Recent paper on climate tipping points

I've not seen the full text of this new paper (the Wang paper), but based on its abstract it doesn't seem hugely inconsistent with my current understanding of tipping points.

There was a high profile paper by Armstrong McKay et al that was published last year. The paper was largely taken to stress the severity of tipping points, (see e.g. this coverage) but when I read the paper, I think the paper is, at least in some ways, quite consistent with what Wang is saying. 

The Armstrong McKay paper listed 16 tipping points, of which

• 4 of them have an estimated timescale of < 50 years
• 3 of them have an estimated timescale of 50 years
• 9 of them have an estimated timescale of > 50 years; 
  • for those 9 tipping points not only is their estimated, but also their minimum timescale is ≥ 50 years, and 2 of them have a minimum timescale ≥ 1000 years 

Hence it seems that the Armstrong McKay paper agrees that "most tipping elements do not possess the potential for abrupt future change within (50) years". (i.e. apparently consistent with Wang)

Also, of the 16 tipping points listed in the Armstrong McKay paper, none of them had a massive impact on the global temperature (i.e. none had more than a 0.6 degree magnitude impact on global temperature). And some of the tipping points actually have a cooling effect.

This again seems consistent with the Wang paper, which says: "Emissions pathways and climate model uncertainties may dominate over tipping elements in determining overall multi-century warming".

One of the things that the Armstrong McKay paper helps to clarify, which doesn't seem to be clear from the Wang paper (as far as I can tell) is that a tipping point might potentially still be quite disruptive even if the global impact is small. (E.g. collapse of the convection in the Labrador-Irminger Seas wouldn't contribute much to global warming -- it actually has a cooling effect -- but it might be significantly disruptive to European and American weather systems).

In short, my understanding (prior to seeing the Wang paper) was that if you're focused on warming (rather than harms) then I largely understood Wang's sanguine-sounding claims to be true anyway.

jackva @ 2023-03-03T02:06 (+4)

Thanks, Sanjay!

Yes, the messaging of the papers is certainly vastly different.

That said, I think there is also a meaningful substantive difference, e.g. Wang et al suggests pretty minimal additional uncertainty through tipping points (less than 0.6 degrees by 2100 on net on plausible emissions pathways).

Would SoGive be interested in looking at these papers comparatively?

Vasco Grilo @ 2023-03-05T16:14 (+8)

Hi Johannes,

I have not looked into the paper you mentioned in the original post, but I wrote about the one linked by Sanjay here. For reference:

Their [McKay's tipping points] most extreme (maximum for positive, or minimum for negative) impact on the global temperature is descrived inTable S3 (see Supplementary Materials):

• Greenland Ice Sheet [collapse]: 0.13.
• West Antarctic Ice Sheet [collapse]: 0.05.
• Labrador-Irminger Sea / SPG Convection [collapse]: -0.5.
• East Antarctic Subglacial Basins [collapse]: 0.05.
• Amazon Rainforest [dieback]: 0.15 (= (0.1 + 0.2)/2).
• Boreal Permafrost [collapse]: 0.3 (= (0.2 + 0.4)/2).
• Atlantic Meriodional Overturning Circulation [collapse]: -0.5.
• Arctic Winter Sea Ice [collapse]: 0.6.
• East Antarctic Ice Sheet [collapse]: 0.6.
• Lowlatitude Coral Reefs [die-off]: not defined (ND).
• Boreal Permafrost [abrupt thaw]: ND.
• Barents Sea Ice [abrupt loss]: ND.
• Mountain Glaciers [loss]: 0.08.
• Sahel & West African Monsoon [greening]: ND.
• Boreal Forest [southern dieback]: -0.18.
• Boreal Forest [northern expansion]: 0.14.
• Boreal Permafrost [gradual thaw]: 0.7.
• Arctic Summer Sea Ice [loss]: 0.25.
• Global Land Carbon Sink [weaken]: ND.
• Ocean Biological Pump [weaken]: ND.
• Marine Methane Hydrates [dissociation]: 0.5.
• Indian Summer Monsoon [shift]: ND.
• South. Ocean Sea Ice [Ind. increase]: ND.
• South. Ocean Sea Ice [Pac./Atl. loss]: ND.
• South. Ocean Sea Ice [bimodality]: ND.
• Equatorial Stratocumulus Clouds [breakup]: 8.
• Antarctic Bottom Water [collapse]: ND.
• Indian Ocean Upwelling [abrupt increase]: ND.
• Tibetan Plateau Snow [abrupt loss]: ND.
• Ocean Deoxygen -ation [global anoxia]: ND.
• Arctic Ozone Hole [abrupt expansion]: ND.
• El Nino Southern Oscillation [permanent / extreme]: ND.
• Northern Polar Jet Stream [instability]: ND.

