So was it the farmers or the factories?

I thought it would be useful to summarise the key points from the evidence and findings in my previous posts:

• The Earth’s climate is driven by orbital forcing, changes in the tilt and orbit of the earth around the sun, which causes predictable changes to climate over the long term because of the variation caused in the amount of sunlight falling on the planet’s surface
• These climactic variations may be detected using methane and carbon captured in polar ice as proxies
• The changes in atmospheric methane and carbon concentrations since the last Ice Age do not follow the predicted model; both would be expected to fall as temperature falls towards the end of the interglacial period but they do not
• Atmospheric carbon starts to rise about 8,000 years BP and methane starts to rise about 5,000 years BP; both gases are now at concentrations significantly above the expected levels and are continuing to rise
• The Neolithic adoption of farming led to widespread forest clearances, which can raise atmospheric carbon levels, and the domestication of animals and the introduction of irrigation, both of which can raise atmospheric methane levels
• Determining the rate at which agriculture spread is necessary as part of establishing a causative link with the anomalous atmospheric gas concentrations
• Agriculture does not only impact the atmosphere; it also changes soils, leaving a permanent geological marker the dates for which tracks its expansion
• Geological events, such as oceanic slides, are potential sources of atmospheric methane and carbon
• Agriculture causes soils and sediments to be transported downslope, creating recognisable geological markers and potentially releasing carbon to the atmosphere

The global impact of humans on the environment clearly started before industrialisation. The markers left by agriculture are widespread, geologically detectable and securely datable. The evidence pulls the start of the Anthropocene well before the inception of industrialisation and the dependence on carbon fuels.

So, to answer the question, the evidence does support the proposition that:

The Anthropocene was started by farmers



Neolithic farmers processing manure (BBC, 2013)

Whether the start of the Anthropocene can be dated to 5000 BP or earlier is not so clear. Ruddiman and other researchers are building a compelling case but there are many alternative factors which might have caused short-term changes in atmospheric methane and carbon and which might have either contributed to the anomaly or caused a tipping point to be reached leading to long-term effects.

 

It is possible that some climate-affecting geological events could be unknown or opaque. For example, the outflowing of Lake Agassiz is associated with initiating the Younger Dryas stadial, but there is disagreement as to whether this huge body of cold fresh water flowed east (Broeker et al., 1989; Hostetler et al., 2000) or west (Murton et al., 2010) so models of the long-term effects on climate have a large margin of uncertainty.

 

We have established a layered structure of processes affecting atmospheric carbon and methane:

• Orbital forcing - a relatively predictable and long term symphony of cycles that span glaciations
• Geological forcing - unpredictable events that do not recur between glaciation cycles, including earthquakes, sub-oceanic slides, bolide impacts, volcanoes, etc.
• Agricultural forcing - the anthropogenic effect that emerged and expanded during the Holocene
• Industrial forcing - the anthropogenic effect that emerged and expanded during the last two centuries

All of these forces have been shown to drive environmental change. The problem for environmental scientists is in disentangling their relative effects.