Nous somme du soleil
— Anderson/Howe, “Ritual”
It’s sad that it’s come to this, but I feel compelled to offer some guidance on the persistent allegation that the Earth is about to enter an ice age. It all started a few days ago, when Matt Drudge added a link to an English-language Pravda (?) story claiming that “a large and compelling body of evidence from within the field of climate science” points to the impending end of the current interglacial period. Never one to care what Drudge is linking to, I tried to ignore it. But then I started getting email.
The most depressing came from someone who was good enough to provide his real name, place of occupation and contact information, none of which I will share with you, except to note that he works in “ag weather forecasting and often get questions when I’m out at farm meetings regarding global warming” and that he and his employer seem to be legitimate. He wrote:
One of my contacts is a big fan of the Milankovitch cycles and says that these offer proof that global warming is not occurring. Do you have an opinion on the Milankovitch cycles and how they should be interpreted?
Given that the email arrived shortly after the Pravda story came to Drudge’s attention, it’s clear that we have a problem. What we need is a primer on Milankovitch cycles. So here goes.
Said cycles are regular changes in the Earth’s orbital characteristics. Specifically, eccentricity (the degree to which the orbit around the sun deviates from a perfect circle), obliquity (the tilt of the Earth’s axis relative to the orbital plane) and precession (or wobble of the tilt). Wikipedia gives a good overview, although all you really need to know is these cycles have different periods of between 19,000 and 100,000 years. Add them up and the effect is a change in “insolation,” or the mount of solar radiation that the Earth absorbs. This is because the Northern Hemisphere currently has most of the land.
Change the insolation factor, and the planetary ecosystem changes the amount of carbon dioxide in the atmosphere, which then changes the temperature of the planet. (Note that just because the change in CO2 levels follows the orbital forcing, we are still left with the fact that increasing CO2 levels leads to a warmer planet.)
As a result, at some points in the collective triple cycle, much of the higher latitudes of the Northern Hemisphere have tended to be covered with ice —;;;; as much as several kilometers thick in some parts. Hence, “ice ages.” In between those phases of the cycles, we get warmer periods called interglacials. The continents only move relatively slowly, and barring an encounter with a passing black hole, the Earth’s orbit isn’t about to change, so we’re stuck the the Milankovitch cycles for the foreseeable future.
Which means that, all other things being equal, we’re headed for another ice age. The only question is when. (Actually, the link between Milankovitch cycles and climate change is not cut and dried, and there are many outstanding questions, but for the purposes of our inquiry, we’ll leave it at that.) The Pravda story doesn’t say exactly when, but its author, Gregory F. Fegel, believes that
Today we are again at the peak, and near to the end, of a warm interglacial, and the earth is now due to enter the next Ice Age. If we are lucky, we may have a few years to prepare for it.
But is Fegel’s opinion shared by climatologists? It should come as no surprise, especially to anyone who has Googles the gentleman’s name, to discover that the answer is no. As usual, a little knowledge is a dangerous thing. If we delve deeper into our understanding, and lack thereof, of the Milankovitch-climate connection, we find that, among other things, not every interglacial lasts the same length of time. This makes sense because the three Milankovitch cycles don’t reach minima and maxima at the same time very often.
An excellent “Perspectives” paper in Science from 2002 lays out some of the issues involved. In An Exceptionally Long Interglacial Ahead? Berger and Loutre discuss the fact that, while back in the 1970s it was thought the last two interglacials were 10,000 years long, that’s not necessarily true. This is important, because, it’s been about 10,000 or 11,000 years since the last ice age ended, and if the current interglacial is scheduled to last only 10,000 years, then we’re in trouble.
But here’s what we know now:
Some assumptions made 30 years ago have since been questioned. Past interglacials may have been longer than originally assumed. Some, including marine isotope stage 11 (MIS-11, 400,000 years ago), may have been warmer than at present. We are also increasingly aware of the intensification of the greenhouse effect by human activities. But even without human perturbation, future climate may not develop as in past interglacials because the forcings and mechanisms that produced these earlier warm periods may have been quite different from today’s.
The small amplitude of future insolation variations is exceptional. One of the few past analogs occurred at about 400,000 years before the present, overlapping part of MIS-11. Then and now, very low eccentricity values coincided with the minima of the 400,000-year eccentricity cycle. Eccentricity will reach almost zero within the next 25,000 years, damping the variations of precession considerably.
Simulations with a two-dimensional climate model, forced with insolation and CO2 variations over the next 100,000 years, provide an insight into the possible consequences of this rare phenomenon. Most CO2 scenarios led to an exceptionally long interglacial from 5000 years before the present to 50,000 years from now … with the next glacial maximum in 100,000 years. Only for CO2 concentrations less than 220 ppmv was an early entrance into glaciation simulated.
As you probably know, the current CO2 concentration is about 385 ppm. But that brings us to the really critical issue, the one where all other things are not equal. Far from predicting an imminent ice age, what we know about Milankovitch cycles and their effect on climate actually leads to the conclusion that, even if the current interglacial doesn’t last several tens of thousands of years, humanity is on the brink of putting an end to the ice age cycle altogether. Berger and Loutre conclude that
Taking into account anthropogenic perturbations, we have studied further in which the CO2 concentration increases to up to 750 ppmv over the next 200 years, returning to natural levels by 1000 years from now. The results suggest that, under very small insolation variations, there is a threshold value of CO2 above which the Greenland Ice Sheet disappears. The climate system may take 50,000 years to assimilate the impacts of human activities during the early third millennium.
In this case, an “irreversible greenhouse effect” could become the most likely future climate.
NASA’s James Hansen came to the same conclusion a while back.
So the next time anyone suggests to you that that we’re headed for another ice age instead of an unbearably hot age, tell them to brush up on their Milankovitch cycles. And you can add that there never was a scientific consensus of an impending ice age. Just a single, one-page article in Newsweek.