It is 'likely' that there is a natural cycle entering a warmer phase, but exacerbated by human activity. CO², Ozone, 'heat sinks' over cities, rain forest reduction, etc..are all PROVEN causes of various changes.

The real point is that, with the stakes being as high as they are, we cannot afford to wait until the worst happens, then analyze a lot of data and nod wisely and say "so THAT's why we are in trouble!"

When it is LIFE we are dealing with, the only sane way is to err on the side of caution. Unfortunately, that's not a popular business model.