Gradual falls and sharp rises in temperature for millions of years have profoundly affected living conditions on the planet and, consequently, our own evolution.
Physics Today 73, 5, 48 (2020); https://doi.org/10.1063/PT.3.4474
Milutin Milanković, a brilliant Serbian mathematician and climatologist, postulated in 1941 that variations in Earth’s orbit could push the planet’s climate in or out of an ice age.1 Vital to that idea is the amount of insolation—incoming solar radiation—at 65° N, a bit south of the Arctic Circle. At that latitude, insolation can vary seasonally by 25%. Milanković argued that reductions in summer insolation allow some winter ice to survive. Each year for thousands of years, ice accumulates around 65° N and eventually forms sheets large enough to trigger an ice age.
Three scientists joined forces 30 years later to verify Milanković’s theory using deep-sea sediment cores collected by the international Ocean Drilling Program. James Hays examined marine microfossils in the cores to estimate past sea-surface temperatures. Nicholas Shackleton measured the oxygen isotope composition in the sediment’s layers, which showed changes in past global ice volume. And the last member of the team, John Imbrie, brought an expertise in time-series analysis to the project. In 1976 they published a seminal paper showing that their climate record contained the same temporal cycles as three parameters, summarized in figure 1, that describe Earth’s orbit: eccentricity, obliquity, and precession.2
Eccentricity describes the shape of Earth’s orbit around the Sun. As Earth experiences a gravitational force from Jupiter, its orbit adjusts during a 96 000-year period from nearly a perfect circle to an ellipse, which causes minor variations in total insolation. Obliquity—the tilt of Earth’s axis of rotation with respect to the plane of its orbit—fluctuates during a period of 41 000 years between 21.8˚ and 24.4˚ and is currently at 23.4˚. A larger obliquity generates a greater difference in the insolation Earth receives during summer and winter.
The third orbital parameter, precession, occurs every 21 700 years and influences Earth’s closest approach to the Sun. During each hemisphere’s summer, precession has the greatest effect in the tropics. Tidal forces of the Sun and Moon, amplified by Earth’s oblate spheroid shape, cause one component of precession. Those forces exert gyroscopic motion on the planet that changes the orientation of its rotational axis. The second component of precession moves Earth’s entire orbit around the Sun in space and resembles the petals of a flower, as shown in figure 1c.
The great ice ages
Over the past 2.5 million years, Earth has undergone some 50 major ice ages and each has substantially changed the planet’s climate.3 During the last one 21 000 years ago, a nearly continuous ice sheet spanned North America. At its thickest, across what is now Hudson Bay, it was more than two miles deep and reached as far south as New York City and Cincinnati, Ohio. The British–Irish ice sheet spread as far south as Norfolk, and the Scandinavian ice sheet extended from Norway to the Ural Mountains in Russia. In the Southern Hemisphere, large ice sheets covered Patagonia, South Africa, southern Australia, and New Zealand. So much water was locked in all those ice sheets that global sea level dropped 120 m, yet if all the Antarctic and Greenland ice melted today, sea level would rise only by 70 m.
How did small wobbles in Earth’s orbit cause those ice ages? Summer temperatures must first decrease a little bit. The consequent accumulation of snow and ice increases Earth’s albedo—the reflection of sunlight to space. Reflecting more sunlight suppresses local temperatures and promotes more snow and ice accumulation, which increases the albedo further. The process, called an ice–albedo feedback, is responsible for building increasingly bigger ice sheets.
Another positive feedback cycle triggers when ice sheets, such as the Laurentide sheet that once covered much of North America, become big enough to deflect atmospheric planetary waves. The change redirects storm paths across the North Atlantic Ocean and prevents the Gulf Stream and its northeastward arm, the North Atlantic Drift, from penetrating as far north as they do today. The surface ocean effects, combined with melt-water increase in the Nordic Seas and the Atlantic, cause a decrease in the sinking of cold, salty water (see Physics Today, April 2019, page 19). As less water in the North Atlantic is driven to the deep ocean, the Gulf Stream pulls less warm water northward, and increased cooling in the Northern Hemisphere expands the ice sheets.
