My theory on what causes ice ages, and why human activity is the reason we're not in an ice age right now.
I've never heard anyone posit an explanation for why we go in and out of ice ages, so here's mine.
Look at the red line in the middle of the chart I posted, you'll see that it has peaks that come regularly, but not so regularly that it would suggest some sort of astronomical event that happens like clockwork, it looks more like a repeating process to me. It follows the same basic pattern each time, but not quite the same schedule.
Since it's a cycle, it just keeps repeating, so I'll start at the point when the earth is coming out of an ice age. As the ice retreats in the summer, there are patches of bare ground where forest which had once been there was killed off by ice. When the sun hits these bare areas, they get warm and heat the air, causing more ice to melt. The ground exposed by the melting ice has no vegetation, so it heats up too and a feedback loop develops, warming the planet until the ice is gone. This is the big spike in temperature you see on the chart, and why it peaks higher than at any other time: the ice is no longer reflecting solar energy and there's no vegetation to cool the ground.
As plant life begins to return, it starts to consume solar energy, but the process takes a long time, since glaciers wiped away a lot of the top soil. Eventually though, forests spread across the earth, covering every piece of open ground, as forests tend to do. The trees become very large, the forests very dense, and they consume enough of the solar energy that the planet tips into a long cooling phase. Eventually ice sheets cover most of the planet, killing most of the forest, then the open ground heats the air and the whole process repeats.
That's why our present warm period just keeps going and going when previous warm periods were much more brief. This time there were humans, and not just humans, civilizations. Thousands of years ago the Middle East and North Africa were covered in forest, but ancient civilizations cut most of it down. That excess heat from that bare ground was sufficient to prevent the earth from tipping into the ice age that would normally be well underway by now. As civilization spread across the globe, so did deforestation, so we've made the earth's climate more similar to the way it normally is during the early part of a warm period.
As such, I don't think we'll be going into an ice age until or unless people stop artificially limiting the forest cover of the planet.