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.

What will be the contribution to sea level rise of the ice sheets in Greenland and Antarctica over the coming century?

Sea level change is one the most outstanding issue in Earth System Science in terms of scientific and societal impact. The largest uncertainty in sea level projections is the rate of melting of ice sheets into the ocean in a warming climate. We are far from being able to make predictions of ice sheet evolution. Progress is urgently needed.

How and why is the cryosphere changing? How are these changes impacting people and eco-systems around the planet? What can be done to ameriolate these changes?

The cryosphere is one of the most dynamic, least sampled and least understood components of the Earth system. Changes in the cryosphere impact the water supply for millions, if not billions of people on the planet and seriously alter the availability of water for industry, irrigation, consumption and sanitation. Changing sea level *will* impact those on the coast, changes in seasonal snow cover will impact regional climate and the Earths albedo. Changes in the Earths polar ice sheets are alarming, but not yet fully understood. Changing ice and snow affects all other aspects of the Earth system – ecology, geology, hydrology, atmospheric science, marine systems, social systems, economic systems and human health. We cannot ignore the cryosphere. Obstacles – perceived relevance to the science funding agencies of tropical countries, politics of high latitudes, resource management challenges. In my opinion decreasing water supplies will be the cause of future international tensions.