A new paper just published in Nature has made a bit of a stir because it has been interpreted as suggesting that global warming has the benefit of avoidance of an Ice Age that was just about to happen. However, the paper does not actually say that, and we already knew that we may have avoided the next ice age, possibly by human activities dating back to the 19th century or before. Also, the paper actually addresses a different question, an important one, but one that may be a bit esoteric for may interested parties.
First, the esoteric question. Simply put, over the last two million years or so, the Earth has gone through a couple of dozen cycles that have ice ages at one end and very warm periods (such as the one we were in in the 19th century) at the other end. The first several cycles were modest, but the most recent have been extreme, with the cold periods involving the growth of major continental glaciers big enough, for example, to cover most of Canada and a chunk of the US. The current warm period, enhanced by anthropogenic global warming, is probably already warmer than the previous really warm periods, and over the next couple of decades will certainly be what has been called a “super-interglacial” with temperatures consistently being above anything during this entire glacial-interglacial cycle.
This cycling of climate is linked to a cycle of how much of the Sun’s energy falls on the earth, when, and where. The simple version of this arises from the fact that land masses, where continental glaciers can form, are concentrated in the Northern Hemisphere. Continental glaciers have their own cooling system (by being bright and reflecting away sunlight, mainly) so once they form they tend to be self sustaining. But it is difficult for then to form to begin with because, well, the Earth is usually too warm. But, if Northern Hemisphere summers are chilled down sufficiently several years in a row, these glaciers can start to form, and this can be part of the onset of a new glacier.
Current and recommended books on climate change.
The Earth wobbles as it rotates. The elliptical orbit of the Earth around the Sun varies in how elongated it is. The location of the Earth on this elliptical orbit during a particular moment in the seasonal round changes over time (so every now and then the solstice, for instance, happens when the Earth is maximally distant from the sun). These three factors change in a regular cycle over different time periods. Every now and then all three factors cause the following thing to be true: late June, the longest and thus sunniest period of the year in the Northern Hemisphere, is also the time when the Earth is farthest from the sun on an elliptical orbit that is as elongated as it ever gets, but the Earth has wobbled up so that the Northern Hemisphere is not as pointed towards the Sun as it could be. When this happens, Northern Hemisphere summers have a minimal amount of the Sun’s energy.
But that difference is probably not enough to start an ice age, and the opposite times, when the Northern Hemisphere’s summers are maximally sunlight, are probably not enough warmer than other periods to kill off an ice age.
During the 1970s and early 80s, the cycles of Sun’s energy variation caused by these orbital quirks were reconsidered (it was a 19th century observation) and correlated with recently obtained isotope data from sea cores indicating glacial cycles. They matched. More NH summer extra sunlight happened during interglacials pinned down by the isotopic data, and NH summer reduced sunlight matched in time with the glacial periods. But, over subsequent years, research tended to show that the changes in sunlight and glacial activity did not correlate exactly. Rather, other causes of the onset or melting of glaciers seemed to be other things.
Over time we have come to realize that the orbital effects, known as Milankovitch Cycles, probably determine the potential for the Earth to be in a glacial period vs. an interglacial period, but other factors actually push the climate system into these new states.
This is like so many other things in nature. You have the right genes to develop perfect pitch, but that does not make you a musician. Growing up in an environment that would encourage one to be a musician is not sufficient to make you a great musician. Having perfect pitch and a music-friendly environment and a few other things, all together in for the same person, might create a David Bowie. Or not. But given millions of people, there will be hundreds of great musicians, and most of them will have most of these factors in place.
The current research is a study that relates atmospheric CO2 changes and Milankovitch changes, and it may be an important contribution to understanding this complex system. I’ve not thought about the paper enough to say this (or not say it), but that is what the paper is about.
Meanwhile, years ago, back in the late 1960s and through the 1970s, paleoclimate experts like John Imbrie and JM Mitchell and others pointed out that greenhouse gasses would likely bring on a “super interglacial” that would obviate an ice age that might otherwise occur very soon. They also noted that after thousands of years following the burning of the last available fossil fuel, or the curtailment of this insane practice, the CO2 added to the atmosphere would likely cycle back into solid form, and the next time orbital geometry matched up with other stuff, we could have our ice age again.
More recently, Bill Ruddiman looked at human activities in recent history and suggested that land clearing practices associated with agriculture, and the early burning of fossil fuels, was sufficient to put off an ice age.
Today we know that the cycling in and out of Ice Ages over the last million years or so is associated with atmospheric CO2 levels well within the range of 200ppm to 300ppm. So, I would guess that once we passed around 300ppm we left the likelihood of an ice age behind. Indeed, it is possible that had we not done that, we might have eventually figured out that we should do it, to avoid an ice age.
But enough is enough. The fact that you like your hamburger cooked does not mean that therefore you should cook it at 10,000 degrees C for a year. You cook it the right amount. More than that ruins it. We might benefit from “cooking” the Earth just a little bit to avoid an ice age (and yes, we do want to avoid an ice age), but we don’t want to overcook the Earth. We passed annual an average CO2 concentration of 400ppm a few months ago. The hamburger, and our goose, is being overcooked.
One outcome of the new research is to suggest that without human perturbation of the climate, we would have skipped this ice age anyway. This assertion is the reason I’m reserving judgement on this paper. I wonder if all the appropriate factors have been taken into account, because I find this assertion difficult to believe. But, I’m not going to make an argument based on incredulity. I’ll just note my incredulity, as someone who has studied Pleistocene climate change, and consider getting back to you on this at a later time.
The paper further suggests that current burning of CO2 will extend that period of time to the next Ice age by double, and that “Our simulations demonstrate that under natural conditions alone the Earth system would be expected to remain in the present delicately balanced interglacial climate state, steering clear of both large-scale glaciation of the Northern Hemisphere and its complete deglaciation, for an unusually long time.”
So, when media report that this study suggests that anthropogenic global warming has put off an ice age, they are talking about shifting a 50,000 year delay to the next ice age (without human effects) to a 100,000 year delay. This would be a new idea, because we were thinking that we had put off an ice age that was just about to happen (over the next centuries). So, the paper actually says nearly the exact opposite of what the press says it says. How could this happen? Can’t imagine…
Ganopolski, A. R. Winkelmann,& H. J. Schellnhuber. 2016. Critical insolation–CO2 relation for diagnosing past and future glacial inception. Nature 529, 200–203 (14 January 2016) doi:10.1038/nature16494.