That's an important point. I was only thinking in terms of the average temperature, but you may be right that the tropical temperatures would remain above freezing. That might provide a safe haven for life even in times of greenhouse gas loss. So I guess my question is, which overall climate is a typical "ice age" closer to-- what we have now, or what we'd have in the absence of greenhouse gases?

Then the entire issue of plate tectonics, raised in this thread, may be a red herring-- if tectonics are not the source of our greenhouse gases. If they are, one gets right back to the stability issue that I'm talking about. If it wasn't regular, then the non-tectonic periods, on thousand-year-timescales, would also suffer from dramatic greenhouse-gas variation, perhaps moreso than what we actually see in our ice-age history. So either we do have non-regular greenhouse forcing, and have to understand what that does to our climate, or the forcing is fairly steady, and we have to understand why that is. It all gets back to the question I asked just above, so until I know that, I can't get any further.

The earth is an average of 14 degrees above zero. A drastic reduction in CO2 would cool the earth by about 3 degrees. However, expanding ice sheets would reduce the earth's temperature further by increasing albedo. Just by how much I don't know. Reductions in albedo due to decreased cloud cover also have to be figured in. But with current levels of solar insolation the earth should not snowball up.

Well there are two kinds of volcanoes on earth. Ones that result from plate tentonics, and ones that don't. The ones found at plate margins release lots of water vapour because the magma has mixed with wet seafloor. Carbon from the seafloor is also released. These volcanos aren't the source of our carbon, but recycle carbon from the seafloor. Volcanoes that aren't found at plate margins, such as the Hawaii ones don't have magma that is mixed with subducted seafloor. They release less gas and tend to be less explody. But they still release some gas, as can be clearly seen if you watch a documentary on Hawaiian eruptions.

When Earth forms a large supercontinent then it looks like that plate tectonic activity does greatly reduce. The ocean traps CO2 and becomes stratified. Only the uppermost layer has oxygen recirculating and can support life, the deeps ocean becomes anoxic. Eventually heat build up under the continent breaks it apart and restarts the tectonics.

here is a recent article: Intermittent Plate tectonics

Presumeably a waterworld would act in the same way, except without any trigger to release C02, so tectonics might not reactivate

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plenty of woo, at the hotel hoagaland...

A feedback I would worry about is the dryness of the atmosphere. CO2 may not warm by more than 3 C at the moment, but H2O certainly warms by much more, perhaps 30 C, I don't recall exactly. But without H2O, the Earth might completely freeze, it seems to me, so if cooler means drier, you could have a runaway loss of the greenhouse effect. Clearly, that can't have happened much to our planet, but is that because the CO2 is maintained, or because it wouldn't happen even without any CO2?

That doesn't matter because I presume that is just a temporary addition to the water cycle. We care more about the creation of the oceans, and then the CO2 balance after that.

The greenhouse effect on earth is about 33 degrees with water vapour accounting for roughly 70% of that. As I understand it, with the current amount of insolation, the earth won't snowball up even with a drastic reduction in greenhouse gases, but you might want to discuss it with someone more knowledgeable on the subject.

Then there's the "young Sun problem", where greenhouse gases are even more important. It's a pretty tricky business, we probably have our hands full with just understanding the perturbations to the current climate conditions on decadel timescales, let alone understanding the full history of the climate over billions of years!

I disagree with the article. Life is much more resilent than our own ideas of that it can do. How long didn't life need to go from microbes to us? A few hundred millions of years? Take Mars for example. It's had liquid water oceans for about 700 million years. That's enough time for an intelligent species to emerge and go extinct long before the water dried up and the Hellas Impact Site happened.

What if a smaller earth had more radioactive elements and was tectonically active for a longer period of time?

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Fields of Space

LOGIC, n.
The art of thinking and reasoning in strict accordance with the limitations and incapacities of the human misunderstanding.

In the Year 2525.

"One small step for (a) man. One giant leap for mankind".

If an astronaut doesn't need good grammar, niether does you.

Host of Seraphim

Not counting the 3 billion or so to get from the start of life to complex prokaryotic microorganisms that formed into the first real multicelled organisms.

It's not just a case of "any germ will do".

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort