Thread by @dvdmckay

We’ve got a new paper out on climate #TippingPoints in @ScienceMagazine: “Exceeding 1.5°C global warming could trigger multiple climate tipping points”
science.org/doi/10.1126/sc…

Explainer: climatetippingpoints.info/2022/09/09/cli…

🧵Thread on what it says, & the implications:

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Climate tipping points (CTPs) have become a major global concern since the first major CTP assessment in 2008, which provided a definition and identified nine parts of the climate system (named "tipping elements") with potential CTPs.
pnas.org/doi/10.1073/pn…

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There have been major climate science advances since then, with improved models, observations, & palaeoclimate records helping understand how and when CTPs could occur.

Based on this, new elements (e.g. permafrost) have been proposed, & others (e.g. monsoons) questioned.

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Estimates of CTP thresholds have also dropped over the years, from mostly 3-5°C in the 2008 assessment to the recent IPCC AR6 report assessing that CTP-related risks emerge above 1°C, become moderate around 2°C, & reach high within 2.5-4°C.

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Recent observations also suggest that some elements are already being destabilised, with some West Antarctic glaciers close to unstoppable retreat, and potential "early warning signals" detected in the Amazon, Greenland ice sheet, & possibly in Atlantic overturning as well.

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Despite these advances, there are still some issues. CTP definitions vary across papers & media, leading to threshold-free feedbacks, abruptly-forced events, & arbitrary thresholds sometimes being labelled as CTPs despite lacking tipping dynamics.

https://twitter.com/ClimateTipPoint/status/1159118858277588994

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Models have improved, but many lack processes & resolution necessary to fully resolve tipping. For example, IPCC AR6 stated that models are biased towards the AMOC being overly stable, and models have also underestimated tropical carbon sink decline.
nature.com/articles/s4158…

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And while there are potential destabilisation signs in some elements, observations are often too short to be able to robustly assess the trend. The AMOC has declined by ~15% over the last 50 years, but this cannot yet be clearly differentiated from natural variability.

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Finally, much CTP discussion still relies on the 2008 assessment, but there have been major advances in our understanding since then.

Based on this, we decided a reassessment of climate tipping elements and their potential tipping points would be timely.

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In this study we assessed over 220 papers on proposed CTPs since 2008, searching for evidence of tipping dynamics & extracting threshold, timescale, and impact estimates. We then categorised them, synthesised estimates, and assigned confidence levels.

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We used the following CTP definition for categorisation:

Tipping points occur when change in a part of the climate system (the ‘tipping element’) becomes: a) self-perpetuating beyond b) a forcing threshold, leading to c) substantial and widespread Earth system impacts.

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(A good metaphor for a tipping point is a seesaw - if a ball is being pushed up a seesaw and the pushing stops before reaching the pivot it would roll back, but after passing the pivot it would carry on rolling until it reaches a new state even if the pushing stopped.)

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Our definition also doesn't require abruptness/irreversibility, despite being common CTP features, as self-perpetuating change can happen over very long timescales (e.g. in ice sheets) and in some special cases can occur across mathematically non-catastrophic thresholds.

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We also differentiate between global 'core' (sub-continental size & key to Earth system current state) and regional 'impact' tipping elements (localised but synchronous tipping across sub-continental area; impacts many people’s wellbeing or highly valued), & remove others.

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Here are the results: we categorised 16 candidates as climate tipping elements (compared to nine in 2008), nine of which as global 'core' tipping elements and seven as regional 'impact' tipping elements:

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New entries include East Antarctic subglacial basins (separated from the land-based EAIS) & convection in the Labrador-Irminger Sea and the subpolar gyre (separated out from the AMOC), while Arctic summer sea ice (but not winter sea ice) and El Niño have been removed.

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A key result are our synthesis estimates for the global warming thresholds that climate tipping points could be triggered beyond based on current evidence – the bars below go from our min. estimate (yellow) through central/best est. (ref, dot lines), to max. (dark red):

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At current warming 5 CTPs are possible but not yet likely in our assessment (possible = above min. but below central estimate): Greenland & West Antarctic ice sheet collapse, tropic coral reef die-off, widespread abrupt permafrost thaw, & Labrador Sea convection collapse.

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At the 1.5°C Paris aim 4 of these become likely (likely = above central estimate) in our assessment, and another 5 CTPs become possible (AMOC collapse, Barents Sea ice collapse, mountain glaciers loss, boreal forest southern dieback / northern expansion).

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Moving further through the Paris range of 1.5-<2°C Labrador-Irminger Sea convection collapse and Barents Sea ice loss also move from possible to likely, adding up to 6 likely and 4 possible below 2°C.

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Current policies could lead to ~2.6°C (1.9-3.7°C): in our assessment this would make 7 tipping points likely (adding mountain glaciers) & 6 possible (adding East Antarctic subglacial basins collapse, Amazon rainforest collapse, and Sahel greening).
nature.com/articles/s4158…

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Comparing our threshold estimates with future emission scenarios, it's clear that in our assessment most but not all climate tipping points are avoidable on the lowest trajectories (SSP1-1.9 & SSP1-2.6), and some CTPs will likely be passed in the coming couple of decades:

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CTPs likely in Paris range could lock in major impacts: sea level rise of 10+m over 1000s years, permafrost thaw amplifying emissions, coral die-off decimating ecosystems & livelihoods of those reliant on them, and Labrador convection collapse disrupting weather patterns.

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In the 2-3°C likely with current policies, CTPs likely in our analysis include committed mountain glacier loss (threatening water supplies of millions) and Sahel greening. Amazon dieback isn't likely until ~3.5°C in our analysis, deforestation could make it happen sooner.

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Beyond 3°C, AMOC collapse would cause major regional cooling & weather disruption, Boreal forest dieback/expansion would impact local communities & regional climate, and permafrost collapse in e.g. Yedoma could add ~0.2-0.4°C to warming (i.e. substantial, but not runaway).

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Our estimates suggest then that Earth may have left a ‘safe’ climate state beyond 1°C, as 5 CTPs become possible beyond this in our assessment.

Beyond 1.5°C 4 of these become likely & 5 more possible in our assessment, with likelihoods increasing through Paris range.

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These results provide strong scientific support for rapid emission cuts in line with the Paris Agreement 1.5°C aim, which would reduce likelihood of triggering CTPs.

Some CTPs are still possible or even likely at this level though, locking in major impacts to adapt to.

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But even if we hit some CTPs, our impact estimates indicate CTPs at Paris levels don't substantially amplify short-term warming & so likely won't trigger 'runaway' climate change, meaning rapid emission cuts now can still reduce the likelihood of passing further CTPs.

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Our study is a 1st attempt at an updated systematic assessment of CTPs, but there are still high uncertainties & some low confidence levels, and so it should be followed by wider community projects (e.g. the new “TIPMIP”) to improve our understanding & monitoring of CTPs.

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Thanks to all my co-authors on this (inc. @ArieStaal @DrJFAbrams @Ricarda_Climate @SarahLizCornell @jrockstrom) and for funding & support from the European research Council-funded ERA project and the Earth Commission @SafeJustPlanet @FutureEarth.

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