PNNL

Abstract

This paper examines the technology implications of limiting the change in mean global surface temperature (GMST) to two degrees Celsius (2oC) relative to preindustrial temperatures. Understanding the implications of this goal is clouded by uncertainty in key physical science parameters, particularly the climate sensitivity. If the climate sensitivity is 2.5oC then stabilization implies stabilization of CO2 concentrations at less than 500 parts per million (ppm) with a peak in global CO2 emissions occurring in the next 15 years and with a decline in emissions to 3.1 petagrams of carbon per year (PgC/y) by 2095. Under such circumstances the value of technology improvements beyond those assumed in the reference case is found to be exceptionally high, denominated in trillions of 1990 USD. The role of non-CO2 greenhouse gases is important. Aerosols could produce significant feedbacks, though uncertainty is significant. If the climate sensitivity is 4.5oC or greater, it may be impossible to hold GMST change below 2oC. On the other hand if the climate sensitivity is 1.5oC, limiting GMST change to 2oC may be a trivial matter requiring little deviation from a reference emissions path until after the middle of the 21st century.