Through the eons, Earth's history has been written by geologic forces. Mountains rose and settled. Seas flooded and ebbed. Continents drifted, iced over and melted.
But in the past couple hundred or maybe thousand years, a new force has emerged - one that many scientists think could rival earthquakes and volcanoes in shaping the next chapter in the planet's story.
Humanity has become such a dominant presence on the globe that it is altering the atmosphere, the oceans and even the panoply of plants and animals roaming the land, they say.
Now, in recognition of those sweeping changes - and out of concern for where they could lead - a small but growing band of researchers has proposed renaming the geologic time in which we live the "anthropocene." It's a melding of the Greek words for "man" and "new," which hews to the scientific protocol for naming such things.
Dutch meteorologist Paul J. Crutzen, who shared in the 1995 Nobel Prize for helping to explain the ozone hole, said the idea came to him suddenly at a scientific meeting in Mexico several years ago as he listened to a discussion of anthropogenic, or human-induced, shifts in global atmospheric chemistry.
"It just struck me we were geologically entering another era, or have entered it 200 years or so ago," he said in a telephone interview. Glacial ice cores show that was the beginning of a buildup in the atmosphere of so-called "greenhouse gases," which many scientists believe are warming the Earth's atmosphere. That's also when the steam engine was invented, launching the Industrial Revolution, he noted.
"I thought it was too good an idea to just keep to myself," Crutzen said. "It's just catching on, I think."
Not with the folks who name geologic time periods - at least not yet.
Although Crutzen, who works at the Max Planck Institute for Chemistry in Mainz, Germany, and a handful of other scientists have weighed in on the idea in a few scholarly journals over the past six years, it hasn't come to the attention of the International Commission on Stratigraphy. The 400 geologists who sit on that panel decide when ages, epochs, periods and eras begin and end. Commission leaders say they've pretty well hashed out the roster of the Earth's phases going back billions of years.
"It's all been named for over 100 years," said Stanley Finney, vice chairman of the commission and professor of geological sciences at California State University at Long Beach. The current epoch, the Holocene, began with the end of the last Ice Age about 8,000 years ago.
Although new evidence can prompt shifts in the boundaries of some geologic times, and a few period labels have even been dropped or replaced, Finney explained, "rarely do we stick in completely new names."
Geologists rely on rocks, sediments and fossils formed over thousands and millions of years to delineate the significant chapters in Earth's history. The signatures of humanity's impact on the planet over the last two centuries that Crutzen and others point to as evidence of a new epoch are much too recent to have established a fossil record.
"My guess is there's not enough sediment in the world right now since the Industrial Age where you could separate it easily and say this represents that new interval of time," said James G. Ogg, professor of earth and atmospheric sciences at Purdue University and secretary-general of the international commission.
But Crutzen and others say it's impossible to ignore the changes a burgeoning humanity has wrought on the planet's atmosphere, on its oceans, rivers and lakes, and on land.
Official or not, "anthropocene" has been picked up by scientists and others concerned about climate change. Crutzen said the president of the Royal Society in England and members of Parliament, among others, have used his term.
It has also crept into the Oxford Pocket Dictionary of Current English, a sign of the frequency with which it is being written and spoken, if not its official acceptance.
It has even generated a ripple of scientific debate. William F. Ruddiman, an environmental scientist at the University of Virginia, contends that the anthropocene began around the end of the Ice Age, when primitive peoples shifted from hunting and gathering to farming. He pointed to indications that greenhouse-gas levels began to rise 8,000 years ago as people cleared forests, burned more wood and domesticated animals. That human-engineered warming of the atmosphere, he argues, has kept the Earth from slipping back into another ice age, in a climate cycle that geologists have charted across the ages.
But Crutzen and others have balked, responding that although early civilizations may have altered the landscape locally or even regionally, only in the last two centuries has humankind really begun to transform the Earth as a whole.
"Yes, humans have affected our environment ever since we've been on the planet," said Will Steffen, an Australian earth scientist. "So have elephants. ... But, in fact, we're looking for a global imprint."
In fact, Crutzen and colleagues have even suggested subdividing the geologically brief period they call the anthropocene. They contend that there's evidence that the pace and scope of human transformation has increased drastically in the last 50 or 60 years, since the end of World War II.
"We call the period from 1950 up until now the Great Acceleration," Steffen said in a telephone interview. To illustrate his point, he has compiled a series of graphs tracking soaring increases in various indicators of human activity - from the levels of nitrogen fouling coastal waters to the number of species extinctions to the construction of McDonald's fast-food restaurants.
The McDonald's graph was "a bit of fun," Steffen concedes, but a lighthearted example of a serious trend: the globalization of commerce.
No matter how impressive the evidence of human manipulation of the Earth, it's unlikely to vault the name anthropocene into the geologic timeline anytime soon, says Andrew Alden, who writes a geology blog for About.com, an online "information portal."
Alden says Crutzen definitely is onto something that concerns geologists, but his idea of giving it a geologic name is little more than a scientific pun, a catchy word portraying an extremely sobering trend.
"We get it, but there's no need to change the time scale, for geologic purposes," Alden said in a phone interview. Still, he says, "it'd be fun to have it on the wall." Only half seriously, he wrote a column proposing adding the Anthropocene Age as a phase in the current Holocene Epoch.
"I leave it to the geologists to decide," Crutzen said of whether to officially recognize the anthropocene. But he and Steffen insist they aren't joking around about humanity changing the natural rhythms of the planet. One worry they have is that the transformation in climate and other natural systems could be irreversible.
Others agree. "If we as a society don't respond to those alarm bells," said Glenn G. Page, director of conservation at the National Aquarium in Baltimore, "we're hearing what our future is going to be. We're going to be in the geologic record as an extinction."
Should that happen, a geologist 10,000 years from now - assuming there are any - may excavate through layers of sediment, bottle caps and rusting metal and "would probably recognize that a major change would have occurred," said Ogg, the epoch-naming commission's secretary-general.
Crutzen and others, though, hope there's still time to put the brakes on the changes.
"We have it in our power to go into the next step of the anthropocene," Crutzen said. "That is, if we treat the Earth in a better way ... If we develop [better] ways of using energy and by not using fossil fuels. Then we have a little bit of paradise ahead. It's possible, but we have to go through a bottleneck in the next couple of decades."