Tying celestial mechanics to Earth’s ice ages Mark Maslin Citation: Physics Today 73, 5, 48 (2020); doi: 10.1063/PT.3.4474 View online: https://doi.org/10.1063/PT.3.4474 View Table of Contents: https://physicstoday.scitation.org/toc/pto/73/5 Published by the American Institute of Physics ARTICLES YOU MAY BE INTERESTED IN Negative carbon dioxide emissions Physics Today 73, 44 (2020); https://doi.org/10.1063/PT.3.4389 Gravitational-lensing measurements push Hubble-constant discrepancy past 5σ Physics Today 73, 14 (2020); https://doi.org/10.1063/PT.3.4424 John Wheeler’s H-bomb blues Physics Today 72, 42 (2019); https://doi.org/10.1063/PT.3.4364 Between complacency and panic Physics Today 73, 40 (2020); https://doi.org/10.1063/PT.3.4453 The speedy secret of shark skin Physics Today 73, 58 (2020); https://doi.org/10.1063/PT.3.4460 Particle beams behind physics discoveries Physics Today 73, 32 (2020); https://doi.org/10.1063/PT.3.4452 Uganda‘s Margherita glacier, in the Rwenzori Mountains, provides the Nile River with some of its water. (Morgan Trimble/Alamy Stock Photo.) 48 PHYSICS TODAY | MAY 2020 Mark Maslin is a professor of Earth system science in the department of geography at University College London. Tying celestial mechanics to Earth’s ice ages Gradual falls and sharp rises in temperature for millions of years have profoundly affected living conditions on the planet and, consequently, our own evolution. Mark Maslin ilutin 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 Msolar 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. MAY 2020 | PHYSICS TODAY 49 EARTH’S ICE AGES FIGURE 1. MILANKOVITCH CYCLES comprise three types years between 21.8˚ and 24.4˚ and is a Eccentricity of variation in Earth’s motion. Eccentricity (a) describes the currently at 23.4˚. A larger obliquity shape of Earth’s orbit around the Sun, which varies from generates a greater difference in the nearly circular to more elliptical with a period of about insolation Earth receives during 96000 years. Obliquity (b) is the tilt of Earth’s axis of rotation summer and winter. 96 000 with respect to the plane of its orbit and oscillates with a The third orbital parameter, pre- years (c) period of some 41000 years. Precession consists of the cession, occurs every 21700 years spin of Earth’s rotational axis and its orbital path over time; b Obliquity and influences Earth’s closest ap- the combined effects of those two components produce proach to the Sun. During each an approximately 21000-year cycle. (Adapted from ref. 3.) hemisphere’s summer, precession 41 000 has the greatest effect in the tropics. years Three scientists joined Tidal forces of the Sun and Moon, amplified by Earth’s oblate c forces 30 years later to ver- spheroid shape, cause one component of precession. Those Precession ify Milanković’s theory forces exert gyroscopic motion on the planet that changes the using deep-sea sediment orientation of its rotational axis. The second component of pre- N cores collected by the in- cession moves Earth’s entire orbit around the Sun in space and ternational Ocean Drilling resembles the petals of a flower, as shown in figure 1c. S 21 700 Program. James Hays ex- years amined marine microfos- The great ice ages sils in the cores to estimate Over the past 2.5 million years, Earth has undergone some 50 past sea-surface temperatures. Nicholas Shackleton measured major ice ages and each has substantially changed the planet’s the oxygen isotope composition in the sediment’s layers, which climate.3 During the last one 21000 years ago, a nearly contin- showed changes in past global ice volume. And the last mem- uous ice sheet spanned North America. At its thickest, across ber of the team, John Imbrie, brought an expertise in time-series what is now Hudson Bay, it was more than two miles deep and analysis to the project. In 1976 they published a seminal paper reached as far south as New York City and Cincinnati, Ohio. showing that their climate record contained the same temporal The British–Irish ice sheet spread as far south as Norfolk, and cycles as three parameters, summarized in figure 1, that de- the Scandinavian ice sheet extended from Norway to the Ural scribe Earth’s orbit: eccentricity, obliquity, and