Greenhouse gases (GHGs) in the atmosphere reinforce ice-sheet feedbacks. Analyses of air bubbles trapped in polar ice indicate that during glacial periods carbon dioxide concentrations dropped by a third and methane by half. Changes in GHGs always precede variations in global temperatures and are therefore a clear driving force of climate change, not a response to it.4
Runaway positive feedbacks froze most of Earth’s water billions of years ago during snowball Earth events, but moisture limitation has prevented a more recent episode. Forming an ice sheet requires a cold, wet climate. But as an ice sheet forces warm surface water farther south, the supply of moisture decreases. By changing the atmosphere and ocean circulation, ice sheets starve themselves of moisture, and that negative feedback loop limits the effects of positive ones.
For the past million years, ice sheets have taken at least 80 000 years to reach their maximum extent, which occurred most recently about 21 000 years ago. However, ice melts much quicker than that: 80% of expanded ice sheets can be lost in 4000 years. Summer sunshine at 65° N triggers deglaciation and starts the melting of Northern Hemisphere ice sheets. Rising concentrations of carbon dioxide and methane in the atmosphere promote climate change and further melt large continental ice sheets. Such processes work against the ice–albedo effect, which acts to keep the ice sheets intact by producing a cooler microclimate.
Ultimately, rising sea levels diminish large ice sheets because the coldest that seawater can be is −1.8 ˚C, whereas the temperature of the ice sheet’s base is −30 ˚C. As seawater melts the ice sheets by undercutting them, ice calves into the ocean. The calving raises sea level further and causes more undercutting (see Physics Today, October 2019, page 14). The sea-level feedback mechanism can be extremely rapid. Once the ice sheets are retreating, the other feedback mechanisms—albedo, atmospheric and ocean circulation, and GHGs—are reversed. That’s why glaciologists and climatologists worry about future climate change: It will activate those feedbacks and cause irreversible instability to the Greenland and West Antarctic ice sheets (see Physics Today, July 2014, page 10).
The eccentricity myth
The last million years of glacial–interglacial cycles, each lasting about 100 000 years, have a saw-toothed pattern with a long period of cooling followed by a short, warm one of rapid melting. More than a million years ago, the cycles were smoother, and each lasted only 41 000 years, as shown in figure 2. That period corresponds to the length of the orbital change associated with obliquity, which controls the heat transfer between low and high latitudes and thus regulates ice growth.
Figure 2. Many glacial–interglacial cycles (red solid line) during the last 5 million years can be seen from measurements of the oxygen isotope composition of lake records. Large ice sheets started to grow in North America 2.5 million years ago during the intensification of Northern Hemisphere glaciation (iNHG). The development of the atmospheric Walker Circulation (DWC) started 1.7 million years ago in the Pacific Ocean and is sustained by a large east-to-west sea-surface-temperature gradient. About 1 million years ago, during the Mid-Pleistocene Revolution (MPR), the polar ice caps expanded toward the equator, and the glacial–interglacial cycle period increased from an average of 41 000 years to 100 000 years. (Adapted from ref. 3.)
For many years, scientists struggled to explain the 100 000-year glacial–interglacial cycles because the 96 000-year eccentricity mechanism has a similar length. But eccentricity is by far the weakest of the orbital variations, and many thought it predominantly modulated precession, so scientists suggested several nonlinear feedbacks to explain the discrepancy. But they found an answer when they realized that the 100 000-year cycle is a statistical artifact.
The average length of the last eight cycles is indeed 100 000 years, but each one varies from 80 000 to 120 000 years. Every fourth or fifth precessional cycle is weak enough that ice sheets can grow bigger and thus more vulnerable to sea-level rise during deglaciation. The next precessional cycle is always much stronger than the previous one and initiates rapid, extreme deglaciation through the sea-level feedback.5 Although the timing of deglaciation seems to better match precession, some researchers have argued that the long glacial–interglacial cycles may correspond to every second or third obliquity cycle.6