A sizable group of researchers (21; all members of the Stratigraphy Commission of the Geological Society of London) has put forward an idea in this month's issue of GSA Today: they suggest that humans have altered the planet enough that it will show up in the geologic record of the future. They suggest, therefore, that we may have already entered a new geologic epoch defined by human alteration. As a result, they've adopted the name originally suggested by Nobel laureate Paul Crutzen: "the Anthropocene." (Crutzen won in 1995, with two other chemists, for his work on the depletion of the ozone layer in the atmosphere.)

The evidence they offer for this assertion is compelling, but it raises a few questions about how we define these stratigraphic breaks in the geologic record.

Here's the only figure from the paper, a temporal comparison between several lines of data (top to bottom): sea level, average global temperature, atmospheric CO2, terrestrial erosion rates, and human population of the planet.

This is a powerful image. The authors note that climate essentially stabilized in the Holocene, the "long summer" of Brian Fagan's phrasing. In a classic display of scientific understatement, they note that this prolonged period of stable climate "has been a significant factor in the development of human civilization."

How will the rise of humanity be remembered by the geologic record? They note that we've accomplished some major changes to the rate of erosion and sedimentation : "directly, through agriculture and construction, and indirectly, by damming most major rivers, that now exceeds natural sediment production by an order of magnitude." I may be missing something here, but it would seem to me that anthropogenic erosion would produce more sediment due to our land use practices, but that less of that sediment would make it to the sea due to the "sediment trap" effect of dammed reservoirs. I mean, the Colorado River doesn't even make it to the ocean anymore.

Then there's temperature. A quote from the paper: "Temperature is predicted to rise by 1.1 °C to 6.4 °C by the end of this century, leading to global temperatures not encountered since the Tertiary." The high end of that estimate is indeed the sort of temperature change that one would think would leave a profound mark in the geologic record. (I find it interesting to note that a cast of 21 stratigraphers persists in using the outmoded and archaic term "Tertiary," by the way. I guess that's as sure a sign as any the Wernerian Chronology still has some kick left in it.)

I think one of the most compelling arguments made in favor of the Anthropocene is the rapid change in the Earth's biosphere. As the authors of the GSA Today paper point out, we've wiped out the majority of the big terrestrial animals, and concomitant wave of extinctions has rippled through the marine realm. Since changes in fossil biota have been the benchmarks of change in the geologic timescale, it seems certain that our tenure will be marked clearly for future paleontologists to see. Not only are species going extinct, those that survive are migrating to new territories as a result of shifting climate.

I'm pleased that the authors also explored changes to ocean chemistry, which will likely be a major source of information to future geologists. They cite Ken Caldiera and Michael Wickett's 2003 study on ocean acidification (which I blogged about last month) which shows that pH in the world's oceans has already dropped by 0.1 unit, and is predicted to continue acidifying so long as there's excess carbon dioxide to absorb from the atmosphere. Of course, add sea level rise to that (as is predicted via accelerated melting of continental ice sheets), and you've got a distinctive stratigraphic signature.

And I guess that brings me to a point that's been on my mind since I started listing these items. Will these changes persist for a long time, or will they be a small but distinct signature, a la the iridium layer at the K/Pg (formerly known as the "K/T") boundary? Another way of putting this: are we seeing the beginning of the Anthropocene's modus operandi, or are we seeing the environmental catastrophe which paves the way for a new, different, and (at this time) unpredictable Anthropocene status quo? At this point, we don't know what the Anthropocene will really look like in bulk. While it makes a lot of sense to point out the accelerated rates of change unfolding in so many geological realms, what it all portends for an as-yet-unattained future equilibrium remains to be seen.

I think papers like this are important. It's both broad in scope and displays some excellent thinking outside the box. I'm curious to see what reaction it provokes in the scientific community. Certainly it's getting some press.

* A side note: Does anybody else find GSA Today to be a weird journal? It always has one main article and then a bunch of stuff about meetings, awards, and the like, of interest to members of the GSA. But the articles featured each month are all over the map. Some, like this month's, are potentially ground-breaking works of scholarship. Others, just seem a bit... fringe. Like the one in December about how a team has shared Denver's geologic story with the public. Or the one about a historical critique of Lord Kelvin. Don't get me wrong: both topics are well and good, but if you're putting out only a single article each month that gets mailed to the entire GSA membership, why those? Sometimes I'm just left perplexed and scratching my head.

References:

Caldeira K., Wickett M.E. 2003. Anthropogenic carbon and ocean pH. Nature. v. 425. p 365. doi: 10.1038/425365a
Fagan, Brian. (2004) The Long Summer: How Climate Changed Civilization. Basic Books. ISBN 0465022812
Zalasiewicz J, Williams M, Smith A, Barry TL, Coe AL, et al. (2008) "Are we now living in the Anthropocene?" GSA Today: Vol. 18, No. 2 pp. 4–8. doi: 10.1130/GSAT01802A.1

Labels: CO2, environmental, global warming, stratigraphy