The future of the earth is dependant on it's internal radioactivity, and the size of the sun i think. basically it's game over when the sun becomes a red giant, but it seems more likely that radioactivity (the main internal energy source of the earth except for some left over from the formation of the solar system) will die down first. The sun is only half way through it's life. I don't think mars was very internally radioactive, and it has a smaller mass so lost the majority of it's atmosphere relatively easily.
as global warming goes, you can think of it as the response of the earth in relieving an irritation, if you are so inclined as to consider james lovelock's gaia theory, whereby the earth is seen as one living system, anyway.
i think the best way to be positive about the future of mankind it is to say, right, we have two possible prosperous futures, and one negative view of it. the positive view is that we will either prevent a huge and unnatural warming of the earth with a mixture of will power, voting, and science, or on the other hand we survive the global warming -->ice age-->???interglacial period? cycle, with, again, will power and science (and of course, more morbidly perhaps, a reduced and more managable population). when in the sh*t, humans tend to group together, and pull amazing things out of the bag, so who knows what we could come up with in such a scenario as near mass extinction?
but despite that, all you really have to do to remain happy about our predicament is to try to live within your means, and according to your own morals, with respect (not just tolerance, an over-used word IMO) for others. in that way you are able to be as flexible about your lifestyle as needs be, and not begrudge it.
it has often got me down too, but i'm happier about it now
