What is the exact sensitivity of the climate system to changing greenhouse gas, dust and aerosol loading in the atmosphere?

The IPCC AR4 has shown that, although we understand better and better how the climate system works, there are still considerable uncertainties in evaluating its future evolution. Not only due to uncertainties in human emissions, but also because of flaws in our physical and biogeochemical understanding of the system. The unpredicted recent evolution of Arctic sea ice or of the Greenland ice sheet are magnificent examples of such flaws.

Although everyone legitimally wants to understand the impacts of climate change (which drives many other global changes such as water resources, biodiversity, agriculture, extreme events, etc…), or wants to develop strategies of mitigation and adaptation to global change, there is little hope in doing a good job in the two latter aspects of global change if one does not improve our physical understanding of the system.

Climate sensitivity is the clear prerequisite to all other physical aspects such as tipping points, feedbacks, thresholds, etc… Solving this question allows one to basically improve the response to all other questions, from the physics to the mitigation, going through the impacts.