The answer to the second question, I think, is "some". It is not 100% and it is not 0%.
Interpreted literally, this statement is obvious but useless. I expect you meant "it is something strictly between 100% and 0% and not particularly close to either one" (i.e. between 10% and 90%), which is a stronger statement, and not obvious a priori (not all trends have multiple unrelated causes). On what basis do you make that claim?
As far as I remember it is usually held that the positive consequences (e.g. better harvests, etc.) overwhelm the negative until the about +2 degrees C rise in the temperature, but it's all very uncertain.
I have heard it claimed that there is a consensus that the effects of warming will be positive through +2 C, but the sources making that claim seem to be quite short of a consensus themselves.
The fifth question, the most interesting of them all, is not in the domain of the climate scientists at all. It is essentially a social and a political question the answers to which are driven by values and trade-offs.
Indeed. That doesn't answer the question, though.
I expect you meant "it is something strictly between 100% and 0% and not particularly close to either one"
Yes, I do.
On what basis do you make that claim?
Earth has been warming up for a while (in terms of centuries) by now -- we're still coming out of the last Little Ice Age. On a more short-term scale for when we have direct instrumental data, I believe (from memory) that the current warming trend started around 1850 which is too early for CO2 to make a noticeable impact. And, of course, the graph of the global temperatures and the graph o...
Note: Please see this post of mine for more on the project, my sources, and potential sources for bias.
I have written a couple of blog posts on my understanding of climate forecasting, climate change, and the Anthropogenic Global Warming (AGW) hypothesis (here and here). I also laid down the sources I was using to inform myself here.
I think one question that a number of readers may have had is: given my lack of knowledge (and unwillingness to undertake extensive study) of the subject, why am I investigating it at all, rather than relying on the expert consensus, as documented by the IPCC that, even if we're not sure is correct, is still the best bet humanity has for getting things right? I intend to elaborate on the reasons for taking a closer look at the matter, while still refraining from making the study of atmospheric science a full-time goal, in a future post.
Right now, I'm curious to hear how you formed your views on climate change. In particular, I'm interested in answers to questions such as these (not necessarily answers to all of them, or even to only these questions).