Controls of the transient climate response to emissions by physical feedbacks, heat uptake and carbon cycling

Richard G Williams,Paulo Ceppi,Anna Katavouta

doi:10.1088/1748-9326/ab97c9

Richard G Williams, Paulo Ceppi + Show 1 more

Open Access

https://doi.org/10.1088/1748-9326/ab97c9

Copy DOI

Abstract

The surface warming response to carbon emissions is diagnosed using a suite of Earth system models, 9 CMIP6 and 7 CMIP5, following an annual 1% rise in atmospheric CO2 over 140 years. This surface warming response defines a climate metric, the Transient Climate Response to cumulative carbon Emissions (TCRE), which is important in estimating how much carbon may be emitted to avoid dangerous climate. The processes controlling these intermodel differences in the TCRE are revealed by defining the TCRE in terms of a product of three dependences: the surface warming dependence on radiative forcing (including the effects of physical climate feedbacks and planetary heat uptake), the radiative forcing dependence on changes in atmospheric carbon and the airborne fraction. Intermodel differences in the TCRE are mainly controlled by the thermal response involving the surface warming dependence on radiative forcing, which arise through large differences in physical climate feedbacks that are only partly compensated by smaller differences in ocean heat uptake. The other contributions to the TCRE from the radiative forcing and carbon responses are of comparable importance to the contribution from the thermal response on timescales of 50 years and longer for our subset of CMIP5 models and 100 years and longer for our subset of CMIP6 models. Hence, providing tighter constraints on how much carbon may be emitted based on the TCRE requires providing tighter bounds for estimates of the physical climate feedbacks, particularly from clouds, as well as to a lesser extent for the other contributions from the rate of ocean heat uptake, and the terrestrial and ocean cycling of carbon.

Highlights

Climate model projections reveal a simple emergent relationship that global-mean surface warming increases nearly linearly with the cumulative amount of carbon emitted since the pre-industrial era (Matthews et al 2009, Allen et al 2009, Zickfeld et al 2009, Gillett et al 2013, Collins et al 2013)
The surface warming response to carbon emissions is diagnosed using a suite of Earth system models, 9 CMIP6 and 7 CMIP5, following an annual 1% rise in atmospheric CO2 over 140 years
Intermodel differences in the to cumulative carbon Emissions (TCRE) are mainly controlled by the thermal response involving the surface warming dependence on radiative forcing, which arise through large differences in physical climate feedbacks that are only partly compensated by smaller differences in ocean heat uptake

Summary

Introduction

Climate model projections reveal a simple emergent relationship that global-mean surface warming increases nearly linearly with the cumulative amount of carbon emitted since the pre-industrial era (Matthews et al 2009, Allen et al 2009, Zickfeld et al 2009, Gillett et al 2013, Collins et al 2013). This relationship is important as the sensitivity of warming to cumulative carbon emission dictates how much carbon may be released before reaching dangerous climate (Meinshausen et al 2009, Zickfeld et al 2009, Matthews et al 2012). For a 2◦C warming target, the maximum permitted emissions extend from

Objectives

Methods

Findings

Conclusion