Stone age humans.
The figure shows temperature, CO2 concentration and insolation for the past 350,000 years, a period that covers four interglacials, shown as yellow columns, of which ours is the most recent. When I first saw it, as described in the previous post, I noticed something odd. In the first three interglacials, temperature follows the same pattern, rising steeply at the beginning then falling until the end. CO2 concentration follows a similar pattern except in the third interglacial, where it oscillates about a constant or slightly rising level.
The pattern in the fourth interglacial is quite different. For the first few thousand years it looks similar to the previous three but then the pattern reverses, with temperature and CO2 rising instead of falling through the rest of the interglacial.
What was different this time? The obvious guess was us. The reversal in the pattern happens at about the time that humans adopted agriculture, resulting in both a large increase in human population and a change in how humans affected the world around them.
While that possibility occurred to me, I did not know enough to tell if it was plausible, if anything humans did prior to recent centuries was large enough to affect global temperature and CO2 levels. Someone in an online discussion pointed me at the work of William Ruddiman, who turned out to have seen the same pattern some twenty years before I did and published on it, proposing what became known as the Early Anthropogenic Hypothesis.
His conjecture was that deforestation, starting about eight thousand years ago, had put enough CO2 into the atmosphere to raise global temperatures. Data on the concentration of methane, another greenhouse gas, showed a similar divergence from the pattern of previous interglacials, this time starting about five thousand years ago. He attributed that to the development and spread of irrigated rice farming, methane being produced by drowned vegetation. The comparison between concentration in the current interglacial and in previous interglacials is shown for CO2 in Figure A, for CH4 in figure B.
Comparison of Holocene trends (red) to stacked averages for previous interglaciations (blue), from Ruddiman et al. (2016) (A) Benthic d18O stack from Lisiecki and Raymo (2005) and B). Dome C dD stack from Jouzel et al. (2003).[1]
His conclusion:
This comparison thus suggests that a glaciation should have begun several thousand years ago in northeast Canada. Early anthropogenic emissions of CO2 and CH4 are the most likely reason that it did not. (Ruddiman 2003, p. 288[2])
Ruddiman’s paper set off a controversy that is still running. Inputs included archaeological evidence on the size of early populations, pollen evidence for the extent of forests, climate modeling, isotope ratio evidence for the source of atmospheric CO2 , and much else. Alternative explanations for the pattern were rejected by supporters of the hypothesis on the grounds that they would have applied to earlier interglacials. Interested readers will find a summary as of 2020 in the review article by Ruddiman et. al. Its updated version of the hypothesis has the CO2 rise starting about seven thousand years ago and includes a variety of complications, none of which change the essential features of the conjecture.
The evidence from MIS 19 [the previous interglacial that most closely mimics the conditions for the current one] suggests that human interference in the operation of the climate system by greenhouse-gas emissions during the Holocene kept ice from accumulating in north-polar regions. The late Holocene was a time in which interglacial warmth persisted only because of early farming. (Ruddiman et. al. 2020)
[1] This is Figure 9 from W.F. Ruddiman, F. He, S.J. Vavrus, J.E. Kutzbach, “The early anthropogenic hypothesis: A review,” Quaternary Science Reviews, Volume 240, 2020, p. 8.
[2] William F. Ruddiman, The Anthropogenic Greenhouse Era Began Thousands Of Years Ago, Climatic Change 61: 261–293, 2003.
