Is there warming in the pipeline? A multi-model analysis of the zero emission commitment from CO2

Abstract

<p>The Zero Emissions Commitment (ZEC) is the change in global mean temperature expected to occur following the cessation of net CO<sub>2</sub> emissions, and as such is a critical parameter for calculating the remaining carbon budget. The Zero Emissions Commitment Model Intercomparison Project (ZECMIP) was established to gain a better understanding of the potential magnitude and sign of ZEC, in addition to the processes that underlie this metric. Eighteen Earth system models of both full and intermediate complexity participated in ZECMIP. All models conducted an experiment where atmospheric CO<sub>2</sub> concentration increases exponentially until 1000 PgC has been emitted. Thereafter emissions are set to zero and models are configured to allow free evolution of atmospheric CO<sub>2</sub> concentration. The inter-model range of ZEC 50 years after emissions cease for the 1000 PgC experiment is -0.36 to 0.29 <sup>o</sup>C with a model ensemble mean of -0.06 <sup>o</sup>C, median of -0.05 <sup>o</sup>C and standard deviation of 0.19 <sup>o</sup>C. Models exhibit a wide variety of behaviours after emissions cease, with some models continuing to warm for decades to millennia and others cooling substantially. Analysis shows that both ocean carbon uptake and carbon uptake by the terrestrial biosphere are important for counteracting the warming effect from reduction in ocean heat uptake in the decades after emissions cease.</p><p> </p><p>Overall, the most likely value of ZEC on decadal time-scales is assessed to be close to zero, consistent with prior work. However substantial continued warming for decades or centuries following cessation of emission is a feature of a minority of the assessed models and thus cannot be ruled out.</p>

A committed fourfold increase in ocean oxygen loss

“…The stabilization of global-mean surface air temperatures after emissions stop by the end of year 2020 is a result of the close cancellation of the warming effect from declining ocean heat uptake and the cooling effect of declining atmospheric CO 2 3 , 5 (Fig. 1a, b ).…”

“…The Earth system is not in equilibrium with current levels of atmospheric CO 2 that have experienced an increasingly rapid growth as a result of anthropogenic CO 2 emissions, with about half of all anthropogenic CO 2 emitted during the past 35 years only 1 . Theory and models nevertheless predict that global-mean surface temperatures would essentially stop rising further and remain relatively stable for many decades to centuries once CO 2 emissions are stopped 2 – 5 . This is also implicit in the concept of transient climate response to cumulative CO 2 emissions (TCRE 6 , 7 ), which provides the scientific rationale for relating temperature targets to remaining carbon budgets.…”

“…According to current models, the remaining radiative forcing during this phase of ocean adjustment to the anthropogenic perturbation is closely balanced by oceanic heat uptake 3 , 4 . The relative stability of global-mean surface temperatures within a few tens of degrees upon ending CO 2 emissions 5 thus comes at the expense of increasing acidification and ocean warming 11 . Ocean warming and associated thermal expansion of seawater adds to committed sea-level rise 12 , 13 , which primarily results from committed melting of inland ice 14 , 15 .…”

Estimating remaining carbon budgets using temperature responses informed by CMIP6

“…It also turns out that the warming is approximately proportional to the size of the lump and not strongly dependent on the background CO 2 concentration. The implication is the linear dependence of GSAT on cumulative emissions, and hence the GSAT will not increase if the emissions stop; the temperature maximum will coincide with the time the annual emissions drop to zero (Matthews et al, 2017;MacDougall et al, 2020).…”

Estimating remaining carbon budgets using temperature responses informed by CMIP6

“…It also turns out that the warming is approximately proportional to the size of the emission increment and not strongly dependent on the background CO 2 concentration. The implication is the linear dependence of GSAT on cumulative emissions, and hence the GSAT will not increase if the emissions stop; the temperature maximum will coincide with the time the annual emissions drop to zero (Matthews et al, 2017;MacDougall et al, 2020).…”