Ice Ages

Water has moderated our temperatures, and allowed life to evolve for the last 3-4 billion years. There has never been a 'Back to the Drawing Board' for life. That means we have always had 'liveable' temperatures.

However, the Earth has gone through some really good temperature cycles. But for time travelers it just means 'bathing suit or parka'. We've had really warm cycles and 'Snowbal Earth' cycles.

This shows some really big cycles, but everybody debates the precision. Poops! There is no precision like today, that's the point. The physics driving these big cycles are the plate tectonic cycles. Sometimes the continents are far apart, and sometimes they are globbed together. Cold is apart, warm is together. There's all sorts of good physics explaining this.

We are currently in a stinky cold cycle. No flying dinosaurs in the jungle at the North Pole. Now, that's global warming! We are in an ice age, or Snowball Earth, and we are happy. We have big continental ice advances every 10,000 years or so, give or take a few tens of thousands.