A far more interesting question for me is the system response.
Let's stipulate, for the purposes of the next bit of discussion, that it a fact that human activity is changing the balance of gases in the atmosphere and causing a net increase in warmth. So the planet is warming up and up and up.
Our planet is a system, and by that I mean it has a number of dynamic processes that all run along that are being influenced by other processes. The basis of a climate model is to capture all of those processes and include them in the model so that given changes to a particular part of the system you can predict how the other parts will respond.
To Vivek's point, the "best" model would let you take all of the data you can get and put that in for the various modeled parameters at any given time and get back an answer. But we don't have one of those models.
For example, none of the IPC's models allow for an ice age even though we know we have had ice ages. I haven't looked recently but the literature on what might be the factor behind the cycle of warm and cold cycles is still unknown.
Second, we have ice cores which suggest that with much lower CO2 levels the planet was warmer than it is now, and which much higher CO2 levels it was cooler than it is now. As a result, the research we've done, the models we've produced, are accurate for a small set of conditions which we have been observing for a very short period of time (even giving credit for the first Sumerian tablets).
We also know that there have been periods of time where this planet was unable to support human life. Not that we hadn't evolved or emerged yet, completely unable to exist. Once, like Mars today, our atmosphere was nearly all CO2!
And the really important bit, we don't know how to control climate change, it is pretty clear we can dump a lot of CO2 into the air and affect it but we don't have the necessary understanding, and models, needed to drive it in one direction or another. As a result, if we cut our carbon foot print to zero, we are still 100% at risk of extinction from climate change. Only then it will be due to one of the other things that we're not in control of doing it instead of us. There isn't a climate scientist out there who has ever gone on the record to say "do this and it will 'end' global warming." They can't because we don't know enough to know what would drive the climate to stay in it's current state forever.