Do cuts in particle pollution accelerate climate change?

By: Richard Allan 

Glorious weather over much of the UK in April 2020 replaced the seemingly relentless winter rain, sodden ground and flooding, making the lockdown bearable for those lucky enough to be able to enjoy time outside. I was struck by the vivid, brilliance of the blue skies and the uncharacteristically pristine air (at least before I was engulfed by smoke wafting up from my neighbour’s barbeque).  Improvements in air quality globally (also seen locally in Reading) resulting from the temporary suppression of economic activity from COVID-19 lockdowns (Figure 1) offer a tantalising glimpse of a cleaner, healthier world possible in a low-carbon future. While human-caused CO2 emissions will temporarily be suppressed there will be no respite from warming of climate – and less particle pollution haze can in fact increase warming as more sunlight reaches the surface.

Figure 1: Reduced column amount of NO2 air pollution over Europe in March/April 2020 compared with 2019 is also a good indicator of changes in harmful particle pollution or aerosols which shade the surface from the sun (Image: © European Space Agency/Copernicus Sentinel-5P satellite data processed by KNMI/ESA)

The effect of longer-term changes in particulate matter (or aerosols) on the atmosphere can already be detected in observations over Europe and China (Figure 2). Measurements show decreasing sunlight reaching the surface (a “dimming”) up to the 1980s as increased particle pollution blocked out the sun. However, cuts in pollution, first in Europe and later in China, can explain a reversal of this trend (a “brightening” at the surface). Aerosols particles act as mirrors by reflecting sunlight back to space but this research also finds changes are linked with how much sunlight is captured (absorbed) by aerosol in the air aloft. These altered heating patterns through the atmosphere and from region to region can further influence rainfall patterns as well as surface temperature.

Figure 2: A schematic depicