Friday, June 24, 2011

The End of the Holocene

Last month the Economist ran a cover story, "Welcome to the Anthropocene," discussing the startling idea that humans had so altered the Earth's surface, oceans, and atmosphere that they had actually inaugurated a new geological epoch. We are all familiar with the anthropogenic increase in atmospheric carbon dioxide, but humans have introduced less well-known but no less profound changes to their physical environment: a 20 percent decrease in the amount of sediment discharged by the world's rivers, for example (thanks to 50,000 or so hydroelectric and flood-control dams), and a 150 percent increase in the amount of nitrogen transferred from the atmosphere to the soil (thanks to synthetic nitrates). Most of these changes are problematic, but they are all the consequence of creating a world economy and technosphere capable of supporting seven billion human beings. Turning back the clock to a "lower-impact" state - one should perhaps say "to the Holocene" - would require massive depopulation, which only a few idealists publicly favor.

As a historian, I have two questions about the "Anthropocene." First, when did this new geological epoch, if we may call it that, begin? One of the commentators on the Economist's website, "Callisthenes," argues that it may be difficult to identify a starting-point using geological rules, but the list of technologies the article provides allows us to identify some significant dates. The Haber-Bosch process of nitrate synthesis was demonstrated in 1909, the construction of large concrete dams began about 1880, and the takeoff in human CO2 production occurred in the latter half of the nineteenth  century. 1900 would seem a good ballpark estimate, but it's fair to say that no-one living at the turn of the twentieth century realized they were bringing the Holocene to a close, just as a fair percentage of humans today would prefer to believe that nothing has changed since the world's creation (in 4004 BC, of course).

My second question is the standard historical significance question: so what? On the time scales most historians work with (a few decades, usually), the inception of a new geological epoch wouldn't seem to leave much of an impact. Taking the past ten thousand years of human history and prehistory as a whole, however, the Economist article suggests an answer to the question "Which was more significant, the Neolithic Revolution [the introduction of human agriculture] or the Industrial Revolution?" Insofar as the Industrial Revolution has reshaped the surface and atmospheric chemistry of the Earth and stamped its geological record, an Earth scientist who accepts the concept of an Anthropocene would argue it was more significant, and that the Neolithic Revolution was primarily important as the precondition for industrialization. Whether historians will be inclined to discuss the matter with their colleagues in the Geology department remains to be seen.

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