The World Bank has just released a new report which warns of the significant costs of a projected 4 degree Celsius increase in average global temperature. The concluding remarks of the report provide a good snapshot of its substance:

A 4°C world will pose unprecedented challenges to humanity. It is clear that large regional as well as global scale damages and risks are very likely to occur well before this level of warming is reached. This report has attempted to identify the scope of these challenges driven by responses of the Earth system and various human and natural systems. Although no quantification of the full scale of human damage is yet possible, the picture that emerges challenges an often-implicit assumption that climate change will not significantly undermine economic growth.15 It seems clear that climate change in a 4°C world could seriously undermine poverty alleviation in many regions. This is supported by past observations of the negative effects of climate change on economic growth in developing countries. While developed countries have been and are projected to be adversely affected by impacts resulting from climate change, adaptive capacities in developing regions are weaker. The burden of climate change in the future will very likely be borne differentially by those in regions already highly vulnerable to climate change and variability. Given that it remains uncertain whether adaptation and further progress toward development goals will be possible at this level of climate change, the projected 4°C warming simply must not be allowed to occur—the heat must be turned down. Only early, cooperative, international actions can make that happen.

The Executive Summary is here, and the full report is here. Also see Will Rogers’ brief summary at the Natural Security blog.