Given that I’ve been talking a lot about how evidence works recently (what might convince me of time travel and something that still hasn’t convinced me of ghosts), I thought it might be fun to talk about something else that is, quite probably, unknowable. There is a hypothesis that human caused climate change started 10,000 years ago with the onset of agriculture. Can we believe that claim?
The hypothesis itself makes sense. Trees are what is known as a carbon sink; trees absorb carbon dioxide from the atmosphere. The pattern is simple: fewer trees, more naturally occurring carbon dioxide. More carbon dioxide does lead to an increase in temperature. So although the early deforestation wouldn’t have added carbon dioxide to the atmosphere, it is plausible that it led to an increase in the amount of carbon dioxide in the atmosphere. But the effect would be small. The effect would be incredibly small, in fact, because it is still up for debate whether most land-plants are effective carbon-sinks at all; because plants have to respire during the night they emit carbon dioxide through the night.
Assuming it is true, how could we know? It is nice to think we can do what the media often tells us we can do: derive global temperatures from somewhere and plot a graph and see what happens. But it is not that simple. Climate data is noisy and that means that little changes are difficult to see. When I say climate data is noisy I mean the data goes up and down over a very short time scale. Take, as an example, the picture I have taken from the Guardian online of the now iconic Hockey Stick Graph.
That pale blue colour, and then red colour after ~1900 is the noise I was talking about: statistical noise. In fact, even the wobble in the dark blue line is statistical noise. We do not need very powerful statistical methods to see that after ~1900 the graph goes up, but before 1900 we don’t really know what the trend is at all (in fact, the trend is often called “stable”, as in there is no trend). But that is due to industrial level deforestation and pollution. We can, in fact, see the inflection point at ~1900 with our eyes.
But the early anthropogenic hypothesis—that we started this process as early as 10,000 year ago—is not going to be as significant as the post-1900 spike we see in this graph. In fact even if it were true I posit to you that the evidence that we may have had is lost to the statistical noise of the past. The evidence—little fluctuations in the graph; move towards an up gradient—is consored by that normal and natural statistical noise, and therefore unknowable.
Evidence that this did happen, other than extrapolating what we know happens when there are fewer trees, does exist. It was first published by Ruddiman and is called the “early anthropocene hypothesis”. Approximately 8,000 years ago the predictable cyclical nature of the carbon dioxide and methane levels in the atmosphere stopped, coinciding with early agriculture. But that is barely enough to be evidence for correlation. But if true, it also leads to another hypothesis called “Overdue glaciationhypothesis”. But that is even harder to show: it is very difficult to show that earth was due another glaciation event, let alone that agriculture was enough by itself to stop it. There does seem to be a cyclical pattern of ice ages, so it’s not ridiculous to claim we were due one. But it’s not exactly well-established.
Statistical noise had made the claim that we affected the climate 8-10,000 years ago near-impossible to see and even if it’s true, we have to commit an extrapolation fallacy to believe it offset an ice age. Although both claims do seem reasonable, they are not knowable.