Glacial Inception Delayed By Humans?
Mixed in with the swirling cloud of misinformation about global warming is also a bit of blather regarding the onset of a new Ice Age. What the promoters of a new big freeze are talking about is a glacial period, since technically we are still in the midst of an ice age—the Pleistocene. A number of scientists have warned that the planet might be headed for a new period of glacial growth based on Earth's recent history. Over the past half million or so years there has been a series of short (15-20k year) warm period sandwiched between longer (70-100k year) glacial episodes. Make no mistake, global cooling is much worse than global warming, so this really matters to our descendants and the whole human race. Now, a new study in Nature says that we maybe off the hook, glacial wise. Not only that, human activity might be the reason.
Some years ago, back in the 1970s, many scientists came to believe that Earth was headed into a new glacial period. Since we had been enjoying the warmth of the Holocene warm period for almost 12,000 years they had historical precedent to back this conjecture. The cyclic nature of warm and cold periods during our current ice age is attributed primarily to changes in Earth's orbital parameters, variations known collectively as the Milankovitch Cycles. I have explained these cycles and the historical climate variations they drive in detail in past posts and will not repeat the descriptions here. What is important to understand is that there have been several interglacials in the past when orbital conditions were quite similar to today's, and that at least one of these interglacials was abnormally long.
People have been modeling the glacial/interglacial cycles for decades, looking at paleoclimate proxy data for a way to predict the future. Among the factors measured has been, not unexpectedly, atmospheric CO2. In a paper in the journal Nature, Ganopolski et al. report modeling studies confirming that we would now be entering an ice age if the concentration had remained at 240 parts per million (ppm). In an accompanying news article, “Climate science: Earth's narrow escape from a big freeze,” Michel Crucifix, from the Université Catholique de Louvain Earth and Life Institute, provides some background information.
In accordance with classical Milankovitch theory, interglacials—warm intervals with the lowest global ice volume—occur during periods of high summer insolation in the boreal latitudes of the Northern Hemisphere. In the past, a decrease in Northern Hemisphere insolation to below its present-day level always led to the end of interglacials and rapid growth of continental ice sheets, accompanied by a reduction in CO2 concentration. However, at present, although summer insolation at 65° N is close to its minimum, there is no evidence for the beginning of a new ice age. On the contrary, sea level, which reflects changes in global ice volume, remained essentially constant over the past several millennia.
Note that there is always a natural reduction in atmospheric CO2 concentration when the planet cools. Conversely, the amount of carbon dioxide always rises during warm periods. This doesn't mean that CO2 is driving Earth's climate but that it varies naturally. However, it probably does play a part in climate regulation, particularly at low concentrations. It is also important to note, for those not familiar with this area of research, that past glacials and interglacials are identified using isotopic data from marine sediments. This has led to a numbering scheme in which isotope “stages” with odd numbers roughly correspond to interglacials. As Crucifix explains: “The authors paid special attention to the glacial inceptions after marine isotope stages 19 and 11, and to the period after marine isotope stage 1 (that is, the Holocene), because insolation evolved in a similar way at those times but led to different outcomes (stage 1 did not produce a glacial inception).”
These comments were prompted by a research report in the same issue of Nature that offers up more insights into the onset of glaciation. Keeping Crucifix's comments in mind, here is the abstract from the actual paper, “Critical insolation–CO2 relation for diagnosing past and future glacial inception,” by Ganopolski et al.:
The past rapid growth of Northern Hemisphere continental ice sheets, which terminated warm and stable climate periods, is generally attributed to reduced summer insolation in boreal latitudes. Yet such summer insolation is near to its minimum at present4, and there are no signs of a new ice age. This challenges our understanding of the mechanisms driving glacial cycles and our ability to predict the next glacial inception. Here we propose a critical functional relationship between boreal summer insolation and global carbon dioxide (CO2) concentration, which explains the beginning of the past eight glacial cycles and might anticipate future periods of glacial inception. Using an ensemble of simulations generated by an Earth system model of intermediate complexity constrained by palaeoclimatic data, we suggest that glacial inception was narrowly missed before the beginning of the Industrial Revolution. The missed inception can be accounted for by the combined effect of relatively high late-Holocene CO2 concentrations and the low orbital eccentricity of the Earth7. Additionally, our analysis suggests that even in the absence of human perturbations no substantial build-up of ice sheets would occur within the next several thousand years and that the current interglacial would probably last for another 50,000 years. However, moderate anthropogenic cumulative CO2 emissions of 1,000 to 1,500 gigatonnes of carbon will postpone the next glacial inception by at least 100,000 years. 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.
I added the emphasis on that last sentence because it represents the take home conclusion from this paper—it is possible that there will be no new glacial period for a long, long time (in human terms). Conversely, there is no expectation that existing glaciers will disappear altogether, ending the Pleistocene Ice Age. But before discussing the significance of the author's findings let's look a some of the details. If we are not at the start of a new glacial period, so called glacial inception, just what is going on? As the authors state:
The most straightforward explanation for the lack of glacial inception at present is that the current insolation minimum is not deep enough because of the low orbital eccentricity of the Earth. However, glacial inceptions have occurred in the past under similar orbital configurations. Marine Isotope Stage (MIS) 11 (about 400,000 years before present, 400 kyr BP) is often considered a close palaeo-analogue for the current interglacial (the Holocene, or MIS1) owing to the similarly low values of the eccentricity of Earth’s orbit and similar CO2 level at that time7 (Fig. 1). The only difference between the insolation minimum at about 400 kyr BP and the present one is a lower obliquity during MIS11.
