Claim: The domestication of rice and clearing of forests beginning near the dawn of civilization likely saved us from an Ice Age.

What the Science Says: This claim is due to paleoclimatologist William Ruddiman, whose book "Plows, Plagues, and Petroleum: How Humans Took Control of Climate", popularized this idea. Denialists picked up on this as re-inforcing the meme that "Man is GOOD for climate". Past ice ages are fairly strongly supported to be due to small changes in the obliquity and tilt of the Earth's axis, and the changing ellipticity of the Earth's orbit - the Milankovitch cycles. These are calculatable with computers with high accuracy. The theory in original form is due to Milankovitch. The strongest correlation with the timing of ice ages comes from the amount of incoming solar energy at the 65N latitude, roughly the Arctic circle. The reason is that this latitude is most sensitive to the gain or loss of ice where it can accumulate mostly on land, and because of ice's ability to self-refrigerate by reflecting sunlight. Ruddiman's idea is significantly flawed, as we see below...

In past Milankovitch cycles, warming induced the release from the oceans and permafrost of CO2, which then added to further warming. Notice that we currently are in an unusually long period when the Milankovich cycles conspire to prevent an Ice Age from happening again. If one assumes the insolation must drop to levels consistent with previous Ice Ages, it may be another 50,000 years and perhaps 130,000 years from now before the next Ice Age.

Support for the Milankovitch theory of Ice Ages comes well-supported reasoning together with the significant correlation between global temperatures and the insolation at the Arctic Circle. However, it is not perfect. Note the lack of interglacials during two insolation cycles ~160,000 years ago. During the last 8,000 years we have not seen the (moderate) cooling that Milankovitch theory predicted. Human causes certainly dominate lately, and perhaps had some influence as early as a few thousand years ago as well.

 

Zooming in - during the past 10,000 years insolation has been dropping to current (moderate) levels, a local minimum, where it will remain roughly constant for the next 10,000 years before rising again.

The periodic Ice Ages of the past half million years resulting from the Milankovic cycles. Note too that based on past interglacial time periods, we have many thousands of years to wait for another Ice Age.

Milankovitch cycles show no new Ice Age for another ~50,000 years (NOAA source) . (NOAA = National Oceanic and Atmospheric Administration. The principle federal science agency on ocean and atmospheric science)

Here is a discussion with Ruddiman on his idea, and Ruddiman's eventual concession that early man's agricultural influence can only account for about 1/3 of claimed Ice-Age avoiding warming. In this post by Ruddiman, you'll see that his hypothesis was refuted by Siegenthaler et al. 2005 and by Spahni et al. 2005. Ruddiman disputes these papers because they aligned data to a past interglaciation when insolation values were at a local maximum, and he says it should instead be aligned with a local minimum since we are at a local minimum in insolation today. This complaint by Ruddiman makes no sense, since it is from 8,000 years ago going forward that he claims early humans prevented an ice age by the development of agriculture, and 8,000 years ago did indeed correspond to a time of local insolation maximum, as the Milankovitch cycle insolation graph above shows. Also, in searching this and other discussions, I am surprised to see no one pointing out that the absolute level of insolation in his favored analog period of 400,000 years ago is only 460 watts/square meter, a very large 20 watts/square meter less than current values, and much lower still than the ~520 watts/square meter which prevailed 8,000 years ago. The paleoclimate record indicates it would seem extremely unlikely than an ice age could be triggered at a time of such high insolation. His idea seems to assume that only the direction of changing insolation matters in inducing an Ice Age, not the absolute amount of heating. Ruddiman's hypothesis was tested recently using one of the many sophisticated climate computer models we have designed - the GENESIS climate model, and found that dropping CO2 and CH4 levels to Ruddiman's estimated non-human-caused levels, and see a 2 deg C temperature drop, which is only 1/3 of the typical temperature drop associated with ice ages, and Ruddiman concedes this. So what was Ruddiman's basis for the dramatic "Ice Age" claim? I have to admit it certainly did make the hypothesis a lurid attention-getter for his popular-level book "Plows, Plagues, and Petroleum: How Humans Took Control of Climate". Note also that we are now no longer in a realm of rapidly dropping Milankovich-induced cooling, and too that CO2 levels now are far higher, due to fossil fuel burning in the last 60 years, than at any time in the past ~800,000 years.

The rapid rise in global temperatures in the last century is the human-caused global warming issue, and therefore Ruddiman's hypothesis is also seen to be a red herring in this regard. What's really interesting is how denialists have latched onto Ruddiman's hypothesis as somehow helping their cause. They couldn't be more wrong. A great article with links on this bizarre logic is here.

The Little Ice Age has several contributing causes; it is not primarily solar. Robock (1979) finds that enhanced volcanism during this period provides the best fit to the data, while fitting to solar acitivity gives a very poor correlation. Also, the Black Plague de-populated and re-forested Europe and the Columbian Exchange (disease from European colonization of the New World) together caused significant de-populating of the Earth, and the resultant re-forestation of the landscape acted as an efficient remover of CO2 (by 10ppm = 3% by observations) from the atmosphere. In any case, we have accurate satellite measurements of insolation for several complete solar cycles now and therefore have a good idea of how to translate observed sunspot numbers and cosmic ray evidence into insolation estimates for past centuries. The result: even another dramatic Maunder Minimum would not induce another ice age, even a Little one (Fuelner and Rahmstorf 2010).

We clearly can't run an experiment on the entire Earth itself from 8000 years ago to today in order to rule out Ruddiman's hypothesis that man-made activity prevented a new Ice Age completely - but the weight of evidence is clear: it doesn't stand up..

In Short: Human activities almost certainly did keep temperatures a bit higher than they would have been, given the dropping insolation over the past 8000 years, but even without human activities, the evidence does not indicate we would have entered a new Ice Age. Regardless, it's amazing that denialists would try and use this issue to argue against doing something about human-caused global warming today. I can only guess that it's another attempt to link "human actions = good for climate" in the public mind, however poorly reasoned.

 

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