How does the Earth manage to stay in this habitable range? Is there a Gaia phenomenon keeping the climate in bounds? A nice idea, but it doesn’t work.
The Earth’s climate becomes more sensitive as it becomes very cold, when an amplifying feedback, the surface albedo, can cause a runaway snowball Earth, with ice and snow forming all the way to the equator.
If the planet gets too warm, the water vapor feedback can cause a runaway greenhouse effect. The ocean boils into the atmosphere and life is extinguished.
The Earth has fell off the wagon several times in the cold direction, ice and snow reaching all the way to the equator. Earth can escape from snowball conditions because weathering slows down, and CO2 accumulates in the air until there is enough to melt the ice and snow rapidly, as the feedbacks work in the opposite direction. The last snowball Earth occurred about 640 million years ago.
Now the danger that we face is the Venus syndrome. There is no escape from the Venus Syndrome. Venus will never have oceans again.
Given the solar constant that we have today, how large a forcing must be maintained to cause runaway global warming? Our model blows up before the oceans boil, but it suggests that perhaps runaway conditions could occur with added forcing as small as 10-20 W/m2.
There may have been times in the Earth’s history when CO2 was as high as 4000 ppm without causing a runaway greenhouse effect. But the solar irradiance was less at that time.
What is different about the human-made forcing is the rapidity at which we are increasing it, on the time scale of a century or a few centuries. It does not provide enough time for negative feedbacks, such as changes in the weathering rate, to be a major factor.
There is also a danger that humans could cause the release of methane hydrates, perhaps more rapidly than in some of the cases in the geologic record.
In my opinion, if we burn all the coal, there is a good chance that we will initiate the runaway greenhouse effect. If we also burn the tar sands and tar shale (a.k.a. oil shale), I think it is a dead certainty.
That would be the ultimate Faustian bargain. Mephistopheles would carry off shrieking not only the robber barons, but, unfortunately and permanently, all life on the planet.