Buy the Book: Ice Age details the history of the discovery of the cycle of ice ages. It covers the people behind it and what causes the Earth to cycle from extended periods of a cooler ice age to a warmer interglacial and back again.
The Notes
- The book does not cover the current situation of man-made climate change and how it might impact the cycle of ice ages.
- In the billions of years of the Earth’s history, polar ice caps are rare. In fact, polar ice caps have only been around for about 10 million years or a fraction of 1% of the Earth’s existence.
- Ocean currents are more important to climate than global weather because currents carry warm water from the equator to the poles. The poles can only freeze if warm water flow is blocked or diverted by landmasses.
- For instance:
- The Gulf Stream, running north from the equator along the US coast, is diverted by Canada and Greenland toward Britain. The product of that diversion is a warming of roughly 6 degrees Celsius of Britain and its neighbors rather than warming the Arctic Ocean.
- Without the diversion, the Gulf Stream would warm the Arctic, melt the northern icecap, but northwestern Europe would cool.
- Melting ice caps would add fresh water to the oceans, lowering its salinity, making it less dense, and possibly slowing, stopping, or reversing the ocean currents.
- A reversal of the ocean currents would flip the climate in most areas of the world. For example, Europe would experience cooler temperatures closer to what Vancouver experiences now. While Vancouver would have weather closer to what Ireland gets today.
- “Consider that the rate at which heat is being transported northward by the Gulf Stream today is more than a million billion watts, as if the current were a real conveyor belt covered with a million million one-bar electric fires, all pouring their heat out into the atmosphere.”
- “Because the real world is much more complicated than the computer models, nobody knows how close the real world is to being changed by such a climatic flip, but the message to absorb is that, thanks to positive feedback, small changes in the heat balance of the Earth can have big effects on climate.”
- We currently live in a warm part of an Ice Age (ice epoch). Small changes in the heat of the planet can push it into a full-blown Ice Age.
- Uniformitarianism — first promoted by James Hutton in the 1790s. It’s the belief that the Earth was shaped by the process of wind, weather, volcano, etc. over millions and billions of years. As opposed to huge catastrophes over shorter periods (catastrophism).
- “On a planet now known to be more than four billion years old, the occasional Ice Age is part of the routine.”
- Louis Agassiz was the first to promote the idea of, and use the term, “Ice Age” in 1837 after concluding that huge glaciers were the reason why giant boulders appeared randomly on the landscape. His idea was not well received.
- Astronomical Theory of Ice Ages
- The idea was first published in 1842 by mathematician Joseph Adhemar. Though his model was flawed, he was the first to correctly tie the cycle of ice ages to the orbit of the earth.
- The Earth orbits around the Sun in an ellipse, with the sun at one focus of the ellipse. So the Earth is closer to the Sun at one end of the orbit than it is at the other end. The change in distance impacts the amount of heat falling on the surface of the Earth. How this affects weather and climate depends on how much the Earth is tilted. The planet leans slightly from vertical — currently 23.5 degrees (the angle also changes, in a range — up or down — of about 3 degrees). This orbit produces the changing seasons throughout the year.
- The Earth also wobbles like a top wobbles as it spins. It’s caused by the gravitational pull of both the Sun and Moon on the Earth.
- “The wobble means that the direction in which the North Pole points traces a circle around the sky once every 26,000 years… In terms of the seasons, the effect of this wobble is to make the dates on which the equinoxes occur shift slowly, in step, around the Earth’s orbit. The effect is slightly counterbalanced by another effect, in which the whole orbit of the Earth around the Sun is shifting around the Sun, so that each ellipse traced by the orbit is rotated slightly, pivoting on the focus occupied by the Sun, compared with the previous ellipse (the effect is caused by the gravitational pull of the other planets). Overall, the effect is called the precession of the equinoxes, and it takes roughly 22,000 years to complete one cycle.”
- “The heat balance of each hemisphere of the Earth doesn’t depend on how many hours of darkness and daylight there are, but on how much heat is absorbed by each square meter of the surface during those hours of daylight, over the course of an entire year.”
- Another piece was added to the theory, thanks to the discovery by French astronomer Urbain Leverrier. The Earth’s orbit is always changing from more elliptical to more circular and back. The pattern repeats at roughly 100,000-year intervals.
- Leverrier’s discovery included a simple formula to calculate the eccentricity of the Earth’s orbit in the past or future. In 1875, James Croll published the results after using the formula to plot the Earth’s orbit 3 million years in the past and 1 million years into the future. He found periods where eccentricity was high for 10,000 to 20,000 years followed by longer periods of low eccentricity.
- “When the orbital eccentricity is low, and the orbit is circular, the amount of heat received by the whole planet from the Sun each week is the same throughout the year; but when the orbit is more elliptical, with high eccentricity, the Earth receives more heat in a week at one end of its orbit, closest to the Sun, and correspondingly less heat, in a week at the other end of its orbit, farthest from the Sun.”
- So a more elliptical orbit creates a wider difference in the seasons depending on which hemisphere you live in than a circular orbit.
