The United Nations' Intergovernmental Panel on Climate Change publishes a report on the consensus view of climate change science about every five to seven years. The first findings of the IPCC's Fifth Assessment Report (AR5) were released on Sept. 27, 2013, in the form of the Summary for Policymakers report and a draft of IPCC Working Group 1's Physical Science Basis. The IPCC does not perform new science but instead authors a report that establishes the established understanding of the world's climate science community.
The report not only includes observations of the real world but also the results of climate model projections of how the Earth will respond as a system to rising greenhouse gas concentrations in the atmosphere. The IPCC's AR5 relies on the Coupled Model Intercomparison Project Phase 5 (CMIP5) effort, an international effort among the climate modeling community to coordinate climate change experiments.
These visualizations represent the mean output of how certain groups of CMIP5 models responded to four different scenarios called Representative Concentration Pathways (RCPs). These four RCPs – 2.6, 4.5, 6 and 8.5 – represent a wide range of potential worldwide greenhouse gas emissions and sequestration scenarios for the coming century. The pathways are numbered based on the expected Watts per square meter – essentially a measure of how much heat energy is being trapped by the climate system – each scenario would produce. The pathways are partly based on the ultimate concentrations of carbon dioxide and other greenhouse gases. The current carbon dioxide concentration in the atmosphere is around 400 parts per million, up from less than 300 parts per million at the end of the 19th century.
The carbon dioxide concentrations in the year 2100 for each RCP are:
RCP 2.6: 421 ppm
RCP 4.5: 538 ppm
RCP 6: 670 ppm
RCP 8.5: 936 ppm
Each visualization represents the mean output of a different number of models for each RCP, because data from all models in the CMIP5 project was not available in the same format for visualization for each RCP. All of the models compare a projection of precipitation changes from 2006-2099 to a baseline historical average from 1971-2000.
Thus, the values shown for each year represent the departure for that year compared to the observed average precipitation from 1971-2000. The IPCC report used 1986-2005 as a baseline period, making its reported anomalies slightly different from those shown in the visualizations.