Why are there ‘Ice Ages’
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A culmination of global forces leads up to it. The timescale of the earth is measured in millions of years. For the same reason it rains ‘sometimes’, there is an ice age ‘sometimes’.
You can call it a ‘correction factor’ or something like that, but that doesn’t really do it justice – more like it happens because it becomes more and more primed to happen, and once it starts, it lasts until it’s over.
Modern Ice Ages (within the past million years) are believed to be primarily caused by natural cycles in the Earth’s orbit called Milankovitch cycles.
Our general climate is at a tipping point where relatively mild orbital changes can trigger an ice age. But with the increasing amount of man made CO2 in the atmosphere may end the cycle of Ice Ages. But at this point we don’t know for sure.
Over thousands of years the Earth orbit changes slightly causing shorter summers. This has a long term knock on effect that causes more and more ice to build up at the poles until it causes a run away effect triggering an ice age.
*On a side note it’s common to refer to only the periods of Glaciation as an Ice Age, but scientists often refer to the period of re-occurring glaciations over Millions of years as one long Ice Age and we just happen to be currently living in a short warm period in a longer ice age.*
Eventually so much ice and glaciers build up in the Northern Hemisphere that entire continents are covered and reflect light back into space making things worse.
The Sea level drops, ocean currents are effected, and the amount of and types of plants that can survive on land are reduced.
Eventually the orbit changes enough that the planet starts to warm up, and again it triggers a runaway effect where all the glaciers start to melt very rapidly ending the ice age.
Ancient Ice Ages (billions of years ago) were caused by different reasons. The Sun had a lower output back then, and the Earths atmosphere had a very different composition than it does now. The continents were also in a different position which affected ocean currents and the weather.
There is something called the Milankovitch cycles https://youtu.be/aBwAkpRtSxA basically over time the tilt of the Earth changes, the Earth moves from a circular orbit to a more oval one and a few other factors mean that the amount of energy reaching the Earth fluctuates over thousands of years.
A truthful answer is, we don’t fully understand why. It’s not entirely clear why there was an ice age for the last 2.5 million years, but not the previous 60.
There are known mechanisms that contribute to them. Lower levels of greenhouse gasses, like CO2 and methane are a major one. You basically cannot have an iceage with higher levels of those, even if everything else lines up perfectly.
Another factor is the distribution of land on earth. More landmass at the poles helps ice start growing. Ice doesn’t like growing on water.
Anything that reflects sunlight helps ice grow. Clouds help lower temperatures. Ice is very reflective too. So some ice can help more ice grow.
Then there’s cycles in earth’s orbit and rotation. They’re called Milankovitch cycles. The earth itself wobbles on it’s axis, which means that it can be more tilted from upright. At that point, the poles get less sunlight in winter, and get colder.
The earth’s orbit isn’t perfectly circular either, it’s an elipse. It’s called eccentricity. So the earth is closer to the sun at one end of the elipse than the other. But this also changes in cycles, with in growing more or less eccentric, putting the earth further away from the sun at it’s furthest.
The earth’s axis also rotates, and because of the tilt, it can mean that which pole faces the sun and which faces away at the point furthest from the sun also changes.
So ice is more likely to grow if CO2 levels are low, there’s a lot of landmass around a pole, and the Milankovitch cycles line up so that average yearly temperatures around the pole are low. But ultimately, when an ice age happens or not doesn’t line up perfectly with predictions based on all of these factors, so we’re missing something.
I’m also not entirely sure which combination of milankovitch actually results in lower temperatures. You’d think “pole as far away from the sun as possible in the winter makes it coldest”, but that also means it’s closest during summer, meaning the warmest summers possible. I don’t know if there even is one correct answer, or does it purely depend on the circumstances.
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