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.

The Fantastic Formation of the Solar System!

July 14th, 2021

Welcome to Megan’s Climate Corner!

            The Universe is ancient, much older than human beings can really fathom, at a whopping 12-13 billion years! We have determined this age range from examining the cosmic microwave background (the “after-glow” of the Big Bang, when electrons began forming the first atoms), estimating the age of different stars in globular clusters (groupings of a million or more stars, all of which are different densities and burn at different rates), and by observing the current rate of expansion to extrapolate back in time. You can read more about the age of the universe here: https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question28.html. Today, we concern ourselves only with our own solar system: our yellow medium star, the sun, and the eight planets that orbit it.

Gravity is a measurable, calculatable force (proportional to mass) that pulls objects toward one another. It’s what pulls everything toward the center of the Earth, and binds all objects in space to a regular orbit. We orbit our sun. Our Milky Way galaxy orbits itself, pinwheel arms spiraling away from its center, where a supermassive black hole resides.

            Our Solar System formed about 4.5 billion years ago from a thick cloud of gas and dust, which condensed rapidly (perhaps from the shockwave of a supernova) and became a swirling spinning disk called a solar nebula, thus creating the sun, the rocky inner planets (Mercury, Venus, Earth, and Mars), and the gaseous outer planets (Jupiter, Saturn, Uranus, Neptune). Read more here: https://solarsystem.nasa.gov/solar-system/our-solar-system/in-depth/

Under the force of this spinning and the force of gravity, the elements of Earth separated out to form distinct layers: the light, silicate minerals floated to the outer shell to form the crust while the heavy, super dense iron and nickel condensed in the center of the planet, into our hot, slightly radioactive core. 

            The sun fuses hydrogen into helium and releases tremendous amounts of energy. This is nuclear fusion. While the sun is a constant supply of energy, it is incorrect to state that, “The sun drives our climate.” Reality is more complex. 

            Our climate is driven by: Earth being a sphere, tilted on its axis, wobbling like a top, and the orbit being sometimes more ovular and sometimes more circular. This is why we have distinct climate zones, opposing seasons in the northern and southern hemispheres, a new North Star every 12,000 years, and why we had regular Ice Ages. We’ll delve into the Milankovitch Cycles in a later edition. 

            Earth is a materially closed, energetically open system. This means that, with some exceptions, like meteors/comets bringing novel minerals to the surface, Earth contains all of the material it will ever contain. We cannot obtain more ore for mining or more water for drinking and farming. Everything we have is everything we'll ever have. Being energetically open, however, means the sun is constantly bombarding us with energy in the form of visible, ultraviolet, and infrared light, and radio waves, x-rays, and gamma rays. We are supplied with energy forever (as far as humans are concerned!), but mind you: the sun is about halfway through its lifespan.

            The Equator is hot and humid because, at the equinoxes, the surface of the equator is at 90 degrees to the sun, thereby receiving twice the amount of energy of the same area at a latitude of 60 degrees. This solar energy causes massive quantities of water to evaporate, which then condenses into clouds. This also creates a low-pressure system where warm air rises, reaches the top of the troposphere, then begins to cool and settle over the subtropics, ~23.5° latitude N and S. In these subtropical high-pressure systems we find dry deserts, where little moisture falls. 

As the seasons change and we progress through the summer and winter solstices, the thermal equator shifts north and south of the true equator, driving the pressure changes that swing winds and weather systems north and south.

The temperate latitudes are at a less direct angle from the sun, experiencing variable solar energy and weather. The poles vacillate between sunless winter and summers when the sun doesn't set at all! This occurs at and above the Arctic Circle (66.5°N) during northern hemisphere summer solstice, and in the Antarctic Circle (66.5°S) during the southern hemisphere summer a.k.a. winter solstice.

The poles are the extremes of our Earth system, experiencing the largest fluctuation in solar energy. They are extremely fragile ecosystems that serve as the proverbial canary in a coal mine with regard to climate change. Stay tuned.

**Thank you to John Porritt for his corrections to a previous draft of this article. See his very helpful, well-articulated email below.**

“Hi Megan,

I'm so glad to have a science-based column addressing climate change in the Trinity Journal and wish you every success.

That said, I find one of your points misleading. The equator is hotter not because it is "physically closest to the sun"; the earth is millions of miles closer to the sun in January than in July. The reason the equator is hotter is because at the equinoxes the surface of the equator is at 90 degrees to the sun, therefore receiving twice the energy of the same area at a latitude of 60 degrees. At all other times, the thermal equator is north or south of the equator, driving the pressure changes and swinging the wind belts north or south. Due to this, Trinity County enjoys half a year of a desert climate and half a Pacific Northwest climate.

I will be following you with interest.

Best regards,

John Porritt”

Teamwork makes the dream work! Thank you, John, for your assistance in clarifying seasonal solar energy budgets on a spherical planet.

Image sourced from: https://www.livescience.com/our-solar-system.html