Humans have a long history, going back well before the beginning of agriculture, of making substantial modification to the planet. The standard view, however, has been that such changes were comparatively modest in scale, affecting regions at most and not having planetary consequences significant enough to modify the operation of the global system. Global scale consequences, those tied to the emergence of conjoined social-ecological systems and significant enough to indicate the dawning of a new geological era (the Anthropocene) have been viewed as comparatively recent -- as a product of the industrial revolution.
The March 25 issue of Nature News, however, reports in The 8000 Year Old Climate Puzzle:
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.
Critics say that human populations were probably too small to support such a hypothesis, and recent studies have raised serious questions about early anthropogenic carbon and methane emissions.
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.
The article goes on to discuss preliminary findings from several studies and to note that supporting evidence is scheduled for publication in a special issue of The Holocene later this year.
For a popular treatments of the Anthropocene, see National Geographic's Enter the Anthropocene: The Age of Man and the Walrus's Age of Breathing Underwater.