Fact-Check: is more than 1.5°C of global warming already locked in?

Claim: The lag between CO₂ emissions and warming means ~0.7°C of warming is yet to come, and aerosols are masking another ~0.7°C, meaning warming of much more than 1.5°C or even 2°C is already locked in even if we stopped all emissions right now.

Reality: If emissions stopped, falling carbon dioxide levels due to natural carbon sinks would counteract the climate lag. Immediately stopping aerosol emissions would cause a warming boost of ~0.2-0.4°C, but a slower partial phase-out can reduce this and can be more than countered by also reducing methane emissions. This means that the responsibility for future warming lies with current rather than past actions.

This is the sixth post in a new climatetippingoints.info series fact-checking claims that various climate tipping points have been crossed, and that sudden catastrophic warming is now inevitable. See the Introduction post for an overview.

It is sometimes claimed (e.g. 1,2,3,4,5,6) that more than 1.5°C or even 2°C of global warming is already geophysically locked-in even if we immediately stopped all greenhouse gas emissions, and that this has been ignored or covered up by scientists.

This is often based on two assumptions: firstly, that the lag between when emissions happen and when warming catches up means ~0.7°C of warming is in the pipeline already; secondly, that another ~0.6-0.7°C masked by cooling aerosols will emerge as emissions fall.

On top of a high estimate of ~1.3-1.4°C of current warming it’s therefore been stated that ~2.7°C of warming is already locked in (and some claim even more), breaching the Paris Targets already and risking triggering extra catastrophic feedbacks.

It’s also sometimes claimed that the planet is already beyond a specific threshold in CO₂ concentrations (e.g. 450500 ppm CO₂e) beyond which more than 2°C of warming becomes inevitable.

In this post of climatetippingpoint.info‘s Fact-Check series, we investigate how much warming is already locked in by climate lag and aerosol dimming. This builds on previous threads and posts on the short-term effects of both the climate lag and aerosol dimming, but here we revisit them together in order to specifically focus on the 1.5-2+°C locked-in claim.

Climate laggard

First of all, we’ll take a look at the warming from recently emitted CO₂ that has not yet had the chance to have its full impact yet. This is sometimes referred to as the climate lag, and has been quoted at +0.5 to +0.7oC on top of the existing warming.

The idea here is that warming from increased CO₂ doesn’t happen immediately – it takes time for energy imbalance from CO₂ to drive warming, time for the vast mass of the oceans to warm up, and time for this all to reach an equilibrium. The time lag between emissions and most (~60%) of the resultant warming is estimated at about 40 years, and the ocean will continue to slowly warm up for hundreds of years longer.

The impact of this was previously estimated by simulating what would happen if CO₂ levels were fixed at a constant value, known as the ‘constant concentrations commitment‘, which in previous model studies led to an extra warming of 0.5 to 0.6°C* by 2100 [*IPCC, AR5, WG1, s12.5]. Another similar study found an extra ~0.3°C by 2200, with gradual extra warming continuing for centuries afterwards, and another that ~2.3°C is locked in when accounting for “pattern effects”.

However, in reality if emissions stopped immediately the climate forcing wouldn’t remain constant like this, with concentrations of short-lived gases like aerosols and methane rapidly dropping and some CO₂ being drawn down into natural carbon sinks.

The 2018 IPCC Special Report on 1.5°C found that if CO₂ emissions ceased now, these negative feedbacks would reduce the warming lag left from the ~0.5-0.6°C to only ~0.1°C by 2100 (see the solid blue line in the figure below). This means warming can still technically be kept to around 1.5°C by 2100 with a CO₂ emission phase out (the dotted blue line below):

Projections of committed warming scenarios from the 2018 IPCC Special Report on 1.5°C (Figure 1.5). If we stopped all emissions now (CO₂, aerosols, and methane) we’d get the yellow curve; if we just stopped CO₂ emissions now we’d get the blue curve. Both stopping CO₂ & aerosol emissions now (green line) or a gradual phase out of just CO₂ emissions (dotted blue line) would keep warming to around 1.5°C by 2100. The differences by 2100 indicate eliminating aerosols leads to ~+0.2C and eliminating methane leads to ~-0.4C.