The figure referred to in the paper is reproduced below. It shows a comparison of Earth’s orbital parameters and CO2 concentrations for MIS1 (green), MIS11 (blue) and MIS19 (black). The vertical dashed line corresponds to the present day for MIS1 and the minima of the precessional component of insolation for MIS11 and MIS19.
What the author's did, based on these data, was to run a number of simulations with an Earth system model of intermediate complexity called CLIMBER-2, which includes the three-dimensional thermomechanical ice sheet model SICOPOLIS15. For more information on the model see “CLIMBER-2: a climate system model of intermediate complexity,” by V. Petoukhov et al. The CLIMBER-2 is a relatively old model, created in 2000, and has been successfully applied for simulating the last eight glacial cycles.
From many simulation runs, tweaking parameters and comparing results, the researchers mapped the threshold value of CO2 leading to glacial inception depending on the maximum summer insolation at 65° N. The results are displayed in Figure 3 shown below.
As the author's explain: “Figure 3a shows that the individual points in the insolation–CO2 space representing different combinations of orbital parameters are clustered around the logarithmic curve. This is consistent with the fact that radiative forcing of CO2 is proportional to the logarithm of CO2 concentration and that in the CLIMBER-2 model, similar to many other climate models, the temperature response to CO2 and orbital forcing is linear within the considered range of CO2 concentrations. The critical summer insolation at 65° N can be described as S = αln([CO2]/280) + β, where α = −77 W m−2 and β = 466 W m−2 and [CO2] is the concentration of CO2 in parts per million.”
Finally the authors conclude that, with current CO2 levels, the planet is not going to slip into a new glacial cycle:
Under three scenarios with cumulative emissions of 500 gigatonnes of carbon (Gt C), 1,000 Gt C and 1,500 Gt C, we simulate the ice volume on the Northern Hemisphere for the next 100,000 years. Even for a total of 500 Gt C cumulative emissions, which is only slightly above the present-day value, the evolution of the Northern Hemisphere ice sheets is affected over tens of thousands of years (Fig. 4). In the 1,000 Gt C scenario, the probability of glacial inception during the next 100,000 years is notably reduced, and under cumulative emissions of 1,500 Gt C, glacial inception is very unlikely within the entire 100,000 years. This confirms our conclusions from the critical insolation threshold for glacial inception. Because all 2013 Intergovernmental Panel on Climate Change scenarios—except Representative Concentration Pathway 2.6 (RCP2.6), which leads to the total radiative forcing of greenhouse gases of 2.6 W m−2 in 2100—imply that cumulative carbon emission will exceed 1,000 Gt in the twenty-first century, our results suggest that anthropogenic interference will make the initiation of the next ice age impossible over a time period comparable to the duration of previous glacial cycles.
Again I have added emphasis on the last sentence. Basically Ganopolski and colleagues say that keeping atmospheric CO2 above 280 ppm would drag the Holocene warm period out to 50,000 years or more. What's more, given the current level of human greenhouse gas emissions, we may even avoid a glacial for 100,000 years. If this is true then we should all celebrate! Indeed, if human emissions are keeping glacial inception at bay then we need to keep on keeping on—drive those SUVs because they are all that stand between humanity and global catastrophe.
Other researchers say the prolonged interglacial may well have been in the cards without human help. The atmospheric level of carbon dioxide just before the Industrial Revolution was 280 ppm, meaning a repeat of MIS11. It should also be noted that MIS11 lasted 28,000 years according to a paper by EPIC researchers that appeared in Nature in 2004. Others claim that Preindustrial Age climate may already have been affected by emissions associated with human activities, namely widespread agriculture. But then, there were no farms 780,000 years ago and MIS19 took place with similar CO2 levels.
Regardless, this does not mean that the future will be uniformly warm or cold. As we have seen over the span of the Holocene, temperatures have been both higher and colder than today. Even a new period of cold similar to the Little Ice Age might not signal the onset of full glacial expansion. Nor does a return to the warmth of the Holocene Climate Optimum or Medieval Warm Period mean we are in for a global meltdown. Undoubtedly scientists will continue to argue for both accelerated warming and a new ice age while the climate varies as it always has.
Of course, this is not the only possible interpretation of the data. The derived model may well have it ass backwards—it may be Earth's climate machinery that is driving CO2 and not the other way around. It certainly seems that has been the case for more than a million years. Claiming rising CO2 concentrations cause a warming climate may be analogous to claiming that spring flowers force the onset of summer warmth.
This is a good point to remind my fellow scientists of the wise words of Richard Feynman regarding scientific theories. He said that it doesn't matter how elegant of wonderful your theory is, it nature disagrees with it your theory is wrong. The same is true of models—the proof always lies in nature.
So heed the words of Feynman and don't fall in love with your computer models, in science nature is the only truth. It could be that we will get a new glacial in spite of the extra CO2 we have been dumping into the atmosphere. Or we might bankrupt our whole species fighting GHG emissions only to have wonderful weather for the next fifty millennia. Only time will tell.
Be safe, enjoy the interglacial and stay skeptical.