A question for the astronomers in the audience...

[andrewducker]

How much longer would the Earth's orbit around the sun have to be for (most of) the planet to be uninhabitable?

Would 10% do it? The poles get pretty damn chilly as it is, if we added another month in there between December and January, would the whole of Europe be similar?

Comments

[cheekbones3]

There are a wide range of hypotheses, there's a rough consensus of 5% closer being lethal, further out though, there's apparently much more potential for variance - atmospheric insulation will be very important.

http://en.wikipedia.org/wiki/Habitable_zone#Solar_System_estimates

[redbird]

I think any answer has to take into account that the planet has supported life for at least 3 billion years, during which time the Sun has grown steadily brighter, and that the Pleistocene/Holocene/Anthropocene is significantly cooler than the Cretaceous, even though the input of solar energy has increased. I'm not convinced of Lovelock's Gaia Hypothesis, but orbital radius clearly isn't the only important factor.

For the specific question of Europe, the variables seem to include plate tectonics as well as ocean currents: consider that London is significantly closer to the pole than Toronto.

[cheekbones3]

Having a nice thick atmosphere is clearly key, to my mind. But then we've probably done pretty well to stay within habitable bounds on the surface for so long with the evolution of the overall system as you say.

It could be hypothesised that biological life geoengineers conditions to suit itself, although that may imply that oxygen-breathing organisms that followed the earlier O2-belching life were omnipotent...

[cheekbones3]

And I must read more on Gaia, I'm aware of it but no more than that. I know a colleague was working on the general area a few years ago at a research institute, I wonder what's happening to that thread.