When I was a kid, there were many who said "global cooling" was a fact.
Similarly, by 1979 we were supposed to run out of aluminum, copper, and other important metals according to many scientists.
I am not convinced that humans are having a discernable effect on the global temperature. 2003, for instance, has been unusually cool here in Michigan.
If one looks back at temperatures for the past 3000 years, they'll find that it isn't even particularly warm right now.
There are numerous obstacles in front of those who worry about global warming.
#1 They have to prove (i.e. really prove -- I know I'm not convinced and I'll put my scientific knowledge up against anyone here) that the recent increase in temperature is a trend that is going to continue into the future. I can be persauded on this but I'm not convinced. There have been similar rises and falls in temperature in the last 10,000 years.
#2 If #1 is demonstrated, then they have to prove that humans are doing so. To a lay person, the amount of CO2 we humans put into the atmosphere sounds huge. But compared to say, a volcanic eruption, it's trivial.
#3 If #2 is proven, then they have to show that global warming is harmful. The planet has, historically speaking, been warmer than it presently is for most of its existence. Sure, we can talk about the ice caps melting as some sort of dooms day worse case scenario but even that is unlikely. And many parts of the world would benefit from it being warmer. I"m not saying that we're better off with global warming, I am saying that we cannot assume that temperatuers of say 5 degrees are going to be definitively worse for humans on the whole.
#4 If they prove #3 then they have to come up with a plan of action that is effective. The Kyoto treaty, for instance, was a joke. India and China, two of the biggest polluters on the planet, weren't even required to do anything getting "developing country" status. My rule of thumb: If you can develop nuclear weapons, you're not a "developing country". A useful treaty would be to have energy used be based on economic output. This is what Bush proposed and was laughed at by environmentalist. But that should be the goal. Efficiency of energy use.
I'm particularly skeptical of #4 because a truly workable solution would require some real sacrifices and sacrifices that wouldn't be proportional. It would also require the same enviromental wackos who worry about global warming to quit blocking nuclear power which is the one high volume energy source that has no significant CO2 emmissions. Crippling the US economy through arbitrary limits on CO2 emmissions is not the answer. It not only won't make much change in how much CO2 humans put out but it would have very negative effects on the rest of the world too.
In other words, the blame America first crowd would need to straighten out and look at this issue as an engineering problem rather than a big political axe to grind.
My general view is that if there is indeed global warming and humans are the cause of it, we better get used to it.
