That's one of the intriguing corollaries of a hypothesis being put forward by William Ruddiman, an emeritus climate researcher at the University of Virginia in Charlottesville. Ruddiman argues that, through deforestation and rice farming, humans have been having a significant effect on greenhouse warming for as much as 8,000 years - not just that last century or so that is the more common view today.

Ruddiman lays out the thesis in a piece in the December Climatic Change, and there are good news stories about his thinking by Richard Kerr in the Jan. 16 Science and by Betsy Mason in yesterday's Nature. (I won't bother linking directly to the articles because of paid subscription juju - if you've got that, you'll know how to find 'em.) New Scientist also ran an article in December, when Ruddiman spoke about this at AGU.

The nut of Ruddiman's argument is that carbon dioxide and methane levels over the last 10,000 years, as seen in ice core records, do not correspond with trends one would expect based on natural solar-induced cycles. The anomalies, he argues, corrolate well with a number of basic innovations in human culture, including the spread of farming across Eurasia, and the rise of rice paddy agriculture.

The plague argument, which is really just a teaser for the much more complex story, goes like this: massive deforestation from the beginning of the age of farming 8,000 years ago across Eurasia is responsible for measured increases in carbon dioxide in the atmosphere, which lead to much more significant and early warming than is currently widely recognized. But plague, by killing a huge portion of the European population, changed that, as large swaths of farmland reverted to forest. Less greenhouse muck in the atmosphere means less warming, and presto! You've got a Little Ice Age.

Ruddiman also pegs another big carbon dioxide decrease to an issue that's much in debate in American archaeology. When Europeans brought their diseases to the Americas in the 1500s, they killed off a more-than-plague-like proportion of the continent's native population, causing a reforestation here that corresponds to a measurable decline in atmospheric carbon dioxide, Mason explains. This goes to the heart of the debate about how truly "wild" the American wilderness was when the first European explorers got here, and how large the native population might have been. One faction among the archaeologists studying this argues that the Native American population was enormous, living, farming, etc., essentially everywhere. It took Europeans several centuries to "explore" the continental interior, according to this argument, and by that time their diseases had preceded them, wiping out the native population. By this argument, places that seemed "wild" - forested, unpopulated by humans - when the Europeans got there had only recently become so, on account of because our diseases had only recently killed all the people who had lived there.