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The Anthropocene
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Why do geological time periods have names rather than numbers?
I come from the meteorological strain (as opposed to the geological strain) of climatology; my education was at the University of Wisconsin's Department of Atmospheric and Oceanic Sciences, just recently renamed from plain and simple "Meteorology Department" when I arrived. This means that the focus was on fluid dynamics: cloud dynamics, large scale atmosphere dynamics, tropical storms and so on. Climate was treated as a sort of background state for all this.
Before my doctorate, and since, I have used computers a lot. I write code. I run code. I write code that runs code. I even write code that writes code. The trouble with computer porgrams to study climate, of course, is that everybody wants to know the future. We know that we are changing the atmosphere and the land, so the future will be unprecedented to some extent. So how can we know when our models get it right?
I am among those who have decided that the right way to model the future is to model the altered climates of the distant past. Only a modeling strategy that captures the climate changes of the past has any credibility in predicting the future and guiding our way through it.
Which brings me to my grumble. As a dynamicist and not a geologist, I am comfortable with numbers. Big numbers (the mass of the ocean), little numbers (the mass of a fog droplet) any numbers. Geologists, the people whom we modelers have to work with to get the past right, have all these poetic names; the Cretaceous, the Jurassic, the Eocene, the Quaternary... I'm starting to recognize some of them, but I'm forever turning them back into numbers in my mind.
The Jurassic (every boy's favorite epoch, the one with the giant dinosaurs); about 200 to 150 million years ago. The Cretaceous after that, until about 70 million years ago. The Eocece, 55 to 34 million years ago. And so on. After a while you get used to these time periods, what they mean, and what we can know about them.
Still, this funny nomenclature drove me slightly mad until recently. I finally cornered a geologist and asked him to explain why they didn't just use, you know, numbers. It turns out there are two very good reasons. First of all, these periods are different. The natural world, not just the plants and animals but the rocks and the sky changed from one period to another. These periods aren't arbitrary. They describe different configurations of the world. Secondly, the changes were known long before they were effectively dated. Some of the transitions were so sharp that they still aren't fully resolved in time. The boundary between the Paleocene and the Eocene, for example, was a time of a great global warming catastrophe accompanied by widespread extinctions.
What about today? Officially we are still in the Quaternary period, which started 1.8 million years ago and is characterized by large ice sheets pulsing forward and retreating on a 100,000 year cycle. The reasons for this pulsing are not fully understood, though the background cooling is easily attributed to a gradual decline of CO2 over the past 60 million years, which in turn is believed related to intense weathering of the unusually high Himalaya mountains.
But, as Andrew Revkin points out in Dot Earth this week, there's a movement afoot to recognize that we are in another great transition, one which will permanently impact the geological record. The Russian geologist Vladimir Vernadsky noted 60 years ago that "Mankind taken as a whole is becoming a powerful geological force." In honor of this change, the present (starting perhaps in 1900, or 1945 or 2000) era is already informally called the "anthropocene" in some circles. This may become the recognized formal name for our time soon enough. There's little doubt that the Quaternary/Anthropocene boundary will be visible in many deposits into the very distant future.
On a road trip through New Mexico last year I spotted an example of an anthropocene process.
Mountain streams have always meandered and changed, especially through soft rocks; mud gets washed into gullies, cliffs tumble. What's different about the anthropocene? Well, in this case there was a parking lot atop the cliff. When the land failed, a half dozen vehicles were washed into the gully along with the usual mud and twigs. They now lie in a half buried state, working their way downhill and toward the Coyote River Basin. These lumps of steel and upholstery are not worth prying out of the mud, so they will become, ever so slowly, rock and mud themselves. Eventually they will be some sort of fossil car, of a form we can't imagine. The rocks around them will be especially iron-rich and red. Perhaps the imprint of a bumpy round steering wheel will appear at a surface, to be sold to young rock collectors of the hundred thousandth generation as particularly interesting specimens. Whatever happens, the earth will be of a different sort than any version of the earth that has ever existed in the past. It is a new era.
Update: Chris Rowan has a very nice essay on this topic as well.
Tags: anthropocene, geological time
2 Comments
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January 29, 2008 10:44 PM
cvj
Hi,
Excellent post, but I'd like a bit more to be convinced of the anthropocene era that may be upon us. I'm looking for scale here. What I mean is that each of the eras we know well, like the Jurassic and so forth, represented the results of huge transformations that affected several layers of interconnected species, giving rise to characteristic fossil records and so forth. Some crashed cars and sunken ships and so forth don't (in and of themselves) constitute such a change. In the bigger scheme of things, might they not show up as mere anomalies, as opposed to major signifiers of a different geological age? I'm playing devil's advocate here, since we can point to things like climate change, and all the different renderings of our biodiversity that will result from that, but in terms of things that an alien species would consider hugely significant impacts... I'm not sure that buried cars or even entire decayed cities are as significant as, say, the fossilized remains of an entire species of group of species that characterize an era (giant dinosaurs, for example)... Now if we manage to nuke ourselves into oblivion or something like that... maybe you've got something there... Other than that, I'm just not sure that we count for much more than a blip (or even a blip on a blip) so far on the geological calendar.
Cheers...
-cvj
January 30, 2008 8:53 AM
Michael Tobis
Clifford, thanks. We can take this up in any of a large number of ways, but I think the car thing is good for capturing the imagination.
In considering planetary surface processes, sources and sinks of individual elements is a useful way to look at things.
The picture is a striking if odd path through the new iron cycle.
The injection of iron out of the ground and into surface processes has been drastically sped up by mining. That's already and anthropocene process. There is far more rust hanging around our environment than is naturally the case, and dust and ocean sedimentation from our era will certainly be detectably iron oxide rich.
We don't really know how most of the iron will eventually be recycled into the earth. Some of it will rust. Some of it will delinerately be buried whole. Some of it may hang around for a very long time tended by humans, or maybe not. The picture is intended to show a third path, of inadvertent burying, which for the time being will be very rare. Hopefully it will stay that way. That's not the point. The point is that all this iron was once rock and will one day be rock again, but the pathway is unprecedented.
As an astronomer, you think of the earth as a sphere. On that scale, what we do matters very little. As a climatologist, I think of the earth as a spherical shell. On a two dimensional view rather than a three dimensional view, humans are a very effective agent of change.
The eras and epochs the geologists talk about are not about the core or mantle processes, which indeed care very little about humans. They are about changing characteristics of surface processes leading to changing characteristics of deposition and rock formation.
We have already made enough changes that if humanity vanishes tomorrow we will have made an impact on the geological record detectable many millions of years hence. Our changes to the carbon cycle are probably the most important, but the signal will be visible in many records.
Whether it's a geochemical blip on the time scale remains to be seen.
Biologically, the last few thousand years have already made a huge and still accelerating impact on the extinction record that will persist as long as wildlife exists. We are already in a major extinction event.
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