This is the second of a multi-part series on climate change at different timescales. The first part dealt with drivers on tectonic scales — millions of years. This part deals with the primary drivers of climate change from hundreds of thousands to thousands of years. Future posts will include millennial and finally short-term (decadal/annual) scales. The earth’s geologic history is divided and subdivided into various eons, eras, and periods, each corresponding to intervals in the earth’s history that are recorded in rocks and the fossils they contain. The flora and fauna of these past periods lived in different climates than we do today– usually warmer, though occasionally colder than the average modern climate. In Part 1 of this series, I explained that we’re living in “icehouse earth” today, because of the permanent ice at the poles. Another major feature of our current period, The Quaternary (2.55 million years ago to today), is the regular cycle of ice ages and warm periods, cyclical swings in the climate system that have caused ice sheets in the northern hemisphere to expand and shrink more than a dozen times in the last two million years.
To grow an ice sheet, all you need to do is toggle the amount of incoming solar radiation (handily abbreviated “insolation”) that enters the top of the atmosphere. Snow and ice always accumulate during the northern hemisphere winters, when the north pole is titled away from the sun (tilt is the reason for seasons, as I’ll explain below). To grow an ice sheet, what you really need is a situation where you have more growth in the winter than you have melting in the summer. You’ll always have some growth in the winter, so ice age winters don’t necessarily need to be all that cold; you just need milder summers to make sure that all that ice accumulation doesn’t melt away. When conditions are favorable and you get ice growth, the accumulating snow and ice starts to deform under its own weight, literally “flowing” from the north. Eventually, the ice itself becomes a major force in the climate system. An ice sheet can be several miles thick at its center, which is high enough to displace the Jet Stream and change weather patterns. All that light-colored ice also creates a positive feedback loop, reflecting a significant portion of insolation back into space, where it can’t warm the earth.
Now that we know how to grow an ice sheet, the next thing we need to start an ice age is how to control that insolation. We know that the ice has advanced and retreated over the last two and a half million years, which means that there must be fluctuations in the amount of insolation hitting to the top of the atmosphere. It turns out that ice age cycles are caused by shifts in the earth’s tilt and orbit — that is, they’re astronomical in origin. There are three particular aspects of the earth’s orbit that, together, control the strength and timing of insolation: tilt, wobble, and the shape of our orbit.
Tilt/Obliquity: If you were to place a pin straight through the north and south poles, you’d see that the earth is tilted; the angle of tilt, or obliquity, is currently 23.5°, but it actually fluctuates from 22.2° and 24.5° over a 41,000-year cycle. It doesn’t seem like much, but it makes a big difference in the amount of seasonal insolation received by the earth. The earth’s tilt is the reason we have seasons — during the northern hemisphere winter, we’re tilted away from the sun, which makes our winter season colder. If we were tilted even further from the sun, we’d have even colder winters, and vice-versa. In other words, the main effect of the earth’s tilt is to amplify or suppress the seasonal differences, or seasonality. Eccentricity: The shape of the Earth’s orbit around the sun also changes on a regular cycle, going from more circular to more of an ellipse or oval. The degree of departure from a perfect circle, or “eccentricity” (more “eccentric” = more like an oval than a circle) naturally changes how close the earth is to the sun. There are two main cycles of this orbital parameter: 100,000 and 413,000 years. The latter is difficult to pick up, but tends to cause the 100,000 year cycles to cluster in periods of stronger or weaker amplitude.
Precession of Solstices and Equinoxes: Solstices are the days during our revolution around the sun when the days are longest (summer solistice) and shortest (winter solstice); equinoxes have equal day length, and occur in spring and fall. In the northern hemisphere, our summer and winter solstices are June and December 20th or 21st, and the equinoxes are March and September 20th or 21st. We think of these dates as relatively fixed, but in fact the timing of those dates within a year have also cycled through time. The timing of solstices and equinoxes shift position with respect to the eccentricity of our orbit (how circular it is), but also with respect to the timing at which the earth is closest to the sun in a given year (the perihelion) and furthest (the aphelion). Confused? The most important thing to remember is that the distance from the earth to the sun has varied with time during each of the seasons. If the sun is closer to the earth during summer, insolation is stronger. If we’re further from the earth during summer, insolation is weaker. Think of it as though the dates are taking a “march” around the sun, like a game of musical chairs; this “precession” in dates is caused by two kinds of motion in the earth’s orbit. The first is our axial precession: the earth has a long-term wobble, like a top. This changes the direction that the north pole is pointed to over time (it’s not always Polaris, the North Star). The second is the precession of the ellipse: the entire elliptical orbit is rotating slowly through time. Together (the wobbling of the axis and the slow turning of our elliptical orbit) combines to form the precession of the equinoxes, which is dominated by a 23,000 year cycle. Once we know these parameters, we can calcuate the amount of insolation entering the atmosphere at any point in time — including the future! In fact, the person who first described these cycles, Milutin Milanković, did exactly that. During his internment in a WWI prison camp, Milancović, a geophysicist and astronomer, worked out these orbital cycles and their impacts on insolation by hand. Decades later, the geologic record was used to validate Milancović’s predictions: when the earth’s insolation is at a minimum, we get ice ages. When it’s at a maximum in the cycle, we get warm interglacials. How do we know all this? The sediment record in the oceans goes back millions of years, and ice core records extend more than 400,000 years. By studying the oxygen isotope records from glacial ice and foraminifera (little single-celled critters who make shells with a fingerprint of the ocean’s chemistry when they were alive), we can reconstruct temperature and ice volume through time (it’s actually pretty complicated, but very cool).
It’s this very sedimentary record that allows us to study the past record of ice ages and warm periods. In the 2.55 million years, the earth is in an ice age more often than a warm period; we tend to think of our modern climate as the norm, when in reality, it’s the exception, rather than the rule, of the Quaternary. Our current warm period started 11,700 years ago; on average, our current cycles are about 100,000 years for an ice age to start and reach its peak, and roughly 10,000 years for a warm period, give or take a few thousand years. There has even been some debate over whether we’re currently due (or overdue) for our next glacial period, and whether anthropogenic climate change has forestalled the onset of the next ice age.
If the earth’s orbital cycles have been relatively stable for hundreds of millions of years, why have we only been getting ice ages in the last two and a half million? Our current climate system is affected by processes at a wide range of timescales, from millions of years to minutes. Some geologically recent events, like the uplift of the Himalayas and the closing of the Isthmus of Panama, cooled the planet and set the stage for permanent ice to form on the poles. In fact, our modern climate system really dates back to about 900,000 years ago, when our ice age cycles shifted from a 60,000-year cycle to a 100,000-year cycle. By added 40,000 years to the ice-age cycle, the ice sheets were able to grow much larger, getting about 10 degrees further south on average. This is also when we saw the development of the Mediterranean climate in southern Europe and north Africa. The cause of this transition has been a source of debate, but recent evidence suggests that a major change in the great oceanic conveyer belt that transports heat was associated with the switch. Modern civilization arose with in our current warm period, the Holocene, and it’s thought that the warmer conditions and increased CO2 levels may have facilitated the spread of agriculture. We have our orbital forcing to thank for much of that, but it’s important to remember that our climate bears the fingerprints of events tens of millions of years ago, too. Orbital cycles don’t just affect ice age cycles, and there are a number of feedbacks in the earth’s climate system driven more by the presence of the ice sheets themselves than insolation cycles. Part 3 of this series will deal with monsoons and orbital-scale interactions in the earth’s climate system, followed by posts on millennial, centennial, and decadal-scale processes.