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 more than 2°C is already locked in even if we stopped all emissions now.
Reality: If emissions stopped now, falling greenhouse gas concentrations would reduce the effects of the warming lag from ~0.6°C to ~0.1°C. Stopping aerosol emissions would cause a warming boost of ~0.2-0.4°C, but a slower partial phase-out can reduce and spread this out. If we stopped all emissions now (including methane) there’d even be an overall cooling by 2100.
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 often claimed (e.g. 1,2,3,4,5) that more than 2°C of global warming is already locked in even if we immediately stopped all greenhouse gas emissions. This is often based on two factors: 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 their high estimate of ~1.3-1.4°C of warming already it’s therefore stated that ~2.7°C of warming is already locked in (and some claim even more), breaking the Paris Targets and risking triggering extra catastrophic feedbacks. It’s also sometimes claimed that there’s a threshold in CO2 concentrations (e.g. 450–500 ppm CO2e) beyond which more than 2°C of warming is 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 2+°C locked-in claim.
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 called 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. This time lag between cause and most (~60%) of the effect is estimated at about 40 years (although 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 emissions stopped immediately and concentrations of CO2 were fixed at a constant value, which led to an extra warming of 0.5 to 0.6oC* by 2100 [*IPCC, AR5, WG1, s12.5].
However, in reality if emissions were to stop 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 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 orange line below) as eliminating short-lived greenhouse gases like methane removes ~0.4°C of warming.
So, if we stopped emitting CO2 now we would still expect around 0.1oC of warming by 2100, with a bit more in store over the next few centuries too, as the climate system approaches its longer-term equilibrium. This assumes an immediate stop to emissions though, rather than a realistic peaking and then wind-down from current emission rates to zero. This is part of why plans to keep below the lower 1.5oC Paris target involve such a rapid and challenging decarbonisation process of net-zero greenhouse gas emissions by 2050 as well as controversial negative emission technologies.
The second claimed factor in locked in 2+oC is that if humans reduced carbon emissions now we’d end up unmasking more warming (often claimed as 0.7oC, or even 2.5oC) 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, aerosols do in fact mask climate forcing equivalent to ~0.6oC of warming, and maybe a bit more. In practice, however, models show a smaller impact as this modelling takes account of the complex Earth system feedbacks that in this case reduce the impact of aerosol reductions. In the earlier figure from the IPCC, the difference between the pink (just greenhouse gas emissions stop) and yellow (both greenhouse gas AND aerosol emissions stop) lines shows how stopping aerosols as well boosts warming by about ~0.2oC, which is equivalent to a about a decade’s worth of the current global warming trend. Alternatively, if CO2 emissions stop but non-CO2 greenhouse gases like methane remain constant (solid blue line), then stopping aerosols as well leads to ~0.4oC of warming (green line) – this difference is due to non-linearities in 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 recent 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 would only lead to ~0.1-0.2oC of warming over that time – the equivalent of around a decade more of the current warming trend – which is preferable to carrying on emitting carbon indefinitely instead. Greater-than-expected indirect cloud effects could boost this warming a bit more, but a controlled phase-out of aerosol emissions would help limit the immediate warming impact and reduce the risk of hitting any climate tipping points in the process.
Finally, the third point: does the current atmospheric content of 500ppm of CO2e (i.e. CO2 equivalent, which is CO2 of ~415ppm plus warming from other greenhouse gases converted into 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 ~2.4°C longer-term (over centuries). [For geological timescales it’d be even higher at ~3.9°C (using an Earth system sensitivity of ~5°C per 2xCO2), but here we’re interested in what’ll happen this century as that’s what most locked-in claims focus on.] However, some claim that 3-4°C is locked in already and that 5°C will be locked in within the next 5 years, which relies on an ECS of 5+°C per 2xCO2 by picking just the recent high-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).
At first the ~2.4°C of long-term warming calculated above seems like it supports the claim that over 2°C is already locked in. But as explained above, these concentrations 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 maintained at this level in the long run after reaching a higher concentration peak when emissions stopped. But stabilising at this level will only happen if emissions keep on going, which is not inevitable in itself, and the non-CO2 components of this value would also rapidly fall if not constantly replenished. If all anthropogenic emissions stopped now methane and aerosols would rapidly drop out and CO2 would gradually fall, and if CO2 stabilised at say ~400ppm then we would expect global warming to temporarily drop to ~0.9°C in the short-run (as methane declines), creep back up to ~1.3°C after a century, and reach ~2°C after several millennia. This means that the current 500ppm of CO2e doesn’t guarantee 2+°C of warming – 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 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 significant amount of extra warming (probably less than ~0.1°C).
While the climate lag and aerosol dimming are real and have an impact on climate change, they don’t lock in huge amounts of warming. If greenhouse gas emissions stopped and concentrations stayed at a fixed level then there’d be another ~0.6°C of warming in the pipeline, but in reality concentrations would fall once emissions stop and cancel out all but ~0.1°C of this warming. Aerosols do 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, minimising its impact. Overall this reduces “locked-in” warming from CO2 and aerosols to around 0.3°C on top of the current (up to 2019) warming of 1.1-1.2°C – well below the extra ~1.4°C sometimes claimed – and can be reduced and compensated for by reducing other short-lived greenhouse gases like methane.
This post was written by Dr. David A. McKay, currently 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.