(1) AN INTRODUCTION TO SCENARIOS ABOUT OUR FUTURE
In AR5 four Representative Concentration Pathways (RCPs) describe scenarios for future emissions, concentrations, and land-use, ending with radiative forcing levels of 2.6, 4.5, 6.0, and 8.5 W/m2 by 2100. Strong mitigation policies result in a low forcing level (RCP2.6). Two medium stabilization scenarios lead to intermediate outcomes: (RCP4.5, RCP6.0).
IPCC's AR5: 4 RCPs
RCP8.5 gets the most attention. It assumes the fastest population growth (a doubling of Earth’s population to 12 billion), the lowest rate of technology development, slow GDP growth, a massive increase in world poverty, plus high energy use and emissions. For more about the RCPs see “The representative concentration pathways: an overview” by Detlef P. van Vuuren et al, Climatic Change, Nov 2011.
RCP8.5 assumes a nightmarish world even before climate impacts, resulting from substantial changes to long-standing trends. It provides AR5 with an essential worst case scenario necessary for conservative planning.
Unfortunately scientists often inaccurately describe RCP8.5 as the baseline scenario — a future without policy action: “a relatively conservative business as usual case with low income, high population and high energy demand due to only modest improvements in energy intensity” from “RCP 8.5: A scenario of comparatively high greenhouse gas emissions” by Keywan Riahi et al in Climate Change, November 2011, This is a material misrepresentation of RCP8.5. Scientists then use RCP8.5 to construct horrific visions of the future. They seldom mention its unlikely assumptions.