they're built on land. Due to erosion, or wearing away by wind and water, these cities will disappear. From a geologic perspective, the most plainly visible human effects on the landscape may just be temporary, Zalasiewicz has observed.

Humans have also transformed the world through farming. Around 38 percent of the planet's ice-free land is now used for agriculture. Here, again, its effects will actually leave behind only small traces at best. Future geologists are most likely to grasp the scale of 21st-century industrial agriculture from the pollen record. They'll see vast stretches of only corn, wheat and soy pollen, which will have replaced the pollen left behind by rain forests or prairies.

The leveling of the world's forests will indicate at least two things to future stratigraphers: the amount of sediment and the extinction of animals who live in trees.

Massive amounts of eroding soil are accumulating as sediment in some parts of the world. That said, the dams we've built on the world's major rivers are holding back sediment that would otherwise be washed to sea. This may make the effects of erosion less obvious to future scientists.

Loss of forest habitat, however, will show major effects. They are a leading cause of animal extinctions. These are now happening at a rate hundreds or even thousands of times higher than during most of the past half billion years.

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 harmless in the short-term. They are released when we burn fuels or wood. Their warming effects, on the other hand, could easily push global temperatures to levels that have not been seen for millions of years.

Global Warming Will Doom Some Species

Some plants and animals are already shifting their ranges toward the Poles, and those shifts will be shown in fossils left behind. Some species will not survive the warming at all. Meanwhile, rising temperatures could raise sea levels six meters (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 obvious. As carbon dioxide warms the planet, it also seeps into the oceans and makes them more acidic. Sometime this century they may become acidified to the point that corals can no longer construct reefs. This 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 many other creatures. 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 impact become geologically significant?

William Ruddiman studies climate history at the University of Virginia. He has proposed that the invention of agriculture some 8,000 years ago, and the deforestation that resulted, led to an increase in carbon dioxide in the atmosphere. It was 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 for the entire Holocene, from about 11,600 years ago to today.

Carbon Dioxide Levels Started To Climb 300 Years Ago

Crutzen has suggested that the Anthropocene began in the late 18th century. This is supported by the study of ice cores, which are samples of ice that has accumulated over long periods of time. These samples show that, 300 years ago, carbon dioxide levels began their uninterrupted rise.

Other scientists put the beginning of the new epoch in the middle of the 20th century. The rates of both population growth and consumption accelerated rapidly then.

Zalasiewicz now heads a working group of the International Commission on Stratigraphy (ICS). It will soon officially determine whether the Anthropocene deserves to be included in the geologic timescale. The decision is likely to take years. That decision may become easier the more we wait.

Some scientists argue that we've not yet reached the start of the Anthropocene. The reason is not because we haven't had a dramatic impact on the planet, but because it will take another few decades.

Crutzen, who started the debate, thinks the term shouldn't just be used to update our geology textbooks. He wants to focus our attention on the consequences of our actions as humans, and on how we might still stop the worst. "What I hope," he says, "is that the term 'Anthropocene' will be a warning to the world."