The warming limit of 2°C has been widely endorsed. The European Union and several prominent international NGOs, including Oxfam and Christian Aid, had advocated the limit for several years before the sixteenth Conference of the Parties (COP 16) to the UNFCCC formally adopted it in Cancun in 2010. The temperature goal is, however, to a large extent arbitrary. Because of cascading uncertainties associated with various feedbacks in the climate system, we are unable to predict if dire consequences owing to warming might befall us well before an equilibrium warming of 2°C. Moreover, whether it is reasonable for least-developed and developing countries to accept a mitigation plan designed to keep warming within that temperature target depends on whether the plan lays costs on them that are likely to prolong or deepen poverty by raising their energy costs. But because it seems possible to limit the warming to 2°C in a manner that would prevent heaping costs on the least-developed and developing countries, and because of the projected economic costs of trying to limit warming at a lower temperature, the 2°C limit has broad credibility. Despite this credibility, international negotiations have so far failed to produce an agreement that would limit emissions of greenhouse gases sufficiently to have a good chance of achieving the warming limit.