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

Abstract:

The Transient Climate Response to Cumulative Emissions (TCRE) is a measure of the climate response to cumulative emissions of carbon dioxide. Recent studies have shown that the TCRE 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 emission rates represented by the current RCP emissions scenarios. Here we use an ensemble of climate model simulations to quantify how the TCRE varies as a function of both the total magnitude and also the rate of CO2 emissions. We show that the long-term temperature response to a pulse CO2 emission (1.81°C per TtC) does not depend on the magnitude of cumulative emissions up to 3 TtC. The transient response to constant emissions, when calculated at the point of doubled atmospheric CO2 (1.66°C per TtC), is also remarkably insensitive to the rate of emissions for emissions as high as 30 GtC per year. There is evidence, however, that for very high emission rates, the warming does not keep pace with the rate of emissions, and the TCRE tends to decrease with time, especially as cumulative emissions also increase. We conclude that the TCRE provides a robust 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 RCP 8.5.