We do know that carbon dioxide and other gases trap and re-radiate heat. We also know that humans have emitted ever-more of these gases since the Industrial Revolution. What we don't know is exactly how sensitive the climate is to increases in these gases versus other possible factors--solar variability, oceanic currents, Pacific heating and cooling cycles, planets' gravitational and magnetic oscillations, and so on. Man-Made Solar Variability, that's the answer. I need a Research Grant.
Given the unknowns, it's possible that even if we spend trillions of dollars, and forgo trillions more in future economic growth, to cut carbon emissions to pre-industrial levels, the climate will continue to change--as it always has.