About 100,000 years ago, the world was comfortably warm like it is now. And then for 80,000 years it slowly slid into a deep ice age, during which the place where New York City is now was covered with ice 1000 feet thick.
Then it began to warm, the ice began to melt and greenhouse gases (GHGs) started to increase. But what caused what? Some think the GHGs caused the earth to warm, but then what caused the GHGs to increase? When people learn that the warming came first, this confuses them, and they think that proves GHGs did not warm the Earth. But here's the way it actually happened, starting from when there was maximum ice.
1. The Earth's orbit and/or axis changed.
Orbital and axis changes happen in complicated cycles. It's not that the earth gets closer to the sun and heats up; we are stuck at our average distance. (Also, since the earth is almost a perfect sphere, small changes in the axis make almost no difference to how much sunlight we get.) What changes is the seasons. If the summers get hotter and the winters get colder then more ice melts in the summer and less ice forms in the winter because when it's very cold the air can't hold enough water to snow much.
So, the orbit/axis change does not make the earth warmer on average it just makes the glaciers covering the top part of the U.S. and Canada melt away and not re-form in the winter.
2. Less ice makes the Earth warmer.
Ice is cold, right? So when it goes away, we warm up. Well, actually, NO. That's not it. It can be bitterly cold with no ice or snow. What makes us warm is absorbing sunlight. If you wear black, you will get hotter in the sun than if you wear white. When the ice melts, the earth starts wearing more green and brown -- not black, but darker than snow white. So the Earth reflects less and absorbs more sunlight and warms up.
3. Warmer waters lose their fizz (CO2)
Pour some Coke in two pans. Get one quite warm on the stove. Cool it off and taste the Coke from both pans. The one you heated has less fizz. Fizz is carbon dioxide / GHG / greenhouse gas / CO2 / CO2. It's great stuff. But (1) it likes to dissolve in water (Coke and oceans), (2) it's what plants breath in to grow, and (3) when it's in the air it helps keep us warm. There is not much fizz in the ocean compared to Coke on a per-can basis. You can't taste it. But oceans are big (and deep) and when they warm a little bit a lot of CO2 can come out. That started happening. The Earth's warming caused more CO2 in the atmosphere. That's why the warming comes first!
4. CO2 causes warming, and around we go
But it works both ways -- warmer oceans release CO2 and CO2 warms the Earth and its oceans. This is a vicious circle. Scientists call it a "positive feedback."
(Positive mean "more," and this feedback loop is a more-more-more loop. Negative feedback is like your home's thermostat: more heat in the room makes the thermostat turn off the furnace and you get less heat from the furnace. Or less room heat causes more furnace heat. So strangely "negative feedbacks" are good because they are stabilizing, and "positive feedback" are bad because they are destabilizing.)
Fortunately the more-CO2-more-warmth feedback loop only tends to be destabilizing. It is not strong enough to get out of control. So this vicious cycle adds to, or amplifies, the warming that gets us out of the ice age. It roughly doubles the warming.
5. That's it for Step 2.
