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- Explain how orbital parameters and the ice-albedo feedback can "combine" to create ice ages
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Ready to add another recipe to your book? Let's write "How to make an ice age."
Let's put the two concepts we just learned together. Roughly every 100,000 years, the eccentricity of Earth's orbit reaches a high value, creating the largest possible differences in Earth-Sun distance over the course of the year. During these times, there's a point in the much faster 19,000-23,000 year precession cycle when Earth is tilted toward the Sun at its closest approach. This makes summers exceptionally warm and winters unusually cold. This effect is further amplified when the obliquity (tilt of Earth's axis) is at its maximum. We focus on the Northern Hemisphere because there's more landmass, allowing snow and ice to form further south. Typically, winter temperatures are cold enough to maintain ice, but very warm summers lead to rapid melting of snow and ice.
In the figure below, we can see the three orbital parameters—precession, obliquity, and eccentricity—plotted over time. Notice how these cycles oscillate at different frequencies: precession is the fastest, followed by obliquity, with eccentricity being the slowest. Also shown in this figure is the summer Northern Hemisphere insolation (solar radiation received at 65°N). If you look very closely, you'll see how this insolation pattern is linked to the orbital cycles: broad changes every 100,000 years from eccentricity, with smaller, faster changes from precession and obliquity. Lastly, the ice core temperature record offers a glimpse into Earth's past climate, where you can clearly see the sawtooth pattern of glacial-interglacial cycles, occurring roughly every 100,000 years.
Let's focus on one glacial-interglacial cycle in particular. First, note that in this graph, time progresses to the left, opposite to what we've usually seen -- there's nothing wrong with this, but you need to flip your brain to think "hey, the present day is on the very left of this plot." Starting at point A, we see a period of very cold temperatures—an ice age—followed by a rapid increase in temperature. This warming coincides with an increase in Northern Hemisphere summer insolation at 65°N, along with a rise in eccentricity and high obliquity and precession values. As described earlier, this combination of factors—greater eccentricity amplifying the effects of precession and obliquity—leads to warmer summers that quickly melt ice, triggering a positive feedback loop that pulls Earth out of the ice age.
At point B, both precession and obliquity are in their negative phases, and eccentricity begins to decrease. This reduces solar radiation at 65°N, leading to warmer winters and cooler summers in the Northern Hemisphere. These conditions are perfect for growing ice sheets: warmer winters allow for more snowfall (since warmer air holds more moisture), and cooler summers prevent accumulated snow from melting. Ice begins to build up, reflecting more solar radiation back to space and causing cooling to spread southward. This is how a glacial period slowly takes hold, lasting tens of thousands of years. The sawtooth pattern of Earth's temperature comes from the fact that melting ice happens faster than its accumulation, leading to quicker warming followed by slower cooling.
Let’s return to the full reconstruction of Earth’s climate history that we looked at earlier (I’ve copied it below—trust me, you’ve seen it before!). You might notice something interesting: these ice ages didn’t start showing up until about 700,000 years ago (the wobbles beginning in the fourth panel). Why weren't there more ice ages in the earlier panels? The mechanism we discussed for glacial-interglacial cycles relies on the formation of large ice sheets, and scientists think that the climate just wasn’t cool enough yet to allow for that. While we can’t say for certain, the leading theory is that the planet needed to cross a cooling threshold for large ice sheets to form. This gradual cooling is thought to have been driven by a slow decline in carbon dioxide concentrations over millions of years. In other words, during the time of the dinosaurs, for example, the planet was simply too warm for the ice-albedo feedback to kick in. Eventually, as the Earth continued to cool with age, it reached that critical point, allowing the 100,000-year cycles we see in blue below (and in the figures above) to begin.
The glacial-interglacial cycles we observe, driven by Earth's orbital changes and amplified by feedback loops like ice-albedo, have only become prominent in the last 700,000 years as the planet's temperature gradually dipped, allowing large ice sheets to form and trigger these dramatic climate shifts. Pretty "cool!"