Abstract
Abstract. The Zero Emissions Commitment (ZEC) is the change in global mean temperature expected to occur following the cessation of net CO2 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. A total of 18 Earth system models of both full and intermediate complexity participated in ZECMIP. All models conducted an experiment where atmospheric CO2 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 CO2 concentration. Many models conducted additional second-priority simulations with different cumulative emission totals and an alternative idealized emissions pathway with a gradual transition to zero emissions. The inter-model range of ZEC 50 years after emissions cease for the 1000 PgC experiment is −0.36 to 0.29 ∘C, with a model ensemble mean of −0.07 ∘C, median of −0.05 ∘C, and standard deviation of 0.19 ∘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 the carbon uptake by the ocean and the terrestrial biosphere are important for counteracting the warming effect from the reduction in ocean heat uptake in the decades after emissions cease. This warming effect is difficult to constrain due to high uncertainty in the efficacy of ocean heat uptake. Overall, the most likely value of ZEC on multi-decadal timescales is close to zero, consistent with previous model experiments and simple theory.
Highlights
The long-term temperature goal of the Paris Agreement is to hold global warming well below 2 ◦C and to endeavour to keep warming to no more than 1.5 ◦C (United Nations, 2015)
The remaining carbon budget can be estimated from five factors: (1) historical human-induced warming to date, (2) the Transient Climate Response to cumulative CO2 emissions (TCRE), (3) the estimated contribution of non-CO2 climate forcings to future warming, (4) a correction for the feedback processes presently unrepresented by Earth System Models (ESMs), and (5) the unrealized warming from past CO2 emissions, called the Zero Emissions Commitment (ZEC) (e.g. Rogelj et al, 2019a)
Ocean carbon uptake evolution is affected by ocean dynamics, changes to ocean biogeochemistry, and changes in atmosphere–ocean CO2 chemical disequilibrium, where the latter is influenced by land carbon fluxes (e.g. Sarmiento and Gruber, 2006)
Summary
The long-term temperature goal of the Paris Agreement is to hold global warming well below 2 ◦C and to endeavour to keep warming to no more than 1.5 ◦C (United Nations, 2015). Rogelj et al, 2018), which represents the total quantity of CO2 that can still be emitted without causing a climate warming that exceeds the temperature limits of the Paris Agreement We present the results of a multi-model analysis that uses the output of dedicated model experiments that were submitted to the Zero Emissions Commitment Model Intercomparison Project (ZECMIP). This intercomparison project explicitly aims to quantify the ZEC and identify the processes that affect its magnitude and sign across models (Jones et al, 2019)
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