Again, we have models out there that look at the carbon cycle and what stabilization scenarios you must reach, in order to get levels of atmospheric carbon dioxide to prescribed levels. For example, you can look at emissions pathways into the future, and you'll see that if you cut emissions by about 50% and stabilize them, you'll stabilize the atmosphere with a carbon dioxide level of about three to four times the pre-industrial level, which is not good. This is very high, and there's a warming that the earth has not seen since humans have been on it, or in fact any time since mammals have been the dominant species. This is not the type of warming we want.
There's a huge amount of data—paleo-environmental as well as model data and basic physics—calling for these changes.
I don't know if I've answered the question. We know how the climate system responds to carbon dioxide.
Even Bill Nigh, the science guy, has a wonderful little experiment in which you put carbon dioxide in one and pure air in another and then look at the effect.
We know what will happen in terms of the broad effects. We don't know exactly which policy paths will take us where we'll be, but this is why we have to make decisions now.