precession.2 Mountains in Russia. In the Southern Hemisphere, large ice Eccentricity describes the shape of Earth’s orbit around the sheets covered Patagonia, South Africa, southern Australia, and Sun. As Earth experiences a gravitational force from Jupiter, its New Zealand. So much water was locked in all those ice sheets orbit adjusts during a 96000-year period from nearly a perfect that global sea level dropped 120 m, yet if all the Antarctic and circle to an ellipse, which causes minor variations in total inso- Greenland ice melted today, sea level would rise only by 70 m. lation. Obliquity—the tilt of Earth’s axis of rotation with respect How did small wobbles in Earth’s orbit cause those ice ages? to the plane of its orbit—fluctuates during a period of 41000 Summer temperatures must first decrease a little bit. The con- 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 Antarctic Northern Hemisphere Maximum volume million years ago, during the Mid- ice cap ice caps of ice caps Pleistocene Revolution (MPR), the polar ice caps expanded toward the equator, and the glacial–interglacial cycle period increased from an average of 41000 years to 100000 years. (Adapted from ref. 3.) Pliocene Pleistocene 2 41-kyr cycles 100-kyr cycles 3 O (parts per thousand) 18 4 δ iNHG DWC MPR 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 TIME (million years ago) 50 PHYSICS TODAY | MAY 2020 a 0.06 0.04 sequent accumulation of snow and ice in- creases Earth’s albedo—the reflection of 0.02 sunlight to space. Reflecting more sun- 0 light suppresses local temperatures and promotes more snow and ice accumula- −0.02 PRECESSION tion, which increases the albedo further. −0.04 The process, called an ice–albedo feed- −0.06 back, is responsible for building increas- ingly bigger ice sheets. b 5 Shallow Another positive feedback cycle trig- Deep gers when ice sheets, such as the Lauren- 4 tide sheet that once covered much of 3 North America, become big enough to deflect atmospheric planetary waves. 2 The change redirects storm paths across the North Atlantic Ocean and prevents LAKE BASINS 1 the Gulf Stream and its northeastward 0 arm, the North Atlantic Drift, from pen- etrating as far north as they do today. The c Eurasian Homo sapiens East African surface ocean effects, combined with H. neanderthalensis South African H. heidelbergensis melt-water increase in the Nordic Seas H. antecessor and the Atlantic, cause a decrease in the H. erectus H. ergaster H. floresiensis sinking of cold, salty water (see PHYSICS H. rudolfensis TODAY, April 2019, page 19). As less water Homo habilis H. denisovan Australopithecus garhi in the North Atlantic is driven to the deep Australopithecus afarensis Ardipithecus H. naledi ocean, the Gulf Stream pulls less warm ramidus A. africanus TRANSITIONS Australopithecus P. robustus water northward, and increased cooling anamensis P. aethiopicus Paranthropus boisei in the Northern Hemisphere expands the HOMININ EVOLUTION ice sheets. Greenhouse gases (GHGs) in the at- 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 mosphere reinforce ice-sheet feedbacks. TIME (million years ago) Analyses of air bubbles trapped in polar ice indicate that during glacial periods carbon dioxide concentra- FIGURE 3. HUMAN EVOLUTION is connected to African climate tions dropped by a third and methane by half. Changes in GHGs change. Large variations in the precession of Earth’s orbit (a) help always precede variations in global temperatures and are there- determine the size and number of deep freshwater lakes in the East fore a clear driving force of climate change, not a response to it.4 African Rift Valley (b). Those lakes, in turn, can be linked to major Runaway positive feedbacks froze most of Earth’s water bil- evolutionary changes in early humans (c). The red dotted line marks lions of years ago during snowball Earth events, but moisture the first dispersal of hominins throughout Africa; solid red lines limitation has prevented a more recent episode. Forming an ice identify the times when hominins dispersed from Africa to Europe sheet requires a cold, wet climate. But as an ice sheet forces and Asia. (Adapted from ref. 8.) warm surface water farther south, the supply of moisture de- creases.