My own preferred statement of The Theory of Global Warming is this:
An increase in the atmospheric concentration of CO2 and other long lived greenhouse gases requires the surface temperature to ultimately increase so as to maintain a balance with the absorbed solar radiation. The increase is amplified by water vapor (also a greenhouse gas), which increases with temperature in such a way as to keep relative humidity approximately constant. Melting of ice will further amplify the warming, particularly in high latitudes. The resulting widespread warming corresponding to a doubling of CO2 will be large enough and rapid enough to be well outside the range of past experience of the human species, by an amount comparable to the difference between a glacial and interglacial climate. Changes in atmospheric cloud properties may somewhat reduce or increase the sensitivity, but do not substantially alter the conclusion.
The last part of the statement of the theory is, of course, the hard part, and the most uncertain.
I have deliberately left the matter of the severity of the impacts of such a climate change out of the hypothesis. Theories regarding the impact are nascent and in many regards still rather ill-formed, in comparison to the theory dealing with the physical dimensions of climate change. Also, insofar as there are uncertainties about the severity of the impacts of climate change, it is a matter for the political apparatus to decide how to deal with the uncertainties, and the extent to which one should pay attention to the worst case vs. the most likely case. The question of how to factor in the uneven distribution of harms (and possibly benefits) across the peoples of the Earth, and between human societies and natural ecosystems, is also at heart a matter of ethics and values. These are questions that can be informed by science, but they are not themselves scientific questions.