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.
10 comments:
I haven't found these arguments to be convincing because they don't account for natural systems. For example, there have always been huge areas of swamp, which do exactly what ice fields are cited for doing. Seasonal floods of large rivers such as the Amazon and Nile also created conditions much like a rice paddie. There are many other examples from peat bogs to beaver ponds that fit as well.
People did cut down and/or burn forests but they also planted trees. Much of the supposed virgin forest of the Amazon has been found to be a created landscape of trees planted for their utility to people. There are similar findings in Africa.
The scale of these natural swampy areas and created forests dwarf the area of Asia where rice farming was practiced.
The timing of climate change is suggestive of human causes, but the explanations have been weak at best. A similar pattern has been cited for more recent climate change. Major cooling seems to follow the introduction of Eurasian germs into the Americas. It is argued that so many people died that land rewilded and that the atmosphere was affected by the vegetative growth, changing the composition and also the planetary albedo.
A complication for causal explanation is that the data is both sparse and speculative. Just so stories.
David, is your argument that anthropogenic climate change in the past saved us from a glaciation so the current far larger and global anthropogenic climate change is saving us from?
From what?
If Ruddiman is right, past anthropogenic warming of about a degree prevented the beginning of a glaciation, so further warming — we are already a degree+ past what he was describing — is probably not needed for the purpose. I've been using the possibility that warming was preventing a glaciation as an example of the sort of very high cost very low probability consequence of preventing warming that gets ignored when similar possibilities on the other side are including in calculations, evidence of bias in the way Nordhaus and others try to estimate net cost of warming. It sounds from this as though that is the sort of consequence people should have been thinking about but, assuming Ruddiman is correct, one they should have concluded wasn't going to happen.
This is quite interesting to me. I have considered the possibility of warming preventing the onset of the next ice age especially, as you mention, as a counterpoint to catastrophic warming scenarios. The more you include extreme possible warming events the more you should think about and also include the possibility of prevented cooling events if you are thinking in a balanced manner
Similarly, warming should overall increase evaporation and thus rainfall and then there's the increased efficiency of water use by plants in a higher CO2 environment. This should probably lead in combination to a relatively high confidence prediction of reduced droughts. I've never heard it considered by non-skeptics and therefore presume that the science is overall unbalanced, much less the media representation of the science
But that a prevented or delayed ice age is already a theory about past human activity I would never have imagined.
@Rob:
I looked at the drought issue in an earlier blog post:
http://daviddfriedman.blogspot.com/2021/10/ipcc-6-drought.html
The IPCC expects more rainfall but that's mainly put in terms of more heavy rains and risks of flood, not in terms of reducing the risk of drought. The fact that warming increases evaporation means the soil losing water faster, which may be the reason for the drought result in the latest IPCC report. How much heavy rainfall helps should depend in part on the existence of reservoirs to store the water in.
David, you’ve unwittingly provided proof of the strong anthropogenic theory - that even small actions by us can have dramatic impacts on the climate. What do you suppose the impact of the 40% increase in CO2 that we’ve caused in the last < 200 years?
@Anonymous:
I'm not sure I would count cutting down a large part of the planet's forests as a small action.
So far as your question, one article I was reading yesterday says we are now safe from another glaciation for at least fifty thousand years.
Compared to what we're doing now, it's small.
We were safe from a glaciation for millennia to come anyway. Funny how 50,000 years from now is more concerning than the next 50.
If Ruddiman is correct, absent the early anthropogenic warming the glaciation would probably have started by now, although not yet have proceeded very far.
Regarding the next 50 years a 2018 IPCC report projected a 2.6% economic cost to unabated warming in 2100 and the climate economic models project 3.6%, and this is in reference to a world economy projected to grow by 100-600% by that time. Taking action to reduce CO2 is smart if we do it in a smart way, but if we assume there's a catastrophe looming we will end up spending world war like levels of resources on inefficient policy
For instance a back of the envelope calculation on the Paris Accord gives it at most a 2-1 benefit - cost ratio and that's with the assumptions that the correct social cost of carbon is the highest in the range given on wikipedia and that policy becomes twice as efficient as it has been historically. The range goes down to 11 cents benefit to dollar of cost
If we do a combination of reasonable carbon taxes and innovation focused policy rather than short term hard emissions cuts policy we will not only save resources but have a much greater effect on CO2 emissions through the century. I know of one projection for direct funding for innovation giving a 10-1 benefit cost ratio going by similar assumptions that get you 11 cents for the Paris Accord
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