But we don't ask who's going to win the Super Bowl. It's that we know that if we let 100 people in one isolated room to watch Super Bowl the room will be always warmer as long as people actually live in it and are allowed to eat (life also involves more heat from the food, assume we add them food all the time). The doubters claim that it doesn't have to happen. It does. We can calculate how much warmer approximately even if we can't be 100% sure for every minute, on the average the curve will match.
It's the same with the Earth. It's as isolated as the isolated room would be -- we know exactly how much heat can dissipate and how much must contribute to the increase of the temperature for the room, and we have quite good estimates for Earth too (less heat dissipates to the space when CO2 increases, and the Sun shines the heat nevertheless - we can nicely calculate even the contributions of the changes in orbit and some clever scientist did that even before the computers were available, it took years of calculations by hand then but it was done). We can be wrong for some particular year, but the direction and the average increase match.
The room is of course simpler than the Earth. Imagine having some water pumps, some fans and some chunks of ice in the room. You know the formulas for the water and ice but you have more to calculate for short term effects. But you still know there will be less ice as the time goes by and more water. And still always a bit warmer.