Forget Global Warming, The Ice Age Is Coming!
Scientists have long suspected that the orbital cycles of our planet are responsible for the periodic climate variation that causes alternating glacial and interglacial periods. Milankovitch's theory of orbital cycles suggests that summer insolation at high northern latitudes drives the glacial cycles. Moreover, statistical analyses have demonstrated that the glacial cycles are indeed linked to eccentricity, obliquity and precession. Now, researchers have confirmed that a combination of two of the Milankovitch cycles conspire to start and stop ice ages. The 100,000-year eccentricity cycle amplifies the influence of the 23,000-year wobble of Earth's spin axis called precession. The new modeling also suggests that the great accumulation of mass by the North American ice sheet causes the abrupt end of glacial periods. CO2 is involved, but is not determinative, in the evolution of the 100,000-year glacial cycles say the scientists.
The striking correlation between Earth's orbital cycles and the waxing and waning of ice ages was first noticed by James Croll, a self-taught, Victorian era, amateur scientist. The inquisitive Scot was the first scientist to attempt to prove the link between Earth's orbit and ice ages (see The Resilient Earth, Chapter 9: “Variations In Earth's Orbit”). In 1875, Croll published his major work on the linkage between Earth's orbital variations and climate, Climate and Time in their geological Relations: A Theory of Secular Changes of the Earth's Climate. Unfortunately, a number of the calculations in that paper proved inaccurate, leading to Croll's theory being dismissed by mainstream science.
The scientist who revived Croll's theory was Milutin Milankovitch, a Serb mathematician and engineer whose initial fame was due to concrete. Eventually, he became a professor at the University of Belgrade, giving up his much higher paying job as an engineer. Milankovitch was a man driven to succeed at whatever he worked on, and he was soon looking around for problems where he could apply his considerable mathematical skills. After reading Croll's work on orbital variation and climate, Milankovitch decided to investigate the causes of climate change. Among the key contributions of Milankovitch's work were the ideas that different latitudes experienced the impact of orbital variation differently, and that the key to the onset of glaciation is cool summer weather, not colder winters as had previously been assumed.
Milankovitch publish his definitive book on climate, Cannon of Insolation and the Ice Age Problem, printed in German in 1941. For a time, this resurgent theory of climate change found wide acceptance, but it was not to last. New dating techniques and geological data began to uncover discrepancies in Milankovitch's predictions. Around the time of his death, in 1957, Milankovitch's theory was out of favor for much the same reasons as Croll's earlier work. But the story does not end there.
In 1970, Wally Broecker and J. van Donk published a paper that detailed temperature changes going back 400,000 years. In this paper, a number of the apparent discrepancies in Milankovitch's theory were resolved. Though he didn't live to see his theory vindicated, Milankovitch's astronomical theory of climate change is now recognized as the best explanation of the cycles of glacial-interglacial change. In his honor, these periodic changes in Earth's orbital orientation are called the Milankovitch cycles.
The only problem is, the slight change in insolation—the energy received from the Sun—caused by the orbital changes is not sufficient to explain the drastic swings in climate. Quoting Shawn J. Marshall, from the University of Calgary's Department of Geography: “The crux of the challenge in modeling glacial cycles is that Earth's response to orbital forcing is entirely out of proportion.” Now, writing in Nature, a group of scientists claim they have discovered the mechanism that links orbital change to climate change. A research letter, “Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume,” reports that a team of researchers led by Ayako Abe-Ouchi, from The University of Tokyo's Atmosphere and Ocean Research Institute, have constructed a new model that reproduces the observed glacial-interglacial cycles. Here is the paper's abstract:
The growth and reduction of Northern Hemisphere ice sheets over the past million years is dominated by an approximately 100,000-year periodicity and a sawtooth pattern (gradual growth and fast termination). Milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles, and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity, obliquity and precession cycles. Yet insolation alone cannot explain the strong 100,000-year cycle, suggesting that internal climatic feedbacks may also be at work. Earlier conceptual models, for example, showed that glacial terminations are associated with the build-up of Northern Hemisphere ‘excess ice’, but the physical mechanisms underpinning the 100,000-year cycle remain unclear. Here we show, using comprehensive climate and ice-sheet models, that insolation and internal feedbacks between the climate, the ice sheets and the lithosphere–asthenosphere system explain the 100,000-year periodicity. The responses of equilibrium states of ice sheets to summer insolation show hysteresis, with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles. The hysteresis loop of the North American ice sheet is such that after inception of the ice sheet, its mass balance remains mostly positive through several precession cycles, whose amplitudes decrease towards an eccentricity minimum. The larger the ice sheet grows and extends towards lower latitudes, the smaller is the insolation required to make the mass balance negative. Therefore, once a large ice sheet is established, a moderate increase in insolation is sufficient to trigger a negative mass balance, leading to an almost complete retreat of the ice sheet within several thousand years. This fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound, which is the lithosphere–asthenosphere response. Carbon dioxide is involved, but is not determinative, in the evolution of the 100,000-year glacial cycles.
Ayako Abe-Ouchi and her colleagues indirectly linked a full-blown global climate model (GCM) with a fairly complex model of the northern ice sheets. They drove the climate model with data on the changing distribution of sunlight driven by orbital variations and the swings in atmospheric CO2 as recorded in ice cores. Then they used the output of the GCM to drive the ice sheet model. When compared with the historical behavior of real ice sheets, the combined models performed well.
