I didn't realize that the vast majority of the time Earth is in an ice age. What causes it periodically like this?

It’s mostly caused by Milankovitch cycles. Which are cyclic variations in our orbit around the sun.

The earth varies in distance to the sun and axial tilt and precession. Like waves, there factors can either overlap and somewhat cancel or they can stack for a larger effect.

ice ages didn't happen until after the asteroid strike. Antarctica moving into the south pole is likely a large part of it, but they don't really understand ice ages that well TBO. Anyhoo, the Earth was a lot warmer prior to the strike and we have been going through extinction events every 100k years.

By "the" asteroid strike I'm guessing you mean the one 66M years ago? There were definitely ice ages before that, likely including one or more Snowball Earth phases (the whole surface frozen): https://en.wikipedia.org/wiki/Snowball_Earth.

The planet is 4.5 billion years old! 400,000 years is just roughly 0.0089% of earth's history (and I rounded up!)

Humans are only about 300 000 years old though, so that part is much more relevant to our species than the rest.

There's no bullet to dodge. Minus human intervention, the next ice age would be in 50,000 years. Humanity will be either some unrecognizable scifi trope or, more likely, long gone by then. Planning for it is pointless.

The time scales for anthropogenic climate change and the previous temperature cycles are so different as to not be comparable.

I’m not saying to plan for it.

I’m just saying that a silver lining of the mess we’ve made is that we probably won’t have that ice age in 50,000 years. We’ve likely disrupted the cycle.

> Minus human intervention, the next ice age would be in 50,000 years. Humanity will be either some unrecognizable scifi trope or, more likely, long gone by then.

I think you are overestimating the probability that humanity would be "long gone by then". Even if a series of catastrophic events + climate change make life as we know it (modern civilization, globalized society/trade) untenable, humans are smart and many places on earth are reasonably forgiving to survival. Not saying it would be comfortable tho.

What I gathered is that if the gulf stream stops, warm water will no longer flow north and the ice sheets will grow explosively, triggering an ice age (the kind where thick ice sheets flow southwards). That would pretty much wipe out anything built in the northern half of Europe... although granted, I don't know if it was like a kilometer high ice sheet rolling our way or how fast it went in previous ice ages, it might be slow enough that it can be stopped and broken down before it causes damage.

Early onset of glaciation via warming instead of cooling. Counter intuitive idea.

It’s unclear that it would play out that way. One could look at what happened the last time it stopped to get some idea, but things are different this time. We have a lot more atmospheric CO2.

This is 99% conjecture. The current climate models do not predict those kinds of changes and most don’t have the fidelity to do so. Even the best models can’t account for simple things like aerosol pollution and they don’t have the resolution to determine the behaviour of ocean or atmospheric currents.

I don’t think many models predict much difference between 3 and 4 degrees, so I don’t think you can describe it as horrific.

IPCC predicts some local tipping points at 4C but global tipping points are much higher.