Climate change is multifaceted. It has long been certain that CO2 emissions are altering the atmosphere, causing heating and disequilibrium — but it is also clear that CO2 is far from the only culprit, or that the Greenhouse effect is the only negative feedback cycle driving the temperature up. Methane emissions, Ice albedo decline, and ocean acidification are all components of Climate breakdown. Widely acknowledged, but also less understood.
However, the story of climate breakdown continues to evolve. The more scientists study its effects and causes, the more ways they find human activity is forcing disequilibrium. Likewise, the more ways in which they find nature is now producing feedback mechanisms outside of our control. This article is a part of a series of summaries covering these newer findings, and brief explanations of their mechanisms. The first article can be found here.
Though all findings here are backed up by peer-reviewed science, none are beyond reproach. Most are relatively new discoveries, and open to further study. Hopefully, the dangers outlined in these new components of climate breakdown will turn out to be less severe than predicted. But if they are exacerbated beyond safe levels, we may truly have opened Pandora’s box.
Atmospheric Aerosols and Global cooling effects
Emissions from burning fossil fuels are the main cause of atmospheric warming. They are also what is helping to cool it down. This idea might sound paradoxical, but it is because different components of fossil fuel emissions affect different mechanisms in the atmosphere. Though it should be noted before discussing further: they do not ‘cancel each other out’. The separate cooling mechanism simply prevents global warming from becoming even more extreme. For now, that is.
When we burn fossil fuels — as well as to a smaller degree biofuels like wood — particulate aerosols are released. These aerosol particles are what make pollution so damaging to humans who live in high emissions zones. But these aerosols also have an impact on the wider climate system. When entering the upper atmosphere, these particles disrupt the path of the sun’s energy and can reflect it back into space. This means heat energy can be prevented from entering the climate system by aerosols from fossil fuel pollution. In aggregate, this effect can help to cool the Earth by scattering shortwave solar radiation. This process does not directly impact the greenhouse effect, which is related to how much heat remains in the atmosphere after entering it.
This may sound like a welcome side-effect, as even though GHG emissions are causing dangerous heating, there is at least a counteractive process at play. Exactly how strong this cooling effect is, remains uncertain. Research by Bellouin et al shows that whilst it is difficult to accurately determine the amount of solar energy that is reflected, it could offset anywhere from a fifth to half of the anthropogenic warming from the greenhouse effect.
However, this poses a significant problem for decarbonization drives in the human economy. The aerosols that cause this cooling are relatively short-lived in the atmosphere, usually falling down to earth within a year. They must be constantly replenished as a byproduct of GHG emissions to continue to have an effect on the climate. Carbon dioxide, on the other hand, is not short-lived. Carbon dioxide from human activities will remain as a warming agent in the atmosphere for hundreds of years. This means that, perversely, a full decarbonisation of the economy could lead to a sudden jump in temperatures. As the aerosol cooling effect wares off, the heating effect of atmospheric carbon will remain for some time, retaining a baseline warming effect without a correlating cooling effect. The significance of this jump is dependent upon how powerful the cooling effect is currently. If the upper estimates are correct, we would experience a rapid (perhaps within a year or two) jump to over 2°C above pre-industrial temperatures.
However, there is new research by Shau-Liang et al that could turn the science on its head: proposing that instead of its main impact being cooling, anthropogenic aerosols may actually be warming the planet.
The Possible Warming Effect of Particulate Aerosols
Anthropogenic aerosols are mainly made up of sulfates and nitrates. Due to their artificial, inorganic origin, they are very fine, thus reaching high into the atmosphere and having a high potential for reflecting solar radiation. Naturally occurring aerosols, such as Saharan dust or marine aerosols, are darker and coarser. As a result, they add to the greenhouse effect by absorbing and re-emitting heat energy, rather than reflecting it. They also fall out of the atmosphere sooner due to their higher mass, having short-lived effects on global temperatures.
Research by Shau-Liang et al shows that the coarseness of anthropogenic aerosols may be higher than was previously predicted, and their climate effects correspondingly different. One mechanism this could occur by is clumping, where finer sulfates and nitrates stick to larger particles. This would cancel out their reflective (thus cooling) properties, but enhance the warming properties of the particle they become attached to. Likewise, the sulfates on their own may simply be coarser than models previously predicted. In this situation, whilst their reflective impact on short-wave radiation (that comes directly from the sun) remains, their absorption and emitting of long-wave radiation (that is already in the atmosphere) is enhanced. As a result, whilst they would retain their cooling properties, they would also act as warming agents, with their own greenhouse effect. The paper does not directly estimate the overall amount of heat increase/decrease due to aerosols. However, if its findings are true, it would mean that at best anthropogenic aerosol cooling is far less significant than predicted, and at worst, that they have a net warming effect on the atmosphere.
Wider Significance
If the cooling effect of anthropogenic aerosols is predominant, then it must be factored into decarbonisation models. It creates a ‘false peak’ in temperatures, with a devesting ‘true peak’ quickly emerging in the immediate aftermath of mass economic decarbonization. It means that the need to mitigate emissions is even higher, as the temperatures we have factored into modeling as ‘acceptable’ may actually be suddenly transgressed even if we do everything possible to reach them.
If the warming effect of anthropogenic aerosols is predominant, this must also be factored into decarbonisation models. It adds another component of warming to an already dangerously overheating system. Likewise, it must be considered by those who propose using artificial aerosols to geoengineer the atmosphere to be cooler. If the warming effect is more significant (or equal) to that of the cooling effect, these geoengineering efforts could be exacerbating the problem they seek to address.
References
- Charlson, R.J., Schwartz, S.E., Hales, J.M., Cess, R.D., Coakley Jr, J.A., Hansen, J.E. and Hofmann, D.J., 1992. Climate forcing by anthropogenic aerosols. Science, 255(5043), pp.423–430.
- Bellouin, N., Quaas, J., Gryspeerdt, E., Kinne, S., Stier, P., Watson‐Parris, D., Boucher, O., Carslaw, K.S., Christensen, M., Daniau, A.L. and Dufresne, J.L., 2020. Bounding global aerosol radiative forcing of climate change. Reviews of Geophysics, 58(1), p.e2019RG000660.
- Visioni, D., Slessarev, E., MacMartin, D.G., Mahowald, N.M., Goodale, C.L. and Xia, L., 2020. What goes up must come down: impacts of deposition in a sulfate geoengineering scenario. Environmental Research Letters, 15(9), p.094063.
- Chen, S.L., Chang, S.W., Chen, Y.J. and Chen, H.L., 2021. Possible warming effect of fine particulate matter in the atmosphere. Communications Earth & Environment, 2(1), p.208.
Teaser photo credit: Aerosol pollution over northern India and Bangladesh. By Jacques Descloitres, MODIS Land Rapid Response Team, NASA/GSFC – http://visibleearth.nasa.gov/view_rec.php?id=2309, Public Domain, https://commons.wikimedia.org/w/index.php?curid=191335
