Scientists have come up with new evidence in support of 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 says 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 say that they have the data to support early anthropogenic climate change.
The argument centres on a curious trend in atmospheric carbon dioxide and methane levels since the last ice age ended about 11,000 years ago and the current Holocene epoch began. In previous interglacial periods, CO2 levels spiked early and then gradually declined until the globe went into another ice age. The Holocene began by following this trend, but then CO2 levels changed course and began to rise around 8,000 years ago. The same thing happened with methane levels around 5,000 years ago. These trends align with the expansion of human agriculture, and Ruddiman, of the University of Virginia in Charlottesville, argues that it is no coincidence — the clearing of land and expansion of irrigation released huge amounts of greenhouse gases into the atmosphere.