Since the Lukewarmers (reasonable climate skeptics) agree on a figure of climate sensitivity that is likely to cover the 2C threshold, now is the time to look at what a 2C increase actually looks like for our home planet. The figure below is taken from the IPCC 4th Report.
(click on the figure to enlarge it)
As the figure shows, there are no set boundaries. These are just projections, rough ideas of what may come about. Things may be better (but see below) or they may be worse. The temperature change that we get from our persistent burning of fossil fuels may be more or (maybe) less.
One reason why 2C is very possible as an option is that the present climatic interglacial age that we have lived in for some 10,000 years is actually 2C cooler than the previous interglacials. Take a look at Fig2 here.
From the figure we see hundreds of millions of people subjected to water problems (droughts and floods). 30% of species, including corals, at risk of extinction. Problems with food production. Yes, some may gain, but others will lose. Damage from floods and storms, especially around the coast. Major health effects.
That is the 2C scenario. Now, in 2013, we are already seeing perturbations of the climate that are pretty serious, which strengthens the credibility of these scenarios.
The projections above, remember, are the effects that will be appearing at the level of temperature increase that is very likely to come about if the "Luke-warmer" climate skeptics are right. If their optimistic projections are wrong (and remember, they are in a tiny minority compared to the general community of climate scientists who think that 3-4C increases are much more likely), then the future looks frankly
alarming in need of a radical decarbonisation of the economic system.