Simulated AMOC projections as part of AMOCMIP project. Results of eight GCMs are shown for the historical period combined with (left column) RCP4.5 and (right column) RCP8.5 and for the experiments without GrIS mass loss, with gGrISmelt and with rGrISmelt. Results are given for AMOC strength at 26°N (below 500 m; (top row) Sv) and for (bottom row) changes (%) in the AMOC strength at 26°N relative to 2006. A 50 year running mean is applied. Depicted RAPID data are an average over all available data between 2004 and 2014 [.,], with uncertainty bars reflecting the year‐to‐year variability (1= 2.2 Sv).

For the first decade (2006–2016) the simulated maximum AMOC strength at 26°N differs substantially between GCMs, but the multimodel mean of 16.5 ± 3.0 Sv ( μ ± 1 σ ) is consistent with the observed mean magnitude of 17.2 ± 2.2 Sv (1 σ of interannual variability for period 2004–2014) [ McCarthy et al ., 2015 ] (Figure 1 ). The simulated AMOC projections indicate similarities, but also significant intermodel differences, both in terms of AMOC weakening as well as AMOC sensitivity to GrIS meltwater. For RCP4.5 all models show a weakening until about year 2100 followed by a stabilization or recovery. By the year 2300 the AMOC has resumed its present‐day strength in two models, while four show a sustained weakening of ~20–40%. Both the ACCESS1.0 and IPSL‐CM5A‐LR simulations show a substantial impact of increased GrIS meltwater in RCP4.5, almost doubling the AMOC weakening compared to the simulations with only global warming. For RCP8.5 all models indicate stronger and longer AMOC weakening compared with RCP4.5 and no significant recovery until year 2300, but the magnitude differs between models.

3.2 Probabilistic AMOC Projections

The results for the intermediate GHG mitigation scenario RCP4.5 indicate an AMOC weakening of 18% by years 2090–2100 (median; compared to 2006) with 3–34% for the 90–100% probability interval and a stabilization after that (Figure 2). In the continued high greenhouse‐gas emission scenario RCP8.5, the AMOC weakens by 37% [−15%, −65%] in years 2090–2100 and by 74% in years 2290–2300, albeit with a large 90–100% probability interval from a 4% strengthening to a 100% weakening. The presented values for the years 2090–2100 are similar to previous probabilistic estimates [Schleussner et al., 2014], providing confidence in the applied methodology. The mean of the GCM‐based AMOC projections shows a similar evolution albeit a somewhat larger decrease compared with the AMOC emulator‐based median (black and red lines in Figure 2, respectively). However, it is important to realize that the two means are not expected to overlap. This is because the number of models included in the GCM‐based mean differs per scenario (RCP4.5 versus RCP8.5) and per time step. On the contrary, the AMOC emulator‐based median effectively uses the available GCM scenarios to extrapolate and fill all missing data for all participating GCMs. Moreover, the relatively small AMOCMIP ensemble may be biased relative to the larger CMIP5 ensemble that is used in the forcing of the emulator.

Figure 2 Open in figure viewer PowerPoint Probabilistic AMOC projections for RCP4.5 and RCP8.5 and impact of individual forcings. AMOC strength changes at 26°N (below 500 m; %) showing (from left to right) the combined impact of increasing temperatures and GrIS mass loss (Climate change including GrIS), GrIS mass loss only (GrIS only) and increasing temperatures only (Climate change excluding GrIS). Results are given for the median and three different confidence intervals (black line and blue shadings, respectively). Also indicated are the multimodel means over the AMOC evolution simulated by the GCMs (red line labeled GCM MMM; averaged over gGrISmelt and rGrISmelt if both exist). Note that the number of models used in the average differs over time and scenario.

The single forcing simulations show that both in the GCMs and the AMOC emulator the warming‐induced AMOC weakening dominates over the GrIS meltwater impact (Figure 2). Nonetheless, the effect of increased GrIS mass loss is nonnegligible and induces an additional median AMOC weakening of about 37% in the years 2290–2300 in RCP8.5. Moreover, in the AMOC emulator the effects of increased GrIS mass loss are largest during the latter half of the projections, since GHG forcing alone leads to a stabilization of the AMOC during the 23rd century, but the simulation including GrIS mass loss shows a continuing decline until year 2300. The latter appears to be in contradiction with the GCM results that show only very little impact of GrIS mass loss in RCP8.5 (Figures 1 and 2); however, a RCP8.5 simulation with the apparently most sensitive GCM (IPSL‐CM5A‐LR) is not available for the GCM‐based mean, thus partly explaining the mismatch. Probabilities for an AMOC collapse in years 2290–2300, defined here as a 90% reduction of its strength, increase from 20% to 44% when GrIS mass loss is included. These results are generally consistent with previous uncoordinated single‐model GrIS mass loss experiments that suggest that the impact of increased GrIS melting is small for rates below ~0.1 Sv [Fichefet et al., 2003; Hu et al., 2009; van den Berk and Drijfhout, 2014; Swingedouw et al., 2015].

In the GCM AMOCMIP experiments, future changes in GrIS solid ice discharge are neglected. We have performed idealized experiments with the AMOC emulator imposing ±1% yr−1 changes of the observed value of ~0.016 Sv yr−1 [van den Broeke et al., 2016] to investigate the impact of such high‐end solid ice discharge changes, high‐end since they translate into a doubling or complete cessation of GrIS solid ice discharge within the next ~70 years (see supporting information for details). The simulated impact of these high‐end GrIS solid ice discharge experiments is an increase (decrease) of the AMOC weakening for more (less) solid ice discharge (Table S2). The impact of increased solid ice discharge is larger than that of a decrease of the same magnitude, which illustrates nonlinearities in the system. The largest effect is seen for increased solid ice discharge in scenario RCP8.5 at year 2300, where the AMOC is decreased by 91% compared to 74% without changes in solid ice discharge. We stress that while these numbers give an impression of the potential importance of future changes in GrIS solid ice discharge, more constraints are required before its effect on future AMOC projections can be fully incorporated.

A more direct connection between the AMOC projections and risk assessments of global climate change can be made by examining yearly AMOC weakening values with respect to the period 1971–2005 as a function of global temperature change (Figure 3). This shows that for a global warming of 2 K above preindustrial levels, often presented as the safe limit, the AMOC is projected to weaken by 15% (90–100% probability interval of 3% strengthening to 28% weakening), and the probability of an AMOC collapse is negligible. This is contrary to a recent modeling study [Hansen et al., 2016] that used a much larger, and in our assessment unrealistic, Northern Hemisphere freshwater forcing (~0.1–0.25 Sv versus ~1–4 Sv, respectively) and finds a substantially larger probability of an AMOC collapse. According to our probabilistic assessment, the likelihood of an AMOC collapse remains very small (<1% probability) if global warming is below ~5 K relative to preindustrial, comparable to cumulative anthropogenic carbon emissions of ~2810 Gt C (using a linear scaling to global temperature change of 0.0016 K/Gt C) [Stocker et al., 2013]. Probabilities increase for greater global warming (11% for 6 K, 19% at 7 K, and 30% at 8 K; Figure 3). Including changes in GrIS solid ice discharge into our assessment only has limited impact on the calculated AMOC collapse probabilities. Whereas the influence of decreases in solid ice discharge is negligible, enhanced rates could increase the likelihood of an AMOC collapse by ~2–22% for global warming of 5–8 K (Figure S5).