re: The Earth
One other factor to put into the whole equation (as if it's even possible to build an equation precise enough to predict more than a minute out) is the possibility that as our population grows and we cram ourselves closer and closer together, that a worldwide pandemic could substantially decrease the human population on Earth. I somehow doubt the Bird Flu is going to be the one that nails us but too much human density is high on my list for what will do us in first.
I'm also pretty sure that effects of global warming will eventually trigger an ice age due to the stopping of the undersea convection stream. Lovelock said the Artic would be a refuge for some. That would hardly be the case in an ice age.
Art