Increase of the transient climate response to cumulative carbon emissions with decreasing CO2 concentration scenarios

Abstract

Near-constancy of the transient climate response to cumulative carbon emissions (TCRE) facilitates the development of future emission pathways compatible with temperature targets. However, most studies have explored TCRE under scenarios of temperature increase. We used an Earth system model (MIROC-ESM) to examine TCRE in scenarios with increasing and stable CO2 concentrations, as well as overshoot pathways in which global mean temperatures peak and decline. Results showed that TCRE is stable under scenarios of increasing or stable CO2 concentration at an atmospheric CO2 concentration (pCO2) double the pre-industrial level. However, in the case of overshoot pathways and a stable pCO2 scenario at a quadrupled pCO2 level, the TCRE increases by 10%–50%, with large increases over a short period just after pCO2 starts to decrease. During the period of pCO2 increase, annual ocean heat uptake (OHU) and ocean carbon storage (CO) (or cumulative ocean carbon uptake from the start of the experiment) exhibit similar changes, resulting in a stable TCRE. During the pCO2 decrease period, after a sudden TCRE increase when pCO2 starts to decrease, the OHU decreases and CO increases (relative to the pCO2 increase period) balance each other out, resulting in a stable TCRE. In overshoot pathways, the temperature distribution when the global mean temperature anomaly cools to 1.5 °C reveals small warming over land and large warming over the oceans relative to the 1% per annum pCO2 increasing scenario, particularly in some high-latitude areas of both hemispheres. The increase in TCRE with overshoot pathways decreases the carbon budget for the temperature anomaly targets in such scenarios. Our analysis showed a 16%–35% decrease in the remaining carbon budget for the 1.5 °C global warming target, in comparison with the reference scenario with a 1% per year pCO2 increase, for pathways peaking at the doubled pCO2 level followed by decline to the pre-industrial level.