Ice Ages: Solving the Mystery
Imbrie, John and Katherine Palmer Imbrie. Ice Ages: Solving the Mystery. 1986. Harvard University Press.
Review by Mike Dubrasich
In 1911 at a coffee house in Belgrade, Serbia, 32-year-old mathematics professor Milutin Milankovitch resolved to “grasp the entire Universe and spread light into its farthest corners”. The occasion was a party, and he was a little drunk. But he kept his word regardless.
It took a while, what with World War I and its aftermath, cruel depression and social disruptions in the Balkans, but in 1938 Milankovitch published “Astronomical Methods for Investigating Earth’s Historical Climate”. His theory, the Milankovitch Theory [here], was that variations in the Earth’s dance around the Sun caused fluctuations in the global climate of the Ice Ages, one such fluctuation being our current Holocene.
Milankovitch described and calculated three tiny astronomical fluctuations: in the shape of the orbit of the Earth around the Sun, in the degree of tilt of the Earth’s axis, and in the direction (into the cosmos) that the axis points toward.
The Earth orbits around the Sun in an elliptical path, but sometimes the ellipse is elongated, and sometimes it is nearly circular. When the orbit is most circular, the Earth receives the more solar radiation during the year than when the orbit is more elongated. This cosmic cycle, from oval to almost round and back to oval, takes 100,000 years.
The Earth’s axis currently tilts at an angle (to the plane of our orbit) by 23.5 degrees, but not always. The angle varies from 22 to 24.5 degrees, and back, over a 41,000 year period. The more the tilt, the more difference there is between summer and winter climates, especially in temperate and boreal regions.
The axis of the Earth’s spin currently points towards the Pole Star, but sometimes it points towards Vega. The direction of the axis slowly rotates, like a spinning top sometimes wobbles, and the imaginary line through the axis describes a circle (actually an ellipse) in the heavens over time. The tilt of the axis towards or away from the Sun causes the seasons. As the axis of tilt rotates, the dates of the equinoxes and solstices change, so this celestial movement is called the “precession of the seasons”. This movement takes place in a double cycle of 23,000 and 19,000 years.
When Milankovitch presented his Theory, it was received with skeptical curiosity. There was no way of dating fossils past 40,000 years or so, the limit of the carbon-14 method, and that method was still in its earliest development, so it was impossible to test his Theory.
By the 1970’s, however, researchers were extracting mud cores from ocean bottoms. The sediment cores were color-banded by different fossil planktons, each assemblage indicating ocean temperatures at the time of deposition. Some long cores dated all the way back to the Matuyama-Brunhes geomagnetic polarity reversal of 780,000 years ago. Thus the cores, and their bands, could be dated. The types of fossil plankton in each band, and later their oxygen isotope ratios, allowed researchers to estimate global temperatures back deep into the Pleistocene.
The researchers came up with a graph of global temperature verses paleo-time [here]. Lo and behold, the graph had fluctuations of 100,000 years, 41,000 years, and in faint signals, 23,000 and 19,000 years, matching the Milankovitch Theory predictions.
This kind of coincidence does not happen in Nature by random chance. The researchers, three in particular, had unlocked a fundamental key to paleoclimate, and indeed, to present and future climate, too. It was a scientific discovery akin to the unraveling of the structure of DNA. The three researchers are John Imbrie, Nicholas Shackleton, and James Hays. There is no Nobel Prize for geo-sciences, but Imbrie [here] and Shackleton [here] subsequently were awarded the Vetlesen Prize, the most worthy substitute. Shackleton was knighted, too!
John Imbrie, together with his daughter-in-law Katherine, wrote a bestselling book about all this, called Ice Ages: Solving the Mystery. It is one of the great scientific discovery books of our age, and a must read. It is better than Watson’s The Double Helix or Hawkings, A Brief History of Time. Your local library has it. So does Amazon. No excuses.
Milutin Milankovitch died in 1958, before his Theory was validated. He never won a prize, but the European Geophysical Society created one and named it after him. Not surprisingly, Imbrie and Shackleton have both been awarded the Milankovic Medal [here].
An additional, concluding comment:
The Earth has been getting colder for 55 million years. For the last 2.5 or so million years the Earth has been so cold that giant ice sheets spread across half the Northern Hemisphere, and reduced much of the rest to tundra and deserts. Our planet has not been this cold since the Permian Era of 250 million years ago.
The cause of the Ice Ages is the tectonic drift of Antarctica over the South Pole. During the Permian it was Pangaea that drifted over the South Pole. Continents over one or both poles prevent the circulation and mixing of polar and equatorial waters, and induce abnormal freeze ups, such as our current Ice Age.
We live in a warm period, a temporary interglacial. This is the 19th or 20th such warm period that has appeared regularly, like clockwork, throughout the Pleistocene. The warm periods reoccur with the same resonance as the astronomical variations in the Earth’s orbit studied by Malinkovitch. It is very unlikely that the two patterns are independent. The attendant mechanisms of earthly climate change are a little more complex, but the astronomical patterns still dominate the paleoclimatic record.
If past history is any guide, our interglacial (the Holocene) is nearly over. It has already outlasted previous interglacials. About 8,000 years ago Holocene global temperatures peaked and have been dropping unsteadily ever since [here]. The point of maximum circularity of the Earth’s orbit occurred 9,000 years ago. We have been getting more elliptical, and cooler, ever since.
The 9,000-year-long cooling trend led to the Little Ice Age of 1450-1850 AD. Medium and high latitude temperatures fell 2-4 degrees C below the Climatic Optimum of 8000 BP. Crops failed in many of the Little Ice Age years, and famines were common in Europe. (When global temperatures shift, the changes are felt most in the higher latitudes. Equatorial temperatures hardly have been affected by glacials and interglacials, while polar latitudes experienced 20 degree C swings.)
The Little Ice Age was the coldest period in the Holocene so far. Global temperatures dropped to a point unseen for over 11,000 years. In some respects, the Holocene ended then and a new 100,000 year glaciation began. We live in the Post-Holocene Era.
The cooling trend in the (late) Late Holocene paused after the Little Ice Age. Global temps have climbed nearly a degree C in the last 150 years. This pause in the cooling trend might have been due to increased atmospheric CO2 from anthropomorphic sources. Whatever caused it, we are lucky. If the cooling had continued, most of North America and northern Eurasia would be frozen wastelands today. Another 2-4 degree C drop would have destroyed most agriculture and forests on both continents. Extreme human suffering, environmental collapse, and mass extinctions were averted, temporarily.
Our recent temperature increase is only temporary, though. The Earth is moving into another Malinkovitch Cycle and another Ice Age glaciation is due. It is written in the stars.
Increased CO2 emissions may be Humanity’s only hope to save Planet Earth from icy doom. In prior epochs atmospheric CO2 concentrations were much higher, as much as 3500 ppm (parts per million) in the Cretaceous and 1000 ppm in the Eocene. If we double current CO2 levels and warm the Earth up 5 degrees C, we will still not be anywhere near the Eocene Optimum, when rainfall was abundant, forests were everywhere, and Life was Fat. Conversely, if we allow the planet to cool down even a degree or two, ice sheets and deserts will expand, and Life will be stretched Cold and Lean, again.
Warmer is Better. Fight the Ice. Life Depends on CO2
