How far optimism can go?

I was reading recently a lot about possible dangers to life on Earth. Just to let myself down a bit.
Let's concentrate on three threats: energetic crysis (fuel depletion), global warming (increased greenhouse gases level), and giant asteroid colliding with Earth ;-)
Energetic crysis is easy. We have measurable data how much oil we use, how much we rely on oil in comparison to other energy sources. We also estimated how big are our oil fields and reserves. The only doubt here is how many alternative sources or new oil fields we can discover in the future. However, we simply cannot predict that. Still, I've read that many people try to manipulate this data or lie about efficiency about alternative energy sources.
We won't get rid of people lying problem in any case. So let just forget about it for now, and move to global warming.

First of all, I feel that I'm not educated enough in natural sciences, to be able to tell who is right in global warming. I can only think in terms of potential cases.
Case 1:
Humans are contributing to global warming.
Under assumption that all computer simulations based on meterological and human activity data would be inaccurate, it is hard to proof that (unless we start World War III and stop human activity at all ;-)). The only proof can be based on measurements and statistical reasoning here. Which is what IPCC is doing.
Case 2:
Humans are insignificantly small contributors.
Even harder to proof. As some proponents of this case say, our data might not include all natural causes. Sure, but this is just putting into doubt, because of some unknown, which is pretty much "guilty, because there are not enough evidence that he is not" reasoning. But to be honest, both cases (1 and 2) in terms of pure logic can be like that, still the only evidence (and a lot of them) are for case 1. Also some anti-warmers tend to provide evidence that natural forces are much bigger contributors, this could be more reliable proof, still as far as I know, all of these were refuted.
Case 3:
The average temperature will actually decrease in near future. I don't know really much about climatology, so cannot say anything about that. I only heard it's very unlikely, because the trend is clear (not only growing, but growing faster and faster, it's harder to say how much it will grow though).
Despite you may see I'm slightly for IPCC, I must clarify that I'm really not sure. I think any conclusions here must be made by specialists. What matters are measurements and numbers you are making intepretation from, and probabilities you put into various scenarios, also since the process is very complex, you need very comprehensive set of measurements (and still you may be missing some).
However, I am always surprised some people (actually surprisingly many) have strong opinion that there is no global warming at all. Some of them tend to use simple logic to proof that (without telling any numbers, one of them is very annoying polish politician, known as JKM). There are also a lot of silly youtube movies trying to fit the data to proof whatever the author wants. I guess strong opinions are usually subject to psychological issues, manipulating or just so called wishful thinking.

Last (but not least) is asteroid threat. My favourite, cause it is so much unpredictable that it is funny (remember my "Cave The Movie"?). There is a NASA program to track dangerous astral objects that can collide with earth. And a lot of effort is done to predict trajectories accurately (but results are far from good enough). You may hear about Russia asteroid-defence program, in case the pessimistic scenario will happen. NASA seem to be more optimistic here.

The real question is, who knows if those threats are real threats or just paranoia ?
I think what could be the worse scenario is: we will do nothing about it and it will happen, all at once.
Again, what seem to be most important here is more and more accurate prediction techniques and monitoring. Who cares if it takes money and human resources ? We will be overpopulated soon, anyway : P