Debate over the Early Anthropogenic Hypothesis

There have been a few mentions of the ‘early anthropocene’ hypothesis recently (cf. the EPICA CO2 results, and Strange Bedfellows). We therefore welcome Bill Ruddiman to RealClimate to present his viewpoint and hopefully stimulate further discussion – gavin.

[Addendum: For a non-technical backgrounder on the 'early anthropocene' hypothesis and its significance in the context of anthropogenic climate change, see Bill Ruddiman's article "How Did Humans First Alter Global Climate?" from the March 2005 issue of "Scientific American" (first two paragraphs available for free; full article must be purchased). -mike]

Guest posting from Bill Ruddiman, University of Virginia

The hypothesis (Ruddiman, 2003) that early agriculture caused large enough emissions of greenhouse gases millennia ago to offset a natural climatic cooling remains controversial. The centerpiece of the hypothesis was a comparison of the increases of CO2 and CH4 values in Vostok ice during the current (Holocene) interglaciation versus the (natural) drops during similar portions of the three previous interglaciations.

EPICA Community Members, 2004). All three papers used the same (seemingly reasonable) approach: they aligned the deglacial warming that initiated the stage 11 interglaciation with the one that initiated the Holocene interglaciation and counted forward in estimated ice-core years to compare the durations. They found that 16,000 more years must elapse before the current Holocene interglaciation reaches the length of the fully natural stage 11 interglaciation (Fig. 1a). The obvious implication is that present-day warmth is accounted for by natural causes, and an anthropogenic overprint is not needed.

It certainly makes sense to turn to stage 11 to find the closest natural analog to climate-system behavior during recent millennia, but I have previously pointed out (Ruddiman 2005) that the EPICA strategy produces an odd alignment of insolation trends for the two interglaciations (Fig. 1b). It positions the present-day northern summer insolation minimum against a stage 11 insolation maximum (409,000 years ago). This inverted alignment raises an obvious question: Does it really make sense to use a past insolation maximum as the ‘closest analog’ to the present-day insolation minimum?

A positive answer to this question would imply that the actual ‘details’ of orbital insolation changes through time can be ignored in testing my hypothesis. Yet the reasoning I used was specifically tied to two well-known theories that invoke orbital changes as central to the operation of the climate system: Milutin Milankovitch’s theory that northern summer insolation plays a prominent role in driving ice volume, and John Kutzbach’s theory that northern summer insolation forces north-tropical monsoons. The inverted insolation alignment arrived at by the EPICA approach (Fig. 1b) ignores the climate-system physics embedded in these theories, as well as the central role they played in my hypothesis. As such, EPICA’s inverted insolation alignment is not a valid test of my hypothesis.

Page 1 of 2 | Next page