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. 450–500 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 +0.7^oC 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):

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:

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.

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 ~2.3–2.4°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:

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.