Highlights of the IPCC Working Group I (WGI) Fifth Assessment Report (AR5) are the essence refined from the researches in the field of climate change physical science in the past seven years. More than half of the observed increase in global average surface temperature since the 1950s was caused by the human influence. Nighty-three percent of the energy resulting from the anthropogenic CO₂ emissions since 1971 is stored in the ocean. Besides, ocean has absorbed about 30% of the emitted anthropogenic CO₂, causing the decrease in pH of ocean surface by 0.1, etc. Based on the CMIP5 models, it is projected that global warming will continue. Relative to 1986-2005, the global mean surface temperature by the end of the 21st century will increase by 0.3~4.8℃. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions. Controlling the warming caused by anthropogenic CO₂ emissions alone with a probability of >66% to less than 2℃ since the period 1861-1880, will require cumulative CO₂ emissions from all anthropogenic sources to stay between 0 and about 1000 Gt C since that period.

In the past two decades, the cognition on the facts, causes and future trends of climate change has been strengthened due to the improvement of observational measures, the enhancement of analysis methods and the development of climate models. It is further confirmed that warming of the climate system is unequivocal, anthropogenic activity is the main cause of global warming since the mid-20th century and further warming will continue in the climate system in future. IPCC assessment reports on climate change have greatly promoted the progress of the physical sciences. They also lay the solid scientific foundations for adapting to and mitigating climate change.

The IPCC assessment reports play an important role in the international climate change negotiation process. The IPCC Fifth Assessment Report (AR5) has been identified as one of the important information sources for the negotiation of a new agreement by the end of 2015 and will have significant influences on the ongoing negotiations on mode of international cooperation to address climate change in the post-2020 period. Based on new and comprehensive observations and advanced model simulations, the newly released IPCC Working Group I (WGI) report reaffirms the fact of global warming trend, strengthens the causality between anthropogenic emissions and global temperature rise and establishes the quantitative relationship between cumulative greenhouse gas emissions and global temperature rise. This paper identifies some key conclusions of the WGI report and analyzes their policy implications, especially their influences on the ongoing negotiation process and the new agreement. It also discusses the role of scientific information in decision-making process, and how this information could be absorbed and used in a scientific manner so that it could better serve the purpose of assisting us in the process of international negotiations and domestic decision making.

Based on a new algorithm to calculate apparent temperature, a novel criterion to evaluate human comfort levels was devised and applied to calculate climate comfort by using daily routine surface observational data from 1 January 1961 to 31 December 2012. Based on daily mortality from circulatory system disease and Nanjing meteorological data from 1 January 2005 to 31 December 2008, characteristics of comfort levels and apparent temperature during this period were analyzed. Then, the relationship between comfort levels and mortality was analyzed. The hot uncomfortable days show an upward tendency, however cold uncomfortable days show a downward tendency (-3.2 d/10a). Early April to mid-May and October are two comfortable periods in Nanjing. Mean apparent temperature has a rising tendency (0.3℃/10a) after 1995. Main comfort levels are comfort and slightly cold in Nanjing during the period of 2005-2008. Peak days of mortality from circulatory system disease occur in cold uncomfortable days. There was a successive high mortality period in 2008, which has a 7-9 d lag compared with the weather disasters process during this period.

The United Nations climate change negotiation is one of the most important multilateral processes in the past 20 years, which has been the center of interest of the international society. The latest debate on legal and procedural issues triggered by Russia, Ukraine and Belarus in 2013 shows that the rules and procedures on decision making process have become one of the major challenges for current and the future negotiations. The analysis is based on the ultimate objective of the UN process as well as its core value. Three different issues have been explored including the mandate, the rules and the decision making approaches. The research concludes that transparency, party-driven and consensus will continue to be the primary principles and decision making rule for the UNFCCC in future.