Humanity's influence on climate began 8,000 years ago, say researchers

Scientists have come up with new evidence in support of the theory that human agriculture was changing the climate 8,000 years ago.

They have found new evidence to back the controversial idea that humanity's influence on climate began not during the industrial revolution, but thousands of years ago.

Proposed by palaeoclimatologist William Ruddiman in 2003, the theory said that human influences offset the imminent plunge into another ice age and helped create the relatively stable climate that we are familiar with today.

It has been repeatedly panned as implausible by palaeoclimate researchers, but eight years on, Ruddiman and others said that they have the data to support early anthropogenic climate change.

One of the studies, led by Jed Kaplan at the Federal Polytechnic School of Lausanne, Switzerland, suggests that agriculture had a much larger impact than previously believed as it expanded in Europe and beyond.

Kaplan built a detailed model to analyse land-use change over time, building in historical and archaeological data where possible. In contrast to most previous estimates, the model assumes that humans cleared more land early on, with only gradual intensification as agriculture improved, reports Nature.

"People are basically lazy. They only intensify their land use when they are forced to," he said.

The result is roughly double the carbon emissions compared with earlier estimates.

Another study in The Holocene by Dorian Fuller, an archaeologist at University College London, explores methane emissions from livestock and the spread of rice agriculture in southeast Asia. Fuller said that the expansion of rice could account for up to 80 pc of the additional atmospheric methane as of 1,000 years ago, and suggests that the expansion of livestock could help to plug the gap in previous millennia.

Both Kaplan and Fuller said that their focus is not so much on Ruddiman's specific hypothesis as on the idea that humans might have influenced climate well before the industrial revolution.

"The human influence is there," said Fuller. "We can see that."

Researchers have plenty of work to do in terms of quantifying early human emissions, added Kaplan, "but it is getting hard to support the idea that anthropogenic influence was negligible before the industrial era". 

Researchers have presented some of the work this week at the American Geophysical Union's Chapman Conference on Climates, Past Landscapes and Civilizations in Santa Fe, New Mexico.