betazed, this started out as a short post but kept growing and growing

, sorry in advance.
I would say that anthropogenic climate change is a fact.
We have changed the albedo of the earth through agriculture and other modifications of the earth's surface as well as our atmosphere (e.g. cloud properties and soot).
We have changed the opacity of the atmosphere to infrared radiation.
We have changed the way elements are cycled through the earth system (notably nitrogen, but also others - the halogens and the ozone hole for example), which impacts the biosphere and thus secondarily affects various drivers of climate.
These are major forcings that drive climate.
The remaining questions are how the earth system will respond, and will that response be good or bad for continued human development. The answer to these questions is unknown in its details.
For example, it may be that we would be in another ice age right now if not for the impact of humans. I cannot say one way or the other.
We are a long way from being able to say what would have happened in the absence of anthropogenic changes. Just as we are a long way from being able to say what the next thousand years will bring that may actually be impossible.
We do better on shorter time scales, mostly because we can constrain so many variables with observational data (direct or indirect). This is the process Carlos mentions above.
I once brought up the Gaia hypothesis in this thread, gene90 sort of scoffed. It originates with a brilliant scientist, James Lovelock, as part of a NASA study on how to detect life on mars. Well, one aspect of the Gaia hypothesis goes as follows: atmosphere warms -> ocean warms -> more bugs grow in ocean -> bugs release more DMS into atmosphere -> DMS oxidizes to sulfate -> sulfate aerosol increases albedo thus offsetting warming.
To properly model this feedback we would need a full life cycle model of nearly the entire oceanic web of life. We are nowhere near being able to do that, even without having to incorporate it into a coupled atmospheric/ocean model.
However, we do currently measure sulfate mixing ratios at many places around the globe, and we measure global cloud properties from space.
This example shows the difference between trying to model this feedback, and monitoring whether it is currently taking place or not. We would know if cloud properties altered significantly (as we saw with Mt. Pinatubo). We did not have all this data even 50 years ago, heck 20 years ago.
Similarly for the sun, we can talk about whether the 11 year sunspot cycle is a good indicator of solar output - but we currently have satellites looking at the sun measuring its output. We can observe the receding glaciers and measure its impact on albedo. We can measure the specific contribution of each trace gas to the overall opacity of our atmosphere to infrared radiation.
The reason for action in the face of such uncertainty about the future is described by me in my first post in this thread (#135), and by Carlos in the end of his post #265. Personally I dont trust Gaia enough to let her run with our anthropogenic changes (as apparently basketcase and gene90 do). I also feel that it is necessary to start creating institutional structures capable of addressing climate change regardless of how the next 50-100 years actually end up.