As scientists get used to the idea that Earth is in a new geological age, that the Holocene – the last geological age – has been replaced by Anthropocene, they're figuring out how it got to be that way.

Two years ago, ecologists Erle Ellis and Navin Ramankutty at the University of Maryland, Baltimore County, released a map of the world's biological areas, traditionally known as biomes. Similar maps were found on science classroom walls across the land, but theirs was different in one very fundamental way: They updated the definition of biome to reflect how human beings used the land.

Ellis and Ramankutty said this was much more relevant to the 21st century, with more than six billion people using more of Earth's water, energy and matter than any other species, than classical biomes that didn't account for humanity's influence. They called their newly-defined areas "anthromes," short for anthropological biomes. It was a map for the anthropocene.

During a subsequent presentation, someone asked the researchers for details on how the anthropocene evolved. To answer that question, Ellis and Ramankutty have come out with a new set of maps that show how anthromes have changed since the beginning of the Industrial Revolution.

"You now have a biosphere that's completely transformed by people. Biology goes on in the human context, not the natural," he said. "And given the idea that most of ecosystem form and process is created by and ruled by human activity, how did it get to be that way?"

Published in the September Global Ecology and Biogeography, the maps show that in 1700, humans had already penetrated almost every habitable area. Then as now, people-free wilderness existed mostly in deserts and tundra. But in 1700, there was lots of "seminatural" land – used by people, but not heavily. Now there's little of that, and much of what remains is embedded within intensively used landscapes.

In future studies, the researchers want to overlay the anthrome map with other ecological metrics, such as biodiversity and biomass production.

The researchers' work raises philosophical and ethical questions about the difference between natural and wild, and the value of nature. In Ellis' view, the difference is irrelevant, and nature has precisely the value assigned it by people.

But whatever the answers are, "The main thing is, people need to be aware of their impacts and benefits on nature," said Ellis.

Images: 1) Global anthrome maps in 1700 and 2000./Global Ecology and Biogeography. 2) Map of land use intensity change between 1700 and 2000./Global Ecology and Biogeography. 3) Graph of global anthrome proportion change between 1700 and 2000./Global Ecology and Biogeography.

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Citation: "Anthropogenic transformation of the biomes, 1700 to 2000." By Erle C. Ellis, Kees Klein Goldewijk, Stefan Siebert, Deborah Lightman, Navin Ramankutty. Global Ecology and Biogeography, Vol. 19 No. 5, September 2010.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.