Physics-based early warning signal shows that AMOC is on tipping course

[Annapurna]

Link post

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.

[Radford Neal]

From the paper:

Under increasing freshwater forcing, we find a gradual decrease (Fig. 1A) in the AMOC strength (see Materials and Methods). Natural variability dominates the AMOC strength in the first 400 years; however, after model year 800, a clear negative trend appears because of the increasing freshwater forcing. Then, after 1750 years of model integration, we find an abrupt AMOC collapse

Given that the current inter-glacial period would be expect to last only on the order of some thousands of years more, this collapse in 1750 years seems a bit academic.

[Charlie Steiner]

See the discussion section.

We have developed a physics-based, and observable early warning signal characterizing the tipping point of the AMOC: the minimum of the AMOC-induced freshwater transport at 34°S in the Atlantic, here indicated by F_ovS. The F_ovS minimum occurs 25 years (9 to 41, 10 and 90% percentiles) before the AMOC tipping event. The quantity F_ovS has a strong basis in conceptual models, where it is an indicator of the salt-advection feedback strength. Although F_ovS has been shown to be a useful measure of AMOC stability in GCMs, the minimum feature has so far not been connected to the tipping point because an AMOC tipping event had up to now not been found in these models. The F_ovS indicator is observable, and reanalysis products show that its value and, more importantly, its trend are negative at the moment. The latest CMIP6 model simulations indicate that F_ovS is projected to decrease under future climate change. However, because of freshwater biases, the CMIP6 F_ovS mean starts at positive values and only reaches zero around the year 2075. Hence, no salt-advection feedback–induced tipping is found yet in these models under climate change scenarios up to 2100 and longer simulations under stronger forcing would be needed (as we do here for the CESM) to find this. In observations, the estimated mean value of F_ovS is already quite negative, and therefore, any further decrease is in the direction of a tipping point (and a stronger salt-advection feedback). A slowdown in the F_ovS decline indicates that the AMOC tipping point is near.

Model year 1750 does not mean 1750 years from now. The model is subtly different from reality in several ways. Their point is they found some indicator (this F_ovS thing) that hits a minimum a few decades before the big change, in a way that maybe generalizes from the model to reality.

In the model, this indicator starts at 0.20, slowly decreases, and hits a minimum at −0.14 of whatever units, ~25 years before the AMOC tipping point.

In reality, this indicator was already at −0.5, and is now somewhere around −0.1 or −0.15.

This is a bit concerning, although to reiterate, the model is subtly different from reality in several ways. Exact numerical values don’t generalize that well, it’s the more qualitative thing—the minimum of their indicator—that has a better chance of warning us, and we have not (as far as we can tell) hit a minimum. Yet.

[Radford Neal]

The logic seems to be:

1. If we do a 1750 year simulation assuming yearly fresh water additions 80 times the current greenland ice melt rate, we see AMOC collapse.

2. Before this simulated collapse, the value of something that we think could be an indicator changes.

3. That indicator has already changed.

4. So collapse of the AMOC is imminent.

Regarding (1), I think one can assume that if there was any way of getting their simulation engine to produce an AMOC collapse in less than 1750 years, they would have showed that. So, to produce any sort of alarming result, they have to admit that their simulation is flawed, so they can say that collapse might in reality occur much sooner. But then, if the simulation is so flawed, why would one think that the simulation’s indicator has any meaning?

They do claim that the indicator isn’t affected by the simulation’s flaws, but without having detailed knowledge to assess this myself, I don’t see any strong reason to believe them. It seems very much like a paper that sets out to show what they want to show.