The simulated ice sheets grew in the same places and at similar rates as the real ones. Over tens of thousands of years, they slowly expanded to volumes as large as seen in the past. This is consistent with the observation that glacial periods manifest gradually. Then, roughly every 100,000 years, they collapsed in a matter of a few thousand years, also consistent with the paleoclimate record. Quoted in Science, paleoclimatologist Maureen Raymo of the Lamont-Doherty Earth Observatory in Palisades, New York, an author on the Nature paper, said “it's doing it all on its own” without contrived climate forcings or grossly simplistic processes in the model. Their results are captured in the figure below.
The upper maps (a) show the equilibrium shapes and surface mass balances of ice sheets when the summer temperature relative to present conditions are respectively (left to right) −2, −1, 0 and 1 K, starting the model runs from large initial ice sheets. Colors indicate the surface mass balance in meters per year. In (b), modeled equilibrium and transient ice volumes as functions of the summer temperature for the North American (left) and Eurasian (right) ice sheets are shown: red dots denote the large-volume equilibrium states if the model runs start from large initial ice sheets, blue dots show the small-volume equilibrium states for small initial ice sheets. The blue areas indicate a positive total mass balance of the ice sheet, red areas indicate a negative total mass balance. The black dots mark the evolution of the transient ice volume every 2 kyr for the last glacial cycle starting 122 kyr before present. The horizontal scales below the figures show the relation between the temperature anomaly and the corresponding insolation at latitude 65° N on 21 June for two given constant atmospheric CO2 concentrations (220 p.p.m. and 280 p.p.m.). Panel (c) same as b but data shown as time series for the past two glacial cycles.
“Our model realistically simulates the sawtooth characteristic of glacial cycles, the timing of the terminations and the amplitude of the Northern Hemisphere ice-volume variations as well as their geographical patterns at the Last Glacial Maximum and the subsequent deglaciation,” the researchers state. “In the frequency domain, our model produces the largest spectral peak at a periodicity of ~100 kyr, as observed in the data, even without the ocean feedback or dust feedback.”
The correlation between the orbital forcings and calculated ice-volume change, expressed as sea-level equivalent, can be seen in the figure below.
Shown in a, insolation forcing (insolation at latitude 65° N on 21 June) with variations in eccentricity, obliquity and precession (black lines); with obliquity fixed at 23.5° (red lines); with eccentricity fixed at 0.02 (blue lines); and with perihelion passage fixed at the spring equinox and no precession (green lines). b, Corresponding spectra of insolation change in a. c, Calculated ice-volume change, expressed as sea-level equivalent (colors same as in a). d, Corresponding spectra of calculated ice-volume change in c.
The researchers also did modeling runs to gauge the impact of CO2 levels. The concluded that “carbon dioxide is involved, but is not determinative, in the evolution of the 100,000-year glacial cycles.” Moreover:
“CO2 variations can result in amplification of the full magnitude of ice-volume changes during the ~100-kyr cycles, but do not drive the cycles. Ice-sheet changes may induce variations in CO2 through changing sea surface temperature, affecting the solubility of CO2, and through changing sea level, affecting the stratification of and CO2 storage in the Southern Ocean. During deglaciation, the melt water may affect ocean circulation, leading to an increase in atmospheric CO2.”
In other words, the glacial-interglacial cycles can drive change in CO2 levels, something widely suspected. Carbon dioxide levels may have played a role in the shift from 41kyr and 100kyr glacial cycles, which happened ~900kyrs ago, but they do not drive the cycles themselves. The bottom line on this is that human carbon dioxide emissions will not stop the onset of the next glacial cycle—like it or not, Earth is going to return to the Ice Age. Here is how Abe-Ouch et al. summarized their findings:
A remarkable conclusion from our model results is therefore that the 100-kyr glacial cycle exists only because of the unique geographic and climatological setting of the North American ice sheet with respect to received insolation. Only for the North American ice sheet is the upper hysteresis branch moderately inclined; that is, there is a gradual change between large and small equilibrium ice-sheet volumes over a large range of insolation forcings. For this reason the amplitude modulation of summer insolation variation in the precessional cycle, due primarily to eccentricity, is able to generate the 100-kyr cycles with large amplitude, gradual growth and rapid terminations.
Other scientists have reviewed the work by Abe-Ouch et al. positively. “This is the best so far, a really nice advance, because they're using more comprehensive models,” says paleoclimate modeler David Pollard of Pennsylvania State University. All the researchers in this field naturally long for bigger, faster computers to run even bigger models on. Why should we care? Again quoting Shawn Marshall: “About 40 such glacial cycles have shaped our planet over the past 2.6 million years (the Quaternary period), representing the most dramatic example of climate variability in Earth's recent history.” This is real climate change.
The good news is that nature has things under control. We see the hubris that some scientists exhibit, claiming that humanity is driving Earth's climate to ruin. All of the calamities predicted by climate change alarmists pale compared with the ravages of a glacial period. CO2 emissions or no CO2 emissions, the next ice age is coming, and it is keeping nature's schedule not mankind's. When that time comes, humanity may well try increasing the greenhouse gases in the atmosphere to keep much of the northern hemisphere from being buried by kilometers of ice. Fittingly, the names of those who cried wolf over anthropogenic global warming will have long been forgotten.Be safe, enjoy the interglacial and stay skeptical.
“It's about time that warm spell ended.”