After Durban: Risk of delay in raising ambition lowers chances for 2°C, while heading for 3.5°C

Summary  The Durban Climate Summit concluded with the groundbreaking establishment of a new body to negotiate a global agreement covering all countries by 2015 (Ad Hoc Working Group on the Durban Platform for Enhanced Action). With a new agreement not scheduled to take effect until 2020 the new agreement appears unlikely to affect the level of action in 2020 already pledged.  As the agreements in Durban do not propose additional action before 2020 the risk of exceeding 2°C remains very high. Action to implement the Durban Agreements will need to be quick to increase emission mitigation and hence have a chance at reaching this goal. Catching up on postponed action is costly and the technological and economic options required to do so are largely untested - or unknown. The Climate Action Tracker estimates that global-mean warming would reach about 3.5°C by 2100 with the reduction proposals currently on the table.  Impacts of climate change are likely to be considerably more severe in all parts of the world for a 3.5°C warming above pre-industrial (compared to 1.5°C or 2°C): o Regional temperature over land areas is projected to increase much more than the global average, and would be strongly amplified in some regions (e.g. 4°C global average means 6-8°C in southern Europe, 2°C global average means 3-4°C in that region) o Crop yields are projected to decline in most regions, while water stress will become a major issue in many regions, especially parts of Africa, Central Asia and parts of Latin America. o Several potential global-scale tipping points are associated with warming beyond 3°C that are not associated with 1.5°C or 2°C, such as:  the possible dieback of the Amazon rainforest  corals reefs dissolving and being irreversibly replaced by algae and sea grass  irreversible long-term loss of the Greenland ice sheets  risk of release of methane hydrates in ocean floor sediments  permafrost thawing due to fast rising arctic temperatures o Approximate estimates indicate that the most extreme costs will be felt in West Africa and South Asia, with 3.5% of regional GDP in residual damage for 2°C warming and 5-6% for 3°C warming. o With a 2°C warming, adaptation costs would be half those associated with a 3°C temperature rise. As with the impacts, regional cost of adaptation and residual damage are highly diverse.