Are people changing geologic time?

For some 4.5 billion years, natural forces such as volcanic eruptions, asteroid strikes, and earthquakes have shaped the Earth.

Now, however, human activity is rewriting geologic history, according to scientists in the February issue of GSA Today, produced by the Geological Society of America.

They blame the industrial revolution for a new geologic epoch, dubbed the Anthropocene. Stresses to the planet's atmosphere, oceans, life forms, and very surface are dramatic enough to end the Holocene epoch, the geologists say. That period began about 12,000 years ago as the last Ice Age melted and the planet warmed enough to allow people to farm and thrive.

The term Anthropocene was popularized six years ago (PDF) by Nobel Prize-winning chemist Paul Crutzen, who fingered the invention of the steam engine in 1784 as the start of the shift.

Until the mid-18th century, only 300 million people walked the Earth--roughly the current population of the United States, according to the United Nations. And the world population could jump from 6.6 billion today to 8 billion by 2030, further stressing the planet.

"The exploitation of coal, oil, and gas in particular has enabled planetwide industrialization, construction, and mass transport, the ensuing changes encompassing a wide variety of phenomena," reported the 21 scientists in GSA Today.

Among the shifts:

• Widespread agriculture, the construction of cities, and damming of rivers have changed the physical makeup of the Earth's surface. Increasing acidity in rising oceans could erode ocean floors.
• Industrialization made carbon levels spike upward by one-third within the last three centuries, and carbon levels are expected to double by the next century. Toxic, man-made chemicals are also affecting the planet's physical character.
• Due largely to rising global temperatures, farming that wipes out plant diversity, as well as other human-triggered forces, half of the life forms that exist today could die off within this century, according to the World Conservation Union.

However, the Anthropocene proposal is controversial. Other geologists argue that the current period of geologic time, if it deserves to be distinguished, is more likely an age rather than an epoch, which is a series of ages.