Prioritize...
When you have finished this page, you should also be able to
- Explain how the orbital cycles discussed in the previous section can lead to large regular swings in the Earth’s temperature between glacial and interglacial periods.
- Define the ice-albedo feedback.
Read...
Glacial-Interglacial Periods
If you had an eagle eye, you might have noticed on the first page of this lesson that the planet's temperature has experienced large regular swings over the past 500,000 years. See the zoomed-in figure below, where the blue and green lines are temperature reconstructions from ice core data in Greenland and northern Russia, respectively. These large swings are called glacial-interglacial cycles. Looking at the time between each peak in temperature, you will see that these cycles occur roughly every 100,000 years. Why do these oscillations occur? Why do they have a regular period of 100,000 years? What explains their “sawtooth-like" shape where the temperature increases rapidly and then decreases slowly? These are all good questions -- let's see if we can answer them!
The previous section of this lesson taught us about three orbital cycles that occur on long timescales (protip: you should pause here and test yourself to see if you can list the three!). The eccentricity of Earth’s orbit, in particular, operates with a period of 100,000 years. This sounds suspiciously like it could be related to the glacial-interglacial cycles! The question is how?
First, let’s think about how these three orbital parameters alter where and when solar radiation hits Earth. In short, we have stronger seasonality when Earth is more tilted, and when precession causes Earth’s Northern Hemisphere to tilt toward the Sun (summer) at a point when it is closer to the Sun. Think about it: when Earth’s axis is tilted more, the poles experience more extreme seasonal differences. If, during this time, Earth is also farther away from the Sun during Northern Hemisphere summer due to precession, summers—while still warmer than winter—become less hot. These impacts are amplified when the eccentricity of the orbit is greater—that is, their role is "turned up" the more oval-shaped Earth's orbit becomes.
When summers are cooler in the Northern Hemisphere due to a combination of these factors, snow and ice are less likely to melt fully. So, what role does ice play in all of this? Excellent question—we need to explore another (very) important feedback in the climate system.
Ice-Albedo Feedback
Let’s dive deeper into the ice-albedo feedback, a process that can amplify the impact of the orbital parameters we’ve just discussed. Remember when we talked about solar radiation earlier in this class? That’s where we first encountered the concept of albedo. To refresh your memory: albedo is a measure of how much sunlight a surface reflects. Snow and ice have a high albedo, meaning they are excellent at reflecting incoming sunlight back into space, which helps keep the surface cool. In contrast, the ocean or land beneath the snow and ice has a much lower albedo, meaning these surfaces absorb more sunlight, which warms the surface.
Now, let’s consider what happens when there’s less ice than usual. In this scenario, more of the low-albedo ocean or land would be exposed, causing more sunlight to be absorbed rather than reflected. More absorption of solar radiation means more "energy in" and a warmer surface. This additional warmth would melt even more snow and ice. As the ice melts, the exposed surface area with low albedo increases, which leads to even more absorption of solar energy. This results in further warming, which melts more ice, and the cycle continues.
Experiment:
In many northern areas of the United States, homeowners who heat their homes with wood often spread dark ash over snow and ice-covered driveways. This not only helps with traction for vehicles but also accelerates the melting process by exposing the underlying darker surface. It's the same concept at play as the ice-albedo feedback! You can try a mini version of this experiment yourself—sprinkle some dark material (like ash from a fireplace or charcoal grill) onto a small patch of snow-covered asphalt and -- once the sun comes out -- observe how much faster it melts compared to the untouched snow!
This process is an example of what we call a positive feedback. Essentially, an initial change (in this case, less ice) triggers a series of events that reinforce and amplify the original change.
The ice-albedo feedback is a powerful mechanism in the climate system. It’s one of the reasons why relatively small changes in Earth's orbital parameters can lead to such dramatic shifts in global temperatures during glacial and interglacial periods. The more ice melts, the more the Earth warms, and vice versa. This feedback loop plays a key role in amplifying the natural variability introduced by the orbital cycles.
Think About It...