- Is the Earth warming?
- Are humans to blame?
- Will the effects of climate change be beneficial or disastrous?
- Can we do anything about it?
Duh. Of course it is. Next question.
I blogged on this point here. The bottom line is that we are virtually 100% certain that humans are contributing to the present warming, and we think it's likely that humans are contributing most of the warming over the last few decades. However, no one credible argues that humans are responsible for ALL of the warming.
Clearly, some people will benefit from warming, while others will suffer. There are a lot of dimensions to this (economic, moral, etc.), but I'll just give the broadest answer. For small warming (e.g., 1 deg C over the next 100 years), current thinking is that harms and benefits are largely comparable, although it is estimated that harms still outweigh the benefits. As the warming increases, the harms get much bigger, and begin dominating over benefits somewhere around 2-3 deg C of warming. That's why 2-3 deg C is often referred to as a tipping point or threshold for dangerous anthropogenic interference. Warmings much greater, say 5 deg C, would be a calamity of Biblical proportions ... real Wrath of God stuff.
I don't know. I think the problem is largely political, but I'm hopeful that we can get our act together in the next decade to make the technical and societal changes necessary to stabilize atmospheric CO2 around 550 ppmv (double pre-industrial levels). If we fail, then we move on to other options (like geoengineering), but I think we have to at least make a legitimate effort to to reduce emissions.