there is no thing as a proof in sciences (except for formal sciences like mathematics and logic), you can only proof that something is wrong, but never that something is right.
While I will agree that this is applicable in some areas of science, like the theory of evolution, gravity, theoretical physics, etc. You can prove things in science, what you can't do is prove a negative which is why the burden of proof is on the person making the affirmative claim (ie global warming exists). And even if you can't prove something you still need to have a proponderance of evidence with which to create a strong theory (like evolution and gravity). To date there simply isn't that proponderance of evidence for man-made global warming. In fact there are scientists out there that have just as much evidence disproving man-made global warming.
Here, take a look at what might happen over the next few years (could take decades or even centuries) as a result of global warming (man-made or otherwise):
1. Global Warming leads to snow and ice melt which in turn leads to increased water levels in bodies of water and a decrease of sunlight reflected back into space leading to more warming.
2. That increased warming causes increased evaporation of the various bodies of water leading to decreased water levels.
3. That increased evaporation leads to increased cloud cover which then leads to increased amounts of sunlight reflected back into space.
4. That increase sunlight relected back into space causes a period of global cooling.
5. The cooling allows the clouds to start releasing more precipitation.
6. The precipitation causes increased snow and and cooling along with precipitation creates increased ice.
7. This returns us to roughly the same place as we were in part 1 of this cycle and the cycle repeats.
Now are all of these steps going to happen, I don't know I'm not a scientist but based on my limited knowledge of science I would say that they certainly could and would render global warming as part of a cycle that will self correct.