- Croll was the first to argue that a feedback loop was needed to build an Ice Age. He was also the first person to recognize the importance of ocean currents on climate and how trade winds are linked to the currents.
- Lastly, Croll predicted correctly that the tilt of the Earth’s axis actually varies over time and affects the intensity of the seasons. Less tilt means less heat received by the poles each year, increasing the likelihood of an Ice Age.
- TLDR: precession, eccentricity, and tilt of the Earth’s orbit factor into the cycle of Ice Ages.
- Milankovitch Model
- Milutin Milankovitch was a Serbian who built off what Adhemar and Croll proposed to create his own model for Ice Ages that could calculate the changes in the amount of heat hitting anywhere on the Earth’s surface at any time. The goal was to measure changes in heat down to square meters. His work was published in 1920.
- Milankovitch used calculations done by Ludwig Pilgrim which detailed how the Earth’s orbit — precession, eccentricity, tilt — changed over the past million years.
- Early results showed “the changes in orbital eccentricity and the precession of the equinoxes can produce effects on the balance of heat at different latitudes big enough to influence the size of the ice sheets on Earth.”
- Wladimir Koppen, in exchanges with Milankovitch, figured out that summer was the key season for an Ice Age to emerge. Ice is more likely to spread if summer is less warm than usual. Reduced warmth in the summer means less winter snow melts. Which starts a feedback loop. If less snow melts in the summer than falls in the winter, then ice sheets grow. Growing ice sheets reflect more heat off the planet’s surface, compounding the process.
- “He worked out in detail the three periodic variations in climate associated with the astronomical influences and identified the geographic regions where they had the greatest influence — the roughly 100,000 year long rhythm associated with the changing ellipticity of the Earth’s orbit, the influence with a rhythm some 41,000 years long associated with the changing tilt (particularly important at high latitudes), and the rhythms about 22,000 years long associated with the precession cycle (less important at the poles, but more important at low latitudes). The combination produces an ever-changing pattern of seasonal heating of different regions of the globe, which Milankovitch could use to calculate how the ice sheets would advance and retreat in response.”
- The Key: “It isn’t so much that Ice Ages occur when the astronomical influences conspire to produce particularly cool summers; rather, what matters is that Interglacials only occur when the astronomical influences conspire to produce unusually warm summers, encouraging the ice to retreat. Without all three of the astronomical rhythms working in step in this way, the Earth stays in a deep freeze. And that is why the actual pattern of climate over the past few million years has been one of long Ice Ages (in fact, a single long Ice Epoch) interrupted by short-lived Interglacials, like the one we are living in now.”
- The Milankovitch Model would fall by the wayside until new dating techniques emerged in the 1960s to verify his measurements. Multiple dating techniques would be used to prove his model correct. A paper on the findings — “Variations in the Earth’s Orbit: Pacemaker of the Ice Ages” — published in Science in 1976 confirmed it.
- “The fact that sea level had been higher at certain times in the past than it is today had long been known — on his voyage round the world on board HMS Beagle, Charles Darwin, who was a geologist before he made a name as a naturalist, was one of several such pioneers who noticed raised beaches, several meters above the present day sea level in different parts of the world… And the obvious cause for changes in sea level is the advance and retreat of ice sheets on land — when more water is locked up in ice, the sea level falls; but when the ice sheets melt, sea level rises.”
- A team from Columbia University dated raised beaches on islands in the Atlantic and Pacific oceans some 120,000 years ago, which were 6 meters higher than today. They found similar evidence about 80,000 years ago.
- “The geological evidence, now well-dated…, shows that during the 60 million years or so from the death of the dinosaurs to the beginning of the present Ice Epoch the temperature of our planet experienced a slow, somewhat erratic decline, as a result of the way the continents were shifting about on the surface of the globe, altering the flow of ocean currents and the way sunlight was absorbed and reflected back into space.”
- Geological Epochs:
- Miocene — 24 million years ago to 5 million years ago.
- Pliocene — 5 million years ago to 1.8 million years ago.
- Pleistocene — 1.8 million years to 10,000 years ago.
- Holocene — began about 10,000 years ago at the beginning of the current interglacial period (also tied to the growth of the human species).
- The Earth was ice-free for 10s of millions of years. Sometime around 13 million years ago, ice began forming in East Antarctica. About 10 million years ago, small glaciers began to form in the mountains of Alaska. Around 6 million years ago, South America and Australia moved away from Antarctica, unobstructed ocean currents around the south pole, and locked Antarctica in a deep freeze.
- About 3 million years ago, South America moved northward, created the land bridge between North America, and diverted ocean currents northward similar to what they are today. The first glaciation around the north pole began about 3.6 million years ago.
- “During an Ice Age, with lowered temperatures there is less evaporation from the oceans, and therefore less rainfall… This means that with the present geography of the globe when Europe experiences an Ice Age, East Africa experiences a drought.”
- Now imagine cooling and warming, ice sheets growing and receding, over 10s of thousands of years, repeating dozens of times, and the impact that might have on the selection process in the evolution of animals and plants around the planet.
