One of the things that comes up when someone talks about climate change is the apparent cyclicity of climatic changes. The Earth has been through several rounds of ice ages and warming in recent millennia, how is this new episode of this warming not just part of that? Well, let’s look at the cycles.
What we see here is a repeating 100 thousand-year cycle of glaciations and warming. We’re in a warm spot, having just come out of an ice age about 10,000 years ago. If we look at the pattern for the last three deglaciations, we see sudden, rapid warming, followed by cooling into another ice age. We’ve already warmed, and have been warm for a while, so we should be cooling down now. That’s why, back in the 1970’s, people were being warned about the coming ice age.
According to the glacial cycles, that’s where we should be heading. Things should be getting cooler. And they were up until about 50 years ago. Then we started seeing increases in annual temperatures. When looking at this graphically, we get what has been referred to as the “Hockey Stick.” You can read more about where the Hockey Stick comes from here.
What causes these glacial cycles? What is this 100,000 year periodicity? This pattern is caused by Milancovitch Cycles, changes in the intensity of the sun that hits the Earth due to properties of the Earth’s orbit and rotation about its own axis. There are three (or four) parts to Milancovitch Cycles.
The first of these is an approximately 21,000 year cycle called precession. This is where the Earth’s rotation axis wobbles, much like how a top wobbles as it spins. This changes the position in the Earth’s orbit at which the equinoxes take place.
Obliquity is a 41,000 year cycle in which the tilt of the Earth’s axis varies from 21.5° to 24.5° from perfectly vertical relative to the plane of the Earth’s orbit around the sun. With greater tilt, the difference between the seasons becomes greater.
The shape of the Earth’s orbit around the sun shifts from being closer to circular to being more oval. This shift is called eccentricity and varies on scales of 100,000 and 400,000 years.
Each of these (precession, obliquity, and eccentricity) have an effect on the amount of sun (insolation) that hits the Earth and therefore Earth’s climate. The term for this is solar forcing. We can take the individual impacts on solar forcing for each of these and add them up to summarize solar forcing at any given time. We can then compare this, and the individual forcings, to the pattern of glaciations. What we see is an approximately 100,000 year cycle of glaciations, which coincides with minima (or low insolation) in the 100,000 year eccentricity cycle.
As we are approaching a minimum in the eccentricity cycle, we might expect to be heading into an ice age – though it might be a few thousand years off. What we are seeing instead is rapid warming. Perhaps we should be concerned.