If all emissions of aerosols, CO₂, and other greenhouse gases stopped now, then after a small initial warming boost there’d actually be an overall cooling of ~0.2°C by 2100 (the yellow line above) as eliminating short-lived greenhouse gases like methane removes ~0.5°C of warming and more than makes up for removing aerosols (which masks ~0.4°C in this model).

A more recent project (‘ZECMIP‘) comparing this ‘zero emission commitment‘ (i.e. how much warming would happen if emissions were to suddenly cease, rather than unrealistically fixing CO₂) in different climate models found similar, with their best estimate being approximately no extra warming after emissions cease (with a range of +/- ~0.3°C either way) after 50 years. After a century there was even a modest cooling of ~0.1°C across the models:

Figure from ZECMIP [MacDougall et al., 2020, Biogeosciences], showing the range of model responses to emissions suddenly stopping. In the top panel bars below the line show the negative radiative forcing (causing cooling) from the ocean (light blue) and land (green) carbon sinks, and bars above the line the positive radiative forcing (the “climate lag”) from slow ocean heat uptake. The bottom panel shows the overall effect of these on global temperatures 50 years later, with models on the left showing net cooling, those on the right net warming, but most in the middle showing minimal change.

They also found that the zero emission commitment was even smaller in more realistic runs where emissions are ramped down rather than stopped immediately. These results were for an emission of ~1000 GtC (vs. ~650 GtC so far), but the land carbon sink will likely weaken at warming levels and unresolved feedbacks from e.g. abrupt permafrost thaw would also likely add extra emissions, so the zero emission commitment would likely grow with further warming.

So, based on current models if we stopped emitting CO₂ right now we would probably get negligible extra warming by 2100. This assumes an immediate stop to emissions though, rather than a realistic wind-down from current emission rates to zero. Any emissions in the meantime would continue to drive extra warming, and given such a wind-down is inevitable this could be thought of as socio-economically rather than geophysically locked-in warming.

Although the difference may seem minor, there’s a big difference between further warming being geophysically locked-in (which would be impossible to stop) and being socio-economically locked-in (which can still be minimised through action, therefore making how much extra warming occurs a societal choice). A negligible zero emission warming commitment means that responsibility for future warming lies with current and future rather than past actions.

This is part of why plans to keep below the lower 1.5°C Paris target involve such a rapid and challenging decarbonisation process of reaching net-zero greenhouse gas emissions by 2050 as well as controversial negative emission technologies in case emissions aren’t cut fast enough.

Aerosolace

The second claimed factor in locked in 2+°C is that if humans reduced carbon emissions now we’d end up unmasking more warming (often claimed as 0.7°C, or even 2.5°C) as a result of something called global dimming.

This refers to the cooling effect of tiny particles called aerosols, which humans emit alongside carbon dioxide from sources like power stations, factories, and fires. This has been posed as an unavoidable paradox, as by closing high emitters like coal-fired power stations in order to reduce greenhouse gas emissions we’d end up increasing temperatures and hitting tipping points anyway.

Visualisation of atmospheric aerosols on 23/8/2018 identified from satellite data. Many aerosols are from natural sources like storms or the deserts, but human activities like induced fires or power stations increase them further. Source: NASA/Joshua Stevens/Adam Voiland

As discussed in more depth in our Fact-Check on Global Dimming any by the recent IPCC AR6 report, aerosols mask climate forcing equivalent to ~0.5-0.6°C of warming, and maybe a bit more if indirect effects are at the higher end of their uncertainty range. In practice, however, models show a smaller impact if this forcing is “unmasked” as this modelling takes account of non-linear feedbacks and the impact of also reducing other short-lived climate forcers like methane.

