A fine question!
I'm not really
qualified to throw numbers around, but I'll take a stab at it. What I can say with confidence is that I believe these figures
are a pretty accurate representation of reality.
Of the climate forcings listed there, the only one not affected by human activity is the sun's output. There are varying levels of uncertainty about exactly how much we affect all the others, but the most clear-cut component is CO2. Increases in CO2 levels are clearly the result of human activity (history of industrialization, isotope ratios, and raw quantities all line up quite nicely), it's the biggest single climate forcing factor, and we're well on our way to doubling CO2 concentrations in the atmosphere. (Doubling is the target for stabilization, and it's an ambitious target that will take a lot of work.)
So, okay, there's a lot going on there, but let's look just at CO2 and solar activity. Solar variability for the 20th century is less than 1%, I gather, so that's about 0.03 W/m2
, while doubling the CO2 component would bring you from 1.5 to 3. So that looks like about 2 orders of magnitude to me, give or take. And that's just CO2; add in the other greenhouse gases and it's even more imbalanced on the side of human activity.
There is definitely a solar component to climate change, but it's comparatively small. This is a question that's been resolved only in the last few years, so there was a lot of argument about it not too long ago. I remember a presentation at work a couple years ago where they were talking about the famous "hockey-stick" graph, and explaining that even including solar variability, the only way the modelers could get the models to track with the observed data at all was by including anthropogenic effects.
Does that summarize pretty well?