When should the Anthropocene start?

As has been widely reported, there are serious moves afoot to define a new geological epoch, the Anthropocene. Although geologists have used the term informally for some time (Paul Crutzen, chemist and Nobel Laureate, popularised it in 2002), the evidence that humans will leave a lasting and significant footprint in the geological record is starting to look robust enough for formal recognition.

From this Guardian article:

The geological signal will be clear from industrial-scale mining, damming, deforestation and agriculture, as well as the concentration of carbon in the atmosphere and nitrates in the oceans. Even the presence of the first human-produced chemicals like PCBs, radioactive fallout and the humble plastic bag could be measured millions of years hence.

Reading between the lines of the various articles, it looks fairly certain this change to the geology textbooks is going to happen, sooner or later. The main debate seems to be around when to set the start date.

I won’t try to second guess the complex deliberations that are no doubt going on in the hallowed virtual halls of the International Commission on Stratigraphy. The Guardian article suggests a compelling reason to set the date at July 16, 1945, with the fallout from the first nuclear weapon detonation generating a worldwide radionuclide sedimentary signature.

But the rash of large and not-so-large animal extinctions caused by humans has been traced back to the end of the last ice age, the late Pleistocene, with consequent alterations in the fossil record going back more than 10,000 years. Pollen stratigraphy shows the imprint of human forest clearance and agriculture going back at least 5000 years. And paleoclimate analysis points towards anthropogenic CO2 level increases dating from a similar period. Perhaps these subtle changes are not enough to define a new epoch.

If you’re wondering what an epoch is, anyway, wikipedia seems ok on this, helpfully defining an epoch as a span of ‘tens of millions of years’. In which case, if we go with a 1945 or industrial revolution-definition for the Anthropocene, the Holocene will only have lasted for some 10,000 years, which seems a little short. Or if we go by CO2 and pollen changes, only 5,000. Use megafauna extinctions as our starting point, and it’s gone completely.

Yet— definitions are only as good as they are useful, and maybe a ridiculously short Holocene is fine if that corresponds to meaningful sedimentary subdivisions useful to actual working geologists, rather than amateurish bloggers. And of course it’s understandable that slices of defined geological time get smaller the closer they get to human observers in the present. And maybe the Nobel laureates and geologists in the news in recent weeks want to underscore the strikingly abrupt, unprecedented nature of the human-induced changes of the last 200 years by suggesting a more recent Anthropocene start date.

[Long sentence alert]

But if the prolonged interglacial that has hitherto been defined as the Holocene has allowed sufficient climatic stability for Homo sapiens to develop agriculture, initiate a series of species extinctions, clear forests and alter landscapes enough— arguably— to alter global CO2 levels, and this process started only a few thousand years from the time the ice caps receded, and laid the groundwork for industrial civilisation and the current acceleration in human impacts on the earth system— I wonder whether our descendants, if there are any, in another 10,000 or a 100,000 years time will bother with the Holocene at all. Wearing deep time goggles, I imagine the Pleistocene will appear to seamlessly merge into the Anthropocene at the end of the last ice age.

Do these definitional issues really matter? I’m not sure. Whatever works for the working geologists, at the end of the day. But setting a later date for the start of the Anthropocene— one closer to the present— smacks of the same kind of temporal bias, the focus on the here and now, that got us into this mess in the first place.

Further reading:
Economist article (linked above)
BBC article (linked above)
Guardian article (linked above)
Realclimate article (W. Ruddiman) on CO2 signature of early land use
GSA Today paper— Zalasiewicz et al, 2008
Some background on anthropogenic signature in pollen stratigraphy


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