In the earlier IPCC figure, stopping aerosol emissions as well as CO₂ results in an extra ~0.4°C of warming compared with just stopping CO₂ emissions (the green vs. blue lines in the figure), whereas stopping CO₂ and non-CO₂ greenhouse gas emissions leads to ~0.5°C of cooling (the pink vs. blue lines).

Stopping both aerosol and non-CO₂ greenhouse gas emissions as well as CO₂ leads to an overall ~0.25°C of cooling (the yellow line), indicating that even though removing aerosols still adds ~0.25°C of warming in this scenario (around a decade’s worth of the current warming trend) stopping non-CO₂ greenhouse gas emissions is more than enough to compensate. The difference in aerosol estimates is due to non-linear interactions with non-CO₂ greenhouse gases, and that tackling them as well reduces the impact of aerosol cuts.

An abrupt end to all aerosol emissions (quickly dumping all the ‘masked’ warming on us) is also very unlikely. Even the dramatic factory and travel shutdowns due to the coronavirus pandemic only led to a temporary ~17% drop in CO₂ emissions (associated with fewer aerosols too), and likely only 4-7% over the whole year. Meaningful decarbonisation will take decades, and aerosols also come from a wide variety of sources beyond fossil fuels such as dust (worsened by deforestation) and cooking fires.

Even a challenging target of halving aerosol emissions in the next couple of decades (similar to what IPCC projections already include, but with no corresponding methane cuts) would only lead to ~0.1-0.2°C of warming over that time. This is equivalent to around a decade more of the current warming trend, which is preferable to carrying on emitting CO₂ indefinitely and locking even more warming instead.

Greater-than-expected indirect cloud effects could boost this aerosol unmasking warming a bit more, but a controlled phase-out of aerosol as well as methane emissions would limit the immediate warming impact and reduce the risk of hitting any climate tipping points in the process.

Locked-out

Finally, the third point: does the current atmospheric content of 500ppm of CO₂e (i.e. CO₂ equivalent, which is the 2020 CO₂ of ~415 ppm plus warming from other greenhouse gases converted into roughly equivalent CO₂ values) lock in 2°C anyway?

Using a standard equilibrium climate sensitivity (ECS) of ~3°C per CO₂ doubling and ignoring aerosol cooling, stabilising at 500ppm CO₂e would give ~1.5°C warming in the short-term (over decades) and ~2.32.4°C longer-term (over centuries). [For geological timescales it’d be even higher at ~3.9°C (using an Earth system sensitivity of ~5-6°C per 2xCO₂), but here we’re interested in what will happen this century as that’s what most locked-in claims focus on.]

However, some people claim that 3-4°C is locked in already and that 5°C will be locked in within the next 5 years. This relies on an ECS of 5+°C per 2xCO₂ by picking just the recent highest-ECS CMIP6 models. These high-ECS models struggle to model historical warming though, and both the full CMIP6 model likely range (1.8-5.6°C, average ~3.9°C) and a recent statistical analysis of multiple lines of evidence (2.6-3.9°C) still support an ECS likely being somewhere between ~2 and 4.5°C per 2xCO₂, with ~3-4°C fitting best.

Even with an ECS of 3°C though, the ~2.4°C of long-term warming calculated above seems to suggest that over 2°C is already locked in. But as explained above, greenhouse gas levels don’t stay fixed if emissions reach zero, as various natural sinks start to absorb or break down some of the greenhouse gases.

500ppm of CO₂e would only guarantee 2+°C of warming if it stabilised at this level in the long run. But this would only happen if enough CO₂ emissions kept on going to counteract removal by carbon sinks, and the non-CO₂ components would fall out even more rapidly if not constantly replenished.

If all anthropogenic emissions stopped now then non-CO₂ greenhouse gases and aerosols would rapidly drop out over a few decades, reducing 500 ppm CO₂e to just the underlying 415ppm CO₂. CO₂ would then fall over the following century and eventually stabilise after ~1000 years, as shown by the zero emission commitment modelling mentioned earlier:

Figure from ZECMIP paper [MacDougall et al, 2020, Biogeosciences] showing how atmospheric CO₂ falls substantially in every model after emissions of 1000 GtC suddenly stop (left – top left are ESMs i.e. Earth System Models, bottom left EMICs i.e. ESMs of Intermediate Complexity) and the resultant change in global temperature in combination with the “climate lag” effect (right).

