From some comments at initforthegold:

Question from Grant - "how can raising the temperature be other than good?" This is an area I'd like to learn more about myself: what are the likely impacts, and what's the scope for us taking action to avoid the negative impacts or adapt to them if we have to?

Here, I avoid discussing ways of measuring impacts, though there are a lot of places to start looking to answer your question. The IPCC, of course, has written entire reports on 'impacts, adaptation and vulnerability', as well as mitigation. The Copenhagen Diagnosis is good place to get an overview of the physical impacts and associated risks.

I think you can avoid a lot of the confusion about measuring impacts by remembering one thing: risk is expensive. Even if we had 100% certainty about global temperature changes, we wouldn't know exactly what the regional impacts were going to be. Starting in the present, this means insurers are calling for action. Without it, as they point out, it will become massively expensive, or just plain impossible, to insure against climate-related outcomes. One thing about insurers: you can be reasonably certain they've looked into the issue pretty thoroughly.

Risk has always been expensive: a study back in the 70s looked at the village of Daiikera in Rajasthan, near Jodphur. Monsoons mean an unreliable quantity of rain. The result: farmers cultivated many distant plots to hedge their bets because they knew only some would produce. The more carbon we put into the atmosphere, the more we face exactly this situation: any one food-producing region is going to be more at risk, and the cost of managing that risk will continue to rise. The Chicago Exchange has its roots in this kind of hedging, for producing egg and butter; the World Bank's work on agricultural risk nicely outlines various ways it can be managed, but none are free. I know I'm repeating myself, but - climate change just makes all this more and more expensive.

Studies of oil shocks also conclude that a lot of the associated cost is due to the risk, not the direct impact. Not knowing what prices are going to be in the near future damages firms' abilities to make choices about investment - and this is the case regardles of the direction the price is going in.

This Wired article has a great overview of the costs involved for flood defenses. It also points out that back in the 50s, Dutch engineers were quantifying the cost as: 'risk = probability of failure x projected cost of damage'. (See previous comment link for more on the associated costs.)

All that's before we get onto the risks of huge movements of people displaced by environmental problems. This is already happening, and has been for decades. Same as with all the other risks, pinning any event on climate change is very, very hard - but knowing that the aggregate numbers and risk will increase is a no-brainer.

So three basic facts to finish with: the carbon we put into the atmosphere will stay there for, I think, between 100 and 200 years. The climate forcing caused by that carbon will take 1000 years or more to equilibriate. So we won't fully know the outcome of our carbon experiment for many centuries, and every bit of carbon we add now is stuck there. The one outcome we do know is that sea-level rise will persist throughout those centuries. Nice quote from Nature: "the climatic impacts of releasing fossil fuel co2 to the atmosphere will last longer than Stonehenge, longer than time capsules, longer than nuclear waste, far longer than the age of human civilization so far."

And, as I say, though we can be certain (within error bounds) of some of the aggregate outcomes, the risk involved will only continue to increase over time - until we stop putting carbon into the atmosphere. But even then, we'll have to wait a loooong time to see the all the consequences play themselves out.