Description

The new study by van Westen et al. is a major advance in AMOC stability science and follows one by Danish colleagues which made headlines last July, both looking for early warning signals for approaching an AMOC tipping point. The study results from a major computational effort using a state-of-the-art climate model to find the AMOC tipping point in a coupled global ocean-atmosphere climate model of good spatial resolution. It is the first systematic attempt to find the AMOC tipping point in a coupled global model, using the quasi-equilibrium approach which was pioneered in 1995 with an ocean-only model of relatively low resolution. The study confirms that the AMOC has a tipping point beyond which it breaks down if the northern Atlantic Ocean is diluted with freshwater, and that this tipping point has been demonstrated for the first time in a state-of-the-art global coupled climate model. It also confirms observational data from the South Atlantic which suggest the AMOC is on tipping course, and proposes a new, physics-based and observable type of early warning signal. The study also highlights concerns about climate models systematically overestimating the stability of the AMOC.