Creationists and those who firmly believe climate change isn't driven by humans miss the point: science isn't about providing certainty. It's about providing uncertainty.
Take gravity. Gravity is something that we can observe pretty easily just by dropping an apple. We can note correlations (massier planets seem to have larger gravitational fields, for example). We can guess that, since the flux per unit area through the surface of a sphere decreases as the inverse square of the sphere's radius, gravity is subject to the inverse square law. We can even posit underlying mechanisms linking gravity to a specific particle, like the Higgs boson. What we can't do is prove that we understand how gravity works, except in terms of other theories (like particle theory and general relativity). We also can't guarantee that gravity functions the same way (or at all!) in places out of our direct experimental reach -- we can just show that the cosmological motions we see match our expectations were gravity to work the same.
These are the same objections that people bring to evolution and climatology: we don't understand much about the underlying mechanisms in either area. We can't show that the same rules that we see operating today are the rules that operated 2,000 or 4,000 or 500,000 years ago. We can say that what we see in the fossil record and among living organisms today strongly suggests a single common ancestor for all life on earth; but we can't rule out the theory that God created the earth 6,000 years ago, because we don't have any objective observers from that time. We certainly can't demonstrate that human activity has caused climate warming, although there do seem to be significant correlations between human activity and climate change. (Note that correlation does not imply causation, though.)
So, why is gravity undisputed (except by Flat Earth people)? And why are climate change and evolution such hot topics? I'm not sure, but I can suggest a few reasons.
Gravity is undisputed today partly because no religion has made the precise mechanism a point of recent dispute. It used to be in dispute, though; remember Galileo? That, ultimately, was a dispute about gravity on the scale of our solar system. Yet no edicts about the Higgs boson, or general relativity, have emanated from the Catholic Church, and Bush doesn't seem to care about gravity.
Another reason that people don't argue much about gravity is that the theory of gravity is predictive. Given a comet's position and momentum, we can tell you pretty much where it's going to go. It's a little harder in atmosphere, but we do it very well -- think ballistic missiles, for example. This predictive power goes a long way towards quieting dissent with the theory, because if you can predict something people will generally believe you understand it pretty well. (We'll come back to this.)
Evolution, for better or for worse, is not in the same position. It's a major point of dispute in at least a few places, and it's not predictive in the least. Even worse, it can't be very specific in predictions, because it's a stochastic theory that is subject to historical contingency. We will never be able to predict what mutations will arise randomly, and we will probably never be able to predict what effect those mutations will have on ecosystems. We might be able to predict general trends, but that is still far away from being an exact science.
Climatology is a much younger science than either the physics of gravity or the study of evolution. Like evolution, and unlike gravity, it seems to be very sensitive to certain kinds of perturbations -- that is, it's "chaotic". Very small changes may have large effects elsewhere. Moreover we don't understand many of the basic processes very well, and we don't have good ways to measure even relatively simple things like energy input from the sun, much less complicated things like CO2 consumption. Climatology is certainly not a predictive science in general, although some things can be predicted, just like in evolution: if you know where a hurricane is today, you can guess pretty well where it's going to be tomorrow.
Climatology is also a big point of contention for economic reasons: global warming, in particular. Corporations don't want to reduce the emissions of greenhouse gasses because they believe that it will have a negative economic impact on them. Therefore they (or their proxies) attack global warming as an unproven theory, in order to undermine its impact on public policy. As with the religiously motivated attacks on evolution, this is definitely bad for science.
If we could predict climate, or predict the effects of evolution, presumably people would regard these theories as being more credible than they are now. Unfortunately it's impossible to turn evolution into a predictive theory, and it's going to be a while before we get a predictive handle on climatology. So both theories are amenable to attack on the charges of being "unproven".
And here we come to the nut: the scientific method can't prove anything, in general. It is is much, much better at disproving theories than it is at confirming them; any working scientist will agree with that! All that an honest scientist can say about gravity, or evolution, or global warming, is that they haven't been disproven yet. There are reasons to believe that gravity and evolution are pretty good theories, scientifically speaking, because they've withstood the test of time. I'm not very knowledgeable about climatology but I do know it's quite a bit shakier in its underpinnings. But attacking any of these theories for not having provided proof is missing the whole point of science, which is to disprove as much as possible.
People -- even many intelligent people who should know better -- frequently get this wrong. Michael Crichton, the prolific author of (among other books) Jurassic Park, gave an interesting lecture at Caltech where he talked about scientist's involvement in political debates on public policy. Nuclear winter and global warming were two examples where a strongly biased view has been pushed strongly and publicly by a relatively small cadre of scientists. Crichton's view seemed to be that scientists were no less fallible than anyone else, which is undeniable (though unpopular among scientists ;). What he missed, and what I think many scientists fail to emphasize, is that thus far the scientific method -- with objective measurements and peer review, in particular -- is the only proven method of discovery known to mankind. We ignore it at our peril.
Scientists can do their part by proudly admitting ignorance. It's not pleasant, but it's undeniable: did you know, for example, that the underlying mechanism by which evolutionary novelty arises is still in dispute? Yep! We still don't really understand how new traits arise! And did you know that the precise reflectivity of the earth -- which is a major determinant of energy input into our climate, and is directly linked to the "greenhouse effect" -- is still not easily measurable? Yep! No long-term trends available! And these are just two things I've worked on -- I'm sure there's an ocean of ignorance out there, just waiting to be publicized. That's science!
The flip side of the coin is that those who critically examine scientific theories should apply the same level of critical analysis to their own beliefs. This applies to postmodern lit-crit as much as it applies to religious believers -- and I think it's as important as science is, as a method for making public policy.
Note to readers: I've been thinking about writing something like this for a while. It's an ongoing project, so please e-mail me at firstname.lastname@example.org if you have thoughts, criticisms, or suggestions.