If we assume similar drawdown proportions for today’s CO₂ excess as in ZECMIP (~35% after 100 years and ~60% after 1000 years) then we’d expect CO₂ to fall to ~370 ppm by 2100 and stabilise at ~335 ppm by 3000. The latter would give an expected long-term warming of ~1.5°C under an Earth system sensitivity of 6°C per 2xCO₂, meaning 1.5°C would still be possible with immediate zero emissions even on longer feedback timescales.

Together with the earlier IPCC modelling then, if all CO₂, non-CO₂ greenhouse gases, and aerosol emissions were to cease now we’d expect a temporary ~0.1-0.2°C boost from aerosols dropping out before global warming likely drops to ~1.0°C by 2100 (as non-CO₂ greenhouse gases reductions outcompete aerosol reductions and CO₂ starts to fall).

Beyond that, warming would gradually creep back up to ~1.5°C after several centuries as a result of slow feedbacks responding to the lower stabilised CO₂. This shows that the current ~500 ppm of CO₂e doesn’t guarantee 2+°C of warming, and that humanity’s future actions can still make the difference as to where CO₂ ends up stabilising in the future.

Finally, there are other climate feedbacks and tipping points that could lock-in some more warming, for example Arctic sea ice retreat and Arctic permafrost thaw. However, as explored previously on climatetippingpoints.info, warming from a summer sea ice-free Arctic (~0.15-0.2°C) is already represented in climate models and so doesn’t add to future projections. And while permafrost is currently thawing, the estimated impact at ~2°C (of an extra ~0.1-0.3°C) suggests that at less than 1.5°C it would not lock-in a substantial amount of extra warming (up to ~0.1°C by 2100, which is partly accounted for in existing carbon budget calculations).

Summary

While the climate lag and aerosol dimming are real and have an important impact on climate change, they don’t lock in huge amounts of warming.

If greenhouse gas emissions stopped but greenhouse gases stayed at a fixed level then there’d be another ~0.5-0.6°C of slow warming in the pipeline, but in reality CO₂ would fall due to natural carbon sinks once emissions stop and largely cancel out this warming.

Aerosols mask ~0.6°C of warming, but even in the unlikely scenario of their sudden elimination models show only ~0.2-0.4°C of extra warming by 2100 as a result. A gradual partial phase-out of aerosol emissions could limit this unmasking effect to ~0.1-0.2°C spread over time, and cuts in non-CO₂ greenhouse gases like methanes could entirely counteract aerosol removal, minimising its impact.

Overall this likely reduces “locked-in” warming from the climate lag and aerosols to a negligible amount on top of the current (2021) warming of ~1.2°C – in contrast to the extra ~1.4°C sometimes claimed – and any short-term warming from aerosol reductions can be reduced and compensated for by reducing other short-lived greenhouse gases like methane.

More recently it’s been claimed that climate scientists knew at the 2015 Paris Agreement talks that 1.5°C was already physically impossible, often based on this quote from an article in The Conversation written by three scientists earlier this year: “We struggle to name any climate scientist who at that time thought the Paris Agreement was feasible. We have since been told by some scientists that… no one thought limiting to 1.5°C was possible“.

However, Dr. James Dyke – one of the authors of The Conversation’s article – has confirmed to climatetippingpoints.info that this quote was “about the profound skepticism about the climate-policy system being able to deliver required rates of mitigation for 1.5°C, not the physical impossibility of limiting warming to 1.5°C“. This means that many scientists at Paris thought that more than 1.5°C was socio-economically rather than geophysically locked-in, in contrast to how it’s sometimes presented.

