At the United Nations Framework Convention on Climate Change Conference in Cancun, in November 2010, the Heads of State reached an agreement on the aim of limiting the global temperature rise to 2°C relative to preindustrial levels. They recognized that long-term future warming is primarily constrained by cumulative anthropogenic greenhouse gas emissions, that deep cuts in global emissions are required, and that action based on equity must be taken to meet this objective. However, negotiations on emission reduction among countries are increasingly fraught with difficulty, partly because of arguments about the responsibility for the ongoing temperature rise. Simulations with two earth-system models (NCAR/CESM and BNU-ESM) demonstrate that developed countries had contributed about 60-80%, developing countries about 20-40%, to the global temperature rise, upper ocean warming, and sea-ice reduction by 2005. Enacting pledges made at Cancun with continuation to 2100 leads to a reduction in global temperature rise relative to business as usual with a 1/3-2/3 (CESM 33-67%, BNU-ESM 35-65%) contribution from developed and developing countries, respectively. To prevent a temperature rise by 2°C or more in 2100, it is necessary to fill the gap with more ambitious mitigation efforts.climate modeling | Coupled Model Intercomparison Project phase 5 | Cancun pledge | climate ethics | geoengineering T he impact of human activities on climate change at global and regional scales, including surface temperature (1), sea-level pressure (2), tropopause height (3), precipitation (4), and ocean heat content (5), has been explored and assessed. Greenhouse gas emissions, mostly CO 2 , are the most important anthropogenic forcing on climate (6). The contribution of greenhouse gas emissions varies widely among nations in both the past and the future. As a result, the United Nations Framework Convention on Climate Change (UNFCCC) reached an agreement that each nation should accept its "common but differentiated responsibilities." This ethical construct demands attribution studies of the historical contribution of emissions to climate change (7). To date, research has tracked the causal chain of climate change from human activities to greenhouse gas emissions, to radiative forcing, and finally to climate change. However, this conventional methodological flow does not consider the reverse process or include feedbacks from climate change to greenhouse-gas concentrations via biogeochemistry or decision-making processes (8). More than 100 countries have adopted a global warming limit of 2°C or below (relative to preindustrial levels) as a guiding principle for mitigation efforts to reduce climate-change risks, impacts, and damage (9, 10). The relationship between the climate policy making and the 2°C target by an appropriate emission pathway has been studied in simple climate models and probabilistic analysis (11, 12). However, climate projection experiments under many emission scenarios, even the latest representative concentration pathways (RC...
