The Early Anthropogenic Hypothesis

Before I continue any further, I feel I have to mention Ruddiman's 2003 paper in which he argues that the Anthropocene began much earlier than the Industrial Revolution: about 8000 years ago. I know it's not directly about population growth, but in my eyes his hypothesis shows what impact even a tiny, 'non-consumer' human population can have (and possibly did have) on the environment and its systems.

He compares the Holocene with previous interglacials and forms three main arguments:

1. That Earth-orbital-driven cyclic variations (phew!) in CO2 and CH4 (methane) during the past 350,000 years would predict a decline in those gas concentrations throughout the Holocene but they began anomalous increases, 8000 years ago for CO2 and 5000 years ago for CH4
2. Possible explanations for those rises based on natural forcing can be refuted by paleoclimatic evidence
3. Archaeological, cultural, historical and geological evidence leads to anthropogenic explanations linked to early agriculture in Eurasia (forest clearance 8000 years ago; rice irrigation 5000 years ago)

source: Ruddiman (2007)

He looks at CH4 concentrations in Vostok ice, which show that they followed the 23,000 year orbital insolation cycle. This supports the orbital-monsoon theory that higher CH4 concentrations had their root cause in orbital precession-dominated summer insolation changes which meant monsoons were more extreme and there was more flooding. However, 5000 years ago, the CH4 signal started to increase, departing from the continued decrease expected from orbital monsoon theory (see figure A above). This risks getting very dry and technical so I'm just going to say that by the industrial era the methane concentration 'should' have been 250 ppb lower than it actually was.

As for CO2 concentration, he compares with previous interglacials and finds that in the last 3, CO2 increase reached a maximum and then dropped steadily for around 10,000 years, whereas in the Holocene CO2 concentrations did peak and start to decline around 10,000 years ago, but then started to rise 8000 years ago (see figure B above). He goes on to prove his point through examining trends at each of the 3 major orbital cycles but I'm not going to go into all of that otherwise this will turn into an essay! If you fancy reading the article (which I recommend), just click on the link above.

He continues by refuting other possible explanations: that natural loss of terrestrial biomass or changes in ocean carbonate chemistry could explain the CO2 rises. They could explain part of the rise, but not nearly enough.

He then proposes his 'famous' idea: that pre-industrial land clearance and rice irrigation are responsible. He points to the domestication of horses and the invention of the ox-drawn plow 6000 years ago, and shows that the pollen sequences in central Europe younger than 6-5000 years are altered enough that pollen analysts view them as unrepresentative of 'natural vegetation'. There was also rapid sediment accumulation in central European lakes between 5000 and 3000 years ago – backing up the claim of forest clearance. Finally the brief periods of CO2 decline could be matched with periods of human pandemics like the bubonic plague. This also backs up the deforestation idea as killing off vast portions of the human population would have meant a halt (or at least significant slowing!) to human activities such as deforestation and as many 'infected' areas were abandoned – a regrowth of forests!

Ruddiman published a response article in 2007, where he addresses the (many) challenges his work has faced. However, he still finds the CO2 and CH4 gas trends anomalous, despite having reviewed the ice timescale used. He still finds that extreme biomass burning (forest clearing) and rice irrigation can explain the anomaly. However, only about 25% of the 'excess' CO2 could come from carbon from deforestation but he proposes that the climate system feedback is responsible e.g. the ocean remained warm because of anthropogenic intervention. Finally pandemics can explain only half the CO2 decreases, but they obviously still had some effect.

Although being precise about these claims is very difficult, there is no doubt that humans did have some effect on their environment, even when they did not possess the technology (or demand) for resource exploitation. It is clear then that humans today have an even greater impact on their environment, and there are many many more of them.