As reaching zero emissions immediately is impossible it is of course inevitable that warming will continue for now and will almost certainly reach at least 1.5°C, but how much more warming happens beyond that will be determined by what people do next and is not a geophysical inevitability.

~

This post was written by Dr. David A. McKay, then a Postdoctoral Researcher at Stockholm Resilience Centre (Stockholm University), where he is part of the Earth Resilience in the Anthropocene Project (funded by the European Research Council) and is researching non-linear climate-biosphere feedbacks. This post was written in his spare time with no funding support for this site, and was proofread and edited by Dr. Rachael Avery.

Update Log: 20/7/20 Some minor post-publication tidying up; 22-23/7/20 clarified aerosol warming numbers, as it can be higher if non-CO₂ GHGs aren’t cut as well, and to clarify climate sensitivity usage and geological-timescale warming; 30/7/20 extra paragraph added on Arctic sea ice/permafrost locked-in warming estimates; 16/11/21 added ZECMIP results, clarified paragraphs describing SR1.5 chart & future CO2 levels, added discussion of recent confusion over scientists thoughts around the feasibility of 1.5C, broke up long paragraphs, and simplified headline message & title.

7 Comments

  1. Interesting article. I have a few questions

    1. In the graph you specify a line for a “50 year phase out CO2”. Can you be more specific about what this means? It cannot mean a linear reduction to zero since the IPCC have given us a budget of about 10 years’ at today’s emissions rates (to avoid 1.5C). So a linear reduction over 50 years would cause us to overshoot that budget by about 150%. You must intend for the vast majority of the reductions to occur within the first 10 of those 50 years. If that is the case it needs to be spelled out clearly so that leaders do not get a false sense of security.

    2. You don’t mention anything about the Arctic permafrost. How much melting is already locked in? How much additional warming would be caused by the carbon that is released when this decomposes? It is my understanding that were it all to decompose aerobically that would produce an amount of CO2 equal to 10 times our budget (for 1.5C). If just a small fraction were to decompose _anaerobically_ the situation would be much more serious as that would release CH4. I don’t think you can leave out the contribution of permafrost melt in considering what is or is not already locked in.

    3. You don’t mention what effect the loss of arctic sea ice would have. How much sea ice loss is already locked in? How much additional warming will be caused by this when it happens?
    You do mention that it takes a long time for increased CO2 levels to warm the oceans. But in addition to lag we must take into account feedback loops – such as the Arctic albedo feedback – that are already in process and have not yet reached their equilibria.

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    1. Hi Alex, glad you found the article interesting. The graph you mention is lifted from IPCC SR1.5 (https://www.ipcc.ch/sr15/chapter/chapter-1/ & search for “figure 1.5”), and is based on using a simplified carbon-climate model to perform some speculative what-if scenarios (e.g. immediate aerosol/methane/CO2 stops) to explore what’s locked-in rather than realistic scenarios. The dotted blue phase out line in particular “… show[s] a case where CO2 emissions are reduced linearly to zero assuming constant non-CO2 forcing after 2020”, which they admit is highly idealised, but is used as a placeholder scenario. Of course later in SR1.5 they develop far more detailed and realistic scenarios for keeping below 1.5C, which indeed do rely on emission cuts being much more front-loaded to the 2020s and lots of negative emissions later on.

      I didn’t mention permafrost and arctic sea ice here as I’ve most commonly found the climate lag and aerosol effects presented as the key reasons that 2+C is locked-in, so focused-in on those for brevity. However, I have also written some in-depth posts on Arctic methane (https://climatetippingpoints.info/2019/05/13/fact-check-is-an-arctic-methane-bomb-about-to-go-off/) and Arctic sea ice (https://climatetippingpoints.info/2019/04/02/fact-check-will-an-ice-free-arctic-trigger-a-climate-catastrophe/) which do explore these effects more – my estimate for permafrost feedback on ~2C warming is an extra 0.1-0.3C globally (so locked-in at current ~1.2C would be towards the bottom end of this), and my estimate for a summer ice-free Arctic global impact (possible some years at ~1.5C, likely permanent at ~2C) is ~0.2C (but this is already represented in climate models, so doesn’t so much add on to what they’re already projecting rather than having that warming arrive a bit earlier than thought). It’s a reasonable point though that underestimated feedbacks add to the locked-in count, and is worth adding in brief to the post, but at ~1.5C it won’t be a huge amount extra (probably~0.1C or so).

