Quantifying the Limits of a Linear Temperature Response to Cumulative CO2 Emissions

Recent studies have shown that the transient climate response to cumulative carbon emissions (TCRE) of the global temperature can be well approximated by a constant value for cumulative emissions up to about 2 TtC. However, there has been little attention given in the literature to how the TCRE varies across the range of emissions rates represented by the current RCP emissions scenarios. The authors use an ensemble of simulations generated using the University of Victoria Earth System Climate Model to quantify how the temperature response to cumulative emissions varies as a function of both the total magnitude and the rate of CO2 emissions. This study shows that the 500-yr response to a pulse CO2 emission (1.81°C TtC−1) does not depend on the magnitude of cumulative emissions up to 3 TtC. The TCRE (1.66°C TtC−1), which relates to the short-term response, is relatively insensitive to constant-rate emissions up to 30 GtC yr−1. This experiment shows that the formal way of estimating the TCRE—that is, at the point of CO2 doubling in an idealized scenario with a 1% yr−1 increase of the atmospheric concentration—is a highly robust measure. The authors conclude that the TCRE provides a good estimate of the temperature response to CO2 emissions in RCP scenarios 2.6, 4.5, and 6, whereas a constant TCRE value significantly overestimates the temperature response to CO2 emissions in RCP8.5.