Adding up all of these, I get a maximum global warming of 10.37 ºC, which is mostly driven by the 8 ºC which could result from the breakup of the equatorial stratocumulus clouds.

So my very tentative conclusion was that the potential breakup of the equatorial stratocumulus clouds is an important consideration. I should note it is still unclear whether this tipping point actually exists, but uncertainty should push us towards acting as if it does exist (unless we expect lots of regression to the mean in further studies). McKay says:

However, this [breakup of the equatorial stratocumulus clouds] has only been resolved in one model so far, and so remains highly uncertain. If further research supports the existence of this tipping point, EQSC would constitute a global core tipping element, albeit one that is unlikely to triggered by anthropogenic warming unless global policy fails.

The part I highlighted above refers to the fact we need 1,200 ppm to trigger that tipping point. From the abstract of the paper which introduced it, Schneider 2019:

In the simulations, stratocumulus decks become unstable and break up into scattered clouds when CO₂ levels rise above 1,200 ppm.

Until reaching 1,200 ppm there would be quite some time to adapt. McKay says that concentration corresponds to "approx. 6.3°C (7-8.9°C) at ECS of 3°C per 2xCO2". However, I was impressed by how fast Schneider 2019 predicts the temperature transition (from 6 ºC of warming to 14 ºC (= 6 + 8) of warming) to be. I did not find information in the text, but there is a movie with a time series in the supplementary information. Here is the print of the cloud cover and temperature over time (sorry, I could not take a print without the play bar).

It looks like an increase of 6 ºC (= 303 - 297) happens in 20 days (= 275 - 255)! This is an underestimate, from the movie description:

The breakup of the stratocumulus clouds is more rapid than it would be in nature because of the unrealistically small thermal inertia of the underlying slab ocean.

That being said, even if the transition takes 10 times as long, 200 days is not much time. Nevertheless, overall, I am still pretty optimistic about extreme climate change (relative to other xrisks) given the low chance of 1,200 ppm.

Sanjay @ 2023-03-03T10:43 (+4)

Sounds like the sort of thing we would enjoy doing in principle. Let me check whether there's capacity within the team.  (I think there's not much capacity, but I'll check)

1. The paper makes the general statements quoted but in the text body clarifies that the evaluation has limitations in scope, omitting:

a) Tipping element cascades

So far, research on cascading behavior has primarily leveraged conceptual modeling rather than process and scenario-based approaches, limiting the applicability of these results for investigating the third question of what the cumulative impacts of transitions by multiple tipping elements might be.

b) Several tipping elements that are warming-dependent

We omit potential carbon fluxes or radiative forcing impacts from other candidate tipping elements (boreal forests, stratocumulus cloud decks, tropical monsoons, AMOC, and Greenland/Antarctic ice sheets ) given higher uncertainty surrounding their potential impacts upon carbon cycling and planetary radiative balance under different warming scenarios. One or more of these tipping elements could add net contributions to warming, however current levels of scientific knowledge and confidence are insufficient to formulate assumptions that aren't largely arbitrary. As stratocumulus cloud deck evaporation remains a novel and uncertain hypothesis, we also omit this mechanism. We assume die-off of tropical coral reefs produces no global climate feedbacks.

c) Several tipping elements that are warming-independent

Our review does not cover the full range of Earth system components that have been proposed to be candidate tipping elements over the last few decades. A number of other systems have been described as potential tipping elements, such as disruptions to the El Niño Southern Oscillation, loss of Antarctic sea ice, changes to snow cover in the Northern Hemisphere, and future shifts in ocean temperatures and oxygen levels, but are not expected to exhibit tipping behavior in response to warming (Ranasinghe et al., 2021).

As such I find this a little untrustworthy as the abstract implies a much higher confidence than is presented through the paper, but they did state it in the text.

2. I agree with what Sanjay said, there is fairly good agreement between the Armstrong McKay and Wang papers. One difference is the Amazon dieback is considered to be much more likely in the latter paper.

The paper broadly conforms to my previously held views on the tipping points discussed, but I would assign more risk to ice melt and AMOC slowdown as these projections don't include new research on grounding line movement that increase sea level rise projections by up to 200% and evolving understanding of crevasse contribution to melting.