Fact-Check: is “global dimming” shielding us from catastrophe?

This is the second post in a new climatetippingpoints.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.

Fact-Check: is “global dimming” shielding us from catastrophe?

Claim: Global dimming (due to cooling by aerosols, as opposed to global warming from greenhouse gases) is masking a large amount of warming (0.7-1.5°C), so if we stopped carbon emissions now we’d get a catastrophic jump in global warming.

Reality: Global dimming is masking around 0.6°C of anthropogenic warming. There are many aerosol sources – including some that cause warming – and so shutting down the worst carbon emitters (like coal power stations) now would not lead to all aerosols disappearing immediately or a sudden, dramatic warming.

It has been claimed that if humans reduced carbon emissions now we’d actually end up causing a rapid jump in warming (by 0.7oC or even 2.5oC in less than a year) as a result of something called global dimming. 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 anyway through global dimming, potentially passing dangerous climate tipping points.

In this post of climatetippingpoint.info‘s new Fact-Check series, we investigate how much warming global dimming is shielding us from, and if this means we’re trapped in to keeping carbon emissions going.

Removing the Mask

Global dimming is the opposite of global warming, with tiny particles we emit called aerosols scattering away incoming solar energy and so cause cooling (as opposed to greenhouse gases, which trap heat and cause warming). They also affect cloud formation, impacting on how much heat those clouds either trap or reflect. Human aerosol sources include sulphate from burning fossil fuels like coal, soot from factories or wood-burning, and dust from roads and land degradation. Natural sources of aerosols include deserts, trees, sea salt, and volcanoes.

atmosphere_geo5_2018235.png
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

Before clean air laws were introduced in the 1970s and 1980s, aerosols may have balanced out warming from carbon emissions and contributed to relatively stable temperatures in th 1950s to 1980s, but this effect has been declining since the 1990s. It’s also been suggested that there was a 1oC warming jump in the days following the September 11th attacks when planes in the US were grounded for a few days, and that this implies a large amount of warming is masked by aeroplanes alone.

In recent decades though greenhouse gas emissions have continued to grow while aerosol emissions have declined (as clean air laws have kicked in), and so overall we have net global warming. The graph below shows all of the factors driving global warming and cooling (in terms of Radiative Forcing (RF), the actual human-driven energy imbalance that leads to temperature change, measured in Watts per square metre of Earth’s surface), leading to the overall warming effect summed up at the bottom:

IPCC_AR5_RF_aerosol
All the different components affecting Radiative Forcing (RF) in the last IPCC report. Aerosols and their impact on clouds are highlighted by the red box. Note how aerosols have a net negative impact on RF, but that black carbon has a positive/warming effect. Also note the significant uncertainty bars. Source: IPCC AR5, and discussed in more detail here.

You can see from the graph above that despite the far larger greenhouse forcing, there’s still a fair bit of global warming being masked by aerosols. Doing the sums from these figures, the overall combined effect of aerosols (including indirect cloud effects) on RF is around -0.8 W/m², which if totally removed (and without accounting for feedbacks) would be equivalent to up to +~0.4oC within a decade or so and up to +~0.6oC longer term. This also fits with the sources of the observed warming so far (with a net non-greenhouse gas forcing of -~0.25oC for 1950-2010, versus -~0.34oC for the same gases now on the graph above). As it’s RF changing rather than temperature, as with CO2-driven warming there’d be a bit of lag before temperature catches up. A couple of recent studies have also suggested that the indirect cloud effects might be towards the greater end of the IPCC range above at around -1 W/m² (boosting the total aerosol effect to ~-1.3W/m² / 0.6-1oC), but the associated cloud dynamics remain highly uncertain.

