Abstract

One of the most prominent climate tipping elements is the Atlantic meridional overturning circulation (AMOC), which can potentially collapse because of the input of fresh water in the North Atlantic. Although AMOC collapses have been induced in complex global climate models by strong freshwater forcing, the processes of an AMOC tipping event have so far not been investigated. Here, we show results of the first tipping event in the Community Earth System Model, including the large climate impacts of the collapse. Using these results, we develop a physics-based and observable early warning signal of AMOC tipping: the minimum of the AMOC-induced freshwater transport at the southern boundary of the Atlantic. Reanalysis products indicate that the present-day AMOC is on route to tipping. The early warning signal is a useful alternative to classical statistical ones, which, when applied to our simulated tipping event, turn out to be sensitive to the analyzed time interval before tipping.
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Climate impacts

The SST changes due to AMOC collapse also affect the atmosphere and global sea-ice distribution. The atmospheric responses (fig. S4) consist of a seesaw pattern in the 2-m surface temperature, a southward intertropical convergence zone (ITCZ) shift, and the strengthening of the Hadley Cell in the Northern Hemisphere. The stronger meridional temperature gradient over the Northern Hemisphere amplifies the subtropical jet, while the opposite happens in the Southern Hemisphere. During the gradual AMOC weakening over the first 1400 model years, there were no significant trends [P > 0.05, two-sided t test (23)] in the global mean surface temperature or in the global sea-ice area. Under the AMOC collapse, the Arctic (March) sea-ice pack extends down to 50°N and there is a gradual retreat of the Antarctic (September) sea-ice pack (fig. S5). The vast expansion of the Northern Hemispheric sea-ice pack amplifies further Northern Hemispheric cooling via the ice-albedo feedback. These findings are qualitatively similar to those in (20), in which AMOC is strongly weakened to 3 to 4 Sv.

The aforementioned ocean, atmosphere, and sea-ice responses strongly influence the regional climates across the globe (Fig. 2). The European climate is significantly different after the AMOC collapse, whereas for other regions only specific months undergo significant changes. The Amazon rainforest also shows a drastic change in their precipitation patterns due to ITCZ shifts, and the dry season becomes the wet season and vice versa. These AMOC-induced precipitation changes could severely disrupt the ecosystem of the Amazon rainforest (7, 24, 25) and potentially lead to cascading tipping (26–28). The Northern Hemisphere shows cooler temperatures after the AMOC collapse, while the opposite is true for the Southern Hemisphere, although not all changes are significantly different (due to large interannual variability).

The European climate is greatly affected (Fig. 3A) under the AMOC collapse. Note that the corresponding changes occur within a relatively short period (model years 1750 to 1850) and under a very small change in surface freshwater forcing. The yearly averaged atmospheric surface temperature trend exceeds 1°C per decade over a broad region in northwestern Europe, and for several European cities, temperatures are found to drop by 5° to 15°C (Fig. 3C). The trends are even more notable when considering particular months (Fig. 3B). As an example, February temperatures for Bergen (Norway) will drop by about 3.5°C per decade (Fig. 3D). These relatively strong temperature trends are associated with the sea-ice albedo feedback through the vast expansion of the Arctic sea-ice pack (fig. S5A).

https://www.science.org/doi/10.1126/sciadv.adk1189