There is not a single, credible climate model that is based on the assumption that mankind is '100% responsible' for climate change.
The fact is that there are a number of climate forcings, both manmade and natural. The question is not so much 'Is the climate changing?' but 'What, if any, impact are human activities having on global climate?'. Both manmade (anthropogenic) and natural forcings (as well as the complex feedbacks that they induce) need to be both understood and accounted for in order to make a credible model of the global climate. I think it would be fair to say that even the most advanced climate models still struggle to produce credible predictions of the future, but on the flip side, there are no credible models that can accurately explain/model observed climate change without factoring in human activity (e.g. atmospheric pollution from burning fossil fuels). In other words, it is practically beyond reasonable doubt that human activity can and does influence global climate, but it is harder to say exactly how and even harder to predict what effect that influence will have.
As Danoff alludes to above, however, perhaps the biggest and most troubling problem with anthropogenic climate change is that the cure could be even worse than the disease. Mankind has been unwittingly/inadvertently influencing global climate for centuries, but, ironically, now that we know that certain activities are contributing to global climate change, it may create the (dangerous) impression that we can or should actively do something to 'fix' the problem (e.g. by spraying cooling aerosols into the high atmosphere on purpose). The key issue here is the difference between 'influence' and 'control'. Some believe that no amount of human activity can influence the climate - they are almost certainly wrong. But many others, myself included, believe that no matter what we do, mankind/technology will prove incapable of actually 'controlling' the climate, and attempting to do so could be utterly disastrous. I think the best we can hope for is that we learn how to mitigate the effects of our activities and learn from the past.