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  2. Hey do you have a real email adress I tried using your contact page but I haven’t gotten a response in a few weeks.

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  3. I would really like to know how much warming feedback loops are likely to cause and, more importantly, how fast? Does it double or triple the temperature increase we’re already causing? Could it run completely out of control? If we miss our climate targets, will we be able to do anything to contain or at least slow down those tipping points?

    In addition, How many species are likely to go extinct at the present trajectory and can Humanity survive a worst case scenario Global climate catastrophe and ecological collapse?

    I know that a lot of these things are impossible to predict with any real certainty, but I’d like to hear your thoughts on it as a scientist.

    I really don’t know what to think, the constant bombardment of overly optimistic and incredibly nihilistic from the internet and the media makes it hard to know fact from fiction.

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    1. Hi Tallach, I’ve actually got a scientific paper summarising some of these potential impacts that’ll be submitted soon, so once that’s in I can provide some more specific numbers. But in the meantime, on short timescales all of these sorts of feedbacks (excluding the very fast ones that are counted as part of regular climate sensitivity to CO2) will cause less warming than human-driven emissions. For example, gradual permafrost thaw – which is one of the strongest potential feedbacks – is projected to emit around 20 billion tonnes (Gt) of carbon (as CO2) by 2100 and ~50GtC by 2300 per degree of global warming, which translated roughly in to temperature increase is 0.1-0.2 degrees extra per degree of warming. This makes it more like a 10-20% feedback on warming, which is far better than say a 200 or 300% feedback but still makes keeping to the Paris targets that much harder. Timescale is important here too – you can from those numbers above that this is happening over hundreds of years, which gives time for e.g. natural or artificial carbon sinks to try and counteract that feedback. Given this is one of the biggest climate feedbacks out there I don’t think warming is likely to go runaway (i.e. more than human emissions, becoming self-perpetuating), and there’ll always be something we can do to try to prevent new tipping points being hit. Having said that, climate feedbacks and tipping points definitely make things a lot more difficult, and make it even more important to try to minimise further warming as far as physically possible (ideally to 1.5C, but even if not then every 0.1C avoided beyond is worthwhile). As for biodiversity, this paper summarises likely climate impacts: https://www.science.org/doi/10.1126/science.aar3646 (behind paywall, but can grab it elsewhere online, and here’s a summary graphic: https://pbs.twimg.com/media/E6PDKZBWQAEof6e?format=jpg&name=small).

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    2. I think the unfortunate truth is that we don’t know how bad it could get, even for a completely specified emissions scenario. The state of the art CMIP6 climate models used by the IPCC showed a range of values from 1.8C to 5.6C for Equilibrium Climate Sensitivity, the equilibrium response to doubling CO2. We can rule out the lower end just by considering what’s already happened: a rise of 1.1C for below 50% increase in CO2, before equilibrium is even reached. But even so, the range is massive and spans “very difficult” through to “total wipeout”.

      What we do know is that warming is likely to be more or less proportional to the total amount of CO2 we release, for the duration of this century. It’s the slope in that graph that’s uncertain.

      You asked about species extinction. Already the rate is 100 to 1000 times the background rate as a result of human activity. Yes that’s a factor of ten uncertainty. Animal population drops are better understood and charted via the Living Planet Index. Since I was born in 1975, population sizes, averaged across species, have dropped by 70% plus or minus a few percent. All that’s happened before climate change has really started to have an effect.

      I think we need to embrace the uncertainty, whilst recognising that we know enough already to know that we have to act and what we have to do.

      PS I’m not a scientist as such but I am studying for an MSc in climate change as a mature student.

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