But seriously, one Peter Kelemen has written very nicely on the hacked CRU emails and the implications, as well as making it clear on what grounds it is thought that 1) human activity has caused a steep increase in CO2 emissions, and that 2) increased CO2 emissions leads to a warmer atmosphere (on page 4).
For these reasons, and based on carbon isotope data, it is all but certain that the present, unprecedented rise in CO2 is due mainly to human output. But one cannot rule out with complete certainty other factors, for example, global warming itself, that could also be significantly contributing to the atmospheric CO2 increase.
CO2 (and methane) in the atmosphere are nearly transparent to UV and visible radiation, but absorb in the infrared, creating a "greenhouse." If it were not for this, the surface of the Earth would be much colder. This leads to the inference that increasing CO2 will lead to increasing warmth. Also, high CO2 concentration in the air leads to ocean acidification, which is probably bad for coral and perhaps also for plankton that make their shells from calcium carbonate (soluble in acid) and form the base of the ocean food chain.