Oh, as to whether we won’t have catastrophic consequences, this whole business of measuring the sensitivity misses a big point. There are thresholds. As the amount of open water in the Arctic at high summer expands, we can get into a positive feedback loop. Warmer summers mean thinner ice next winter, which then melts sooner next spring, leading to more heating as sunlight strikes open water (dark) rather than ice (reflective). Past some point, no further radiative forcing due to increased CO2 is needed; the system will proceed on its own to a new equilibrium with a lot of open water in the summer.

Once that happens, the Greenland ice cap loses the protection it had up til now, that air coming down from the pole came down across ice, winter or summer, and was thus always cold. Washed by less-cold waters and bathed in less-cold air, the icecap becomes vulnerable.

Again, there may well be a threshold. Past some point, the Greenland ice cap fails even if CO2 levels go no higher. And that makes the world that much less shiny, once again. And thus, that much warmer.

Where this all ends is far from clear. What is clear is that there is the potential for step increases in equilibrium temperatures from small increases in CO2. If one of those steps takes us across the final threshold, in which sulfur-metabolizing bacteria win access to the sunlit layers of the ocean, we’re in very big trouble.

Hydrogen sulfide emissions from the seas

Kump, Pavlov and Arthur (2005) have proposed that during the Permian-Triassic extinction event the warming also upset the oceanic balance between photosynthesising plankton and deep-water sulfate-reducing bacteria, causing massive emissions of hydrogen sulfide which poisoned life on both land and sea and severely weakened the ozone layer, exposing much of the life that still remained to fatal levels of UV radiation.[47][48][49]

From the Wikipedia article on extinction events