The Milankovitch Cycles Drove the Ice Ages for Millions of Years, Long Before Humans Arrived
Welcome back, readers!
The Milankovitch Cycles are named after Milutin Milankovitch, who correctly calculated the collective (and varying) amounts of solar insolation reaching the mid-latitudes (30-60 degrees) attributed to three specific orbital cycles. Rather than whirl around like a stationary seat on a merry-go-round, Earth actually wobbles like a top as it orbits the sun, sometimes more steeply tilted on its axis, and sometimes making more of an oval-shaped orbit instead of a perfectly circular orbit. These cycles are referred to as precession, obliquity, and eccentricity, respectively.
We humans cannot feel these astronomical movements because they take a long time to unfold. The shape of Earth’s orbit (eccentricity) becomes more ovular or more circular over the course of 100,000 years. This is because Jupiter and Saturn, the largest gaseous planets in the solar system, have a strong enough gravitational pull to warp our Earthly orbit. “When Earth’s orbit is at its most elliptic, about 23% more incoming solar radiation reaches Earth at our planet’s closest approach to the Sun each year than does at its farthest departure from the Sun. Currently, Earth’s eccentricity is near its least elliptic (most circular) and is very slowly decreasing.” (https://climate.nasa.gov/news/2948/milankovitch-orbital-cycles-and-their-role-in-earths-climate/).
Meanwhile, Earth’s axial tilt (obliquity) ranges from 22.1 to 24.5 degrees every 41,000 years, which doesn’t sound like a lot to us, but makes a HUGE difference at the poles. When the Earth is less tilted (closer to 22.1 degrees), the sun’s rays hit the poles at a low, indirect angle. This promotes glaciation, i.e. the growth of continental ice sheets via the reduction of summer melt such that snow and ice can chronically accumulate. Twenty- to ten-thousand years ago, continental ice sheets covered North America down to New York and Washington state! In contrast, when Earth is tilted at 24.5 degrees, more direct sunlight can hit the polar regions, which contributes to summer melt such that it outpaces snow and ice accumulation, i.e. deglaciation. “Earth’s axis is currently tilted 23.4 degrees, or about half way between its extremes, and this angle is very slowly decreasing.” (Ibid.)
Finally, Earth wobbles like a top over the span of 26,000 years. This has several effects. First, seasons become more extreme in one hemisphere and less extreme in the other. The northern hemisphere has more land mass for continental ice sheets to grow upon. Therefore, the amount of solar radiation reaching the northern hemisphere specifically drives the glaciation/deglaciation cycles. Milder seasons in the northern hemisphere promote ice growth. Second, axial precession also changes the timing of the seasons, causing them to begin earlier over time. Hey, our Gregorian calendar is just a social construct, after all! Third, precession causes us to point to new North Stars. Currently they are Polaris and Polaris Australis, but several thousand years ago, they were Kochab and Pherkad. (Ibid).
Don’t panic! The Milankovitch Cycles were easily the most difficult concept for me to grasp as an undergraduate. The main takeaway is that humans have absolutely no ability to influence our orbital movements, and these orbital movements drove the advance and retreat of the ice ages for millions of years before we even arrived on scene. What humans most assuredly DO have the ability to influence is atmospheric chemistry. We alter the carbon cycle every day on a massive scale. This is why we have extraordinarily fast warming, light-speed geologically speaking. We should be entering a gradual cooling period, slipping into the next ice age, but instead we are sky-rocketing in the opposite direction, with global CO2 and temperature increasing exponentially. We know precisely why.
On an unrelated note, I checked the Climate Prediction Center again. It looks like we have equal chances (50:50) forecasted for a normal precipitation year! Let’s hope we receive some of the La Niña moisture expected for the Pacific Northwest. We are technically part of the Cascades, overlying the subduction zone between the Juan de Fuca and North American tectonic plates. Did you know Lassen Peak is the southernmost Cascade volcano? I will continue to hope for plentiful precipitation this winter, ideally in the form of snow.
Be well in the meantime.
