Fri May 13th, 2011 at 07:37:04 AM EST
Climate science is a fascinating and rapidly evolving field. It is now clear to all, except to those who don't want to understand, that human-induced climate change is already a problem, and a rapidly worsening one, that requires political and economic action. Not to save the world (that would be rather arrogant and pretentious), but perhaps to save humanity.
Palaeoclimatology, the study of the history of the world's climate, is a vital vector for understanding where we are now and where we are going : if we can closely model past climate trajectories, we can more closely constrain the possible futures.
The major vector of human influence on the climate has been the huge pulse of gases released by the burning of fossil fuels, starting with coal in the 19th century and worsening with the current orgy of oil and gas. The many gigatonnes of carbon released in only a couple of centuries constitute a cataclysmic climate event, orders of magnitude greater than any previous human influence. So, by convention, pre-industrial human influence was more or less politely ignored. It was easily demonstrated, by running a few computer simulations, that land-use changes due to agriculture might have had a marginal effect, but human population was so small that the climate effect could only have emerged from background noise by about the 18th century, so that it was almost immediately swamped by the fossil fuel effect.
However... what does the climate record actually have to say on the subject? When does the Anthropocene commence - with the industrial age, or ten thousand years earlier?
front-paged by afew
The earth's current geological epoch, the Holocene, is the eighth interglacial period in the current Ice Age, the Quaternary glaciation, also known as the Pleistocene glaciation. Eccentricity, wobble and tilt of the earth's orbit around the sun, plus the current configuration of the continents, combine to produce periodic expansion and contraction of the planet's icecaps.
It's been about twelve thousand years since the end of the last glacial period. With no human influence on the climate, this interglacial period would be expected to end in about another 15 000 years. This would be accompanied, in a hypothetical non-human-influenced climate, by a fall in atmospheric CO2 to 210 ppm.
The concentration of CO2 (and also of methane, another powerful greenhouse gas) in the atmosphere has been rising for 5000 years. This is surprising in light of previous interglacial periods; it would be expected that, after the initial rise of C02 to about 260 ppm which accompanied the end of the last glacial period, the level of CO2 would decline slowly until the beginning of the next.
There are two views on this : it's a natural phenomenon, for which several mechanisms have been postulated (top graph), or it's due to anthropogenic influences, counteracting the natural trend (bottom graph).
Bill Ruddiman was the first prominent palaeoclimatologist to postulate significant human impact on the climate before the industrial age. But this was perhaps more a matter of intuition than persuasive scientific evidence; until recently, it was difficult to get the numbers to stack up.
His book Plows, plagues and petroleum (2005, revised edition 2010) postulates agriculture and pastoralism as the prime movers in climate impact, based largely on their apparent synchronicity.
The book seemed to some to mark him out as a dissenter from scientific concensus on global warming; I was rather alarmed to find a warm recommendation for the book on junkscience.com. It's true that those who are more interested in the politics of climate than in the science may find the thesis an uwelcome distraction (or a welcome distraction, for denialists) from the message about the need to master greenhouse gas emissions. For one thing, it pretty much abolishes the baseline "natural" CO2 level against which industrial-epoch emissions can be evaluated; for another, it opens the question of what an "appropriate" or "good" level of CO2 would be.
An article by Ruddiman on Realclimate.org summarizes (in a not necessarily unbiased way) the current state of the play :
RealClimate: An Emerging View on Early Land Use
An August 2011 special issue of the journal The Holocene will help to move this discussion forward. All scientists who have been part of this debate during the last decade were invited to contribute to the volume. The list of those invited was well balanced between the two views, both of which are well represented in the issue. The papers have recently begun to come online, but unfortunately behind a paywall.
The article, and especially the courteous and erudite discussion which follows on RealClimate, illustrate that Ruddiman's thesis is no longer a fringe view, but one supported by a growing body of evidence.
Increasingly, it appears that Ruddiman may have initially been right for the wrong reasons. The evidence now points towards deforestation by hunter-gatherers, largely by fire, as the biggest per-capita human contribution to greenhouse gases. This would explain the "front-loading" of CO2 increase with respect to population levels. Ruddiman's original agriculture-only thesis has been robustly debunked by Stocker et al, who demonstrate that you can't get the pre-industrual CO2 levels from agriculture alone. Ruddiman acknowledges the imbalance in the carbon budget, and points to ongoing work on the impact of fire by hunter-gatherers and early agriculturalists. Colonisation of new territories in the historical era (New Zealand, where the first humans arrived around the 12th century, is a striking example) illustrate the huge land-use impacts that can arise from relatively tiny human populations.
The history of agriculture is one of ever-increasing intensity of land use. Hunting and gathering support only very low population densities. Agriculture, from its inception, will first colonise all of the arable territory, then will inevitably intensify, as its success supports a growing population. Populations must innovate or perish; and thus, the per-capita human impact on greenhouse gases has followed a diminishing trend -- until the industrial age.
Jed Kaplan's high-resolution simulations of European land use is one paper that I wouldn't mind paying for, though it isn't yet available on line. A summary here seems to indicate that it addresses a still-controversial aspect of Ruddiman's thesis : the postulate that historical variations in climate (for example, the "little ice age" and "medieval warm period") can be tied to human land-use changes : for example, the Black Death, the Mongol invasion, and the 90% decline in Amerindian populations after European contact provoked re-forestation which would have had a strong cooling influence. However Julia Pongrantz's work suggests that global impacts from such events would be practically negligeable, with only the Mongol invasion registering, just barely.
I have no fixed view on all this, I'm just marvelling at the expansion of our understanding. It's all good, as long as it's honest science. What do you-all think?
Late addition : This article on Nature News summarizes all this rather better than I...