Thursday, October 28, 2021

How Humans Held Back the Glaciers

  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 o