So the +0.7oC quoted at the start of this article is not too far off our maximum estimate here of an eventual warming of ~0.6oC. However, larger masking effects are sometimes claimed. One model study has projected up to 1oC warming resulting from strong aerosol emission cuts, but this represents what would be masked by 2100 after continued moderate CO2 emissions rather than what is being masked now. An even higher figure of 2.5oC is also sometimes claimed, but this is hard to reconcile with observed RFs and heat budgets. This figure is also based on just one model, with an estimate for the strength of aerosol-cloud feedbacks far outside the range of other models and observations.

Shutting Down Debate

The big difference between aerosols and greenhouse gases though, is that aerosols have a very short lifespan in the atmosphere, falling out of the atmosphere after only a few weeks. This has led to the fear that shutting down heavy carbon emitters like coal power stations as part of decarbonisation would lead to a rapid warming spike as all the aerosols disappear, risking passing imminent climate tipping points. This scenario is sometimes called the dimming paradox, or the “McPherson Paradox” after one well-known and controversial proponent.

But human-made aerosols have many sources, from power plants to cars, fires, and agriculture. An immediate removal of human-made aerosols over only a few years is extremely unlikely, with decarbonisation likely taking decades. It’s also worth remembering that some aerosols, such as black carbon (soot from incomplete burning) in smog and “brown clouds“, actually cause warming as well (by absorbing some incoming solar energy rather than scattering it), and so driving down aerosol emissions from industrial smog would reduce a source of warming as well as cooling.

As a result, any warming from aerosol reduction would be more gradual and partial than portrayed in the dimming paradox scenario. Even if an immediate phase-out of aerosols along with CO2 happened, more realistic modelling for the 2018 IPCC Special Report on 1.5oC showed that the actual aerosol warming bump would be at most a temporary ~0.2oC warming over a few decades (the green versus dotted-blue curves below), which is equivalent to a about a decade’s worth of the current global warming trend. This is less than the ~0.4-0.6oC calculated above, as this modelling takes account of the complex Earth system feedbacks that in this case reduce the impact of aerosol reductions. Greater than expected indirect cloud effects could boost this warming bump 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.

Projections of committed warming scenarios from the 2018 IPCC special report on 1.5 degrees. If we stopped all emissions now, we’d get the yellow curve; if we stopped everything but aerosols we’d get the pink curve, and if we stopped CO2 & aerosols but not methane we’d get the green curve. A gradual phase out of CO2 emissions would keep warming to around 1.5 degrees by 2100 (dotted blue line).

And what about the often-mentioned temperature jump in the US after September 11th, 2001? This was actually more to with the impact of the disappearance in contrail clouds (the thin strips of cloud left behind by planes) on the temperature difference between night and day (with warmer days and cooler nights), with no evidence for an overall warming effect. It also represents an aerosol-cloud effect that is specific to aeroplanes, and wouldn’t apply in the same way to shutting down power stations.

Finally, a key problem with avoiding limiting aerosol emissions by keeping heavy polluters going is that further carbon emissions guarantees further warming anyway, even if part of it continues to be hidden by aerosols. Overall it’s probably worth experiencing a decade’s worth of global warming from reduced aerosols now instead of inexorably building up ever more decades of warming from continued carbon emissions in the future.

Summary

While the total removal of human-made aerosols theoretically leads to a short-term warming of up to ~0.4oC (and ~0.6oC by 2100), an abrupt end to all aerosol emissions at once is very unlikely and other feedbacks would reduce this warming bump too. Decarbonisation will take decades, and aerosols come from a wide variety of sources beyond just the heaviest emitting power stations. Recent IPCC modelling projects that even a rapid phase out of aerosol emissions would only lead to a temporary ~0.2oC of warming – the equivalent of around a decade more of the current warming trend – which is preferable to carrying on emitting carbon indefinitely instead and is less likely to trigger any tipping points.

~

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: 14/1/20 Details of IPCC SR1p5 modelling of the impact of immediate aerosol emission cessation added, along with details of recent studies suggesting greater indirect cloud effects towards the larger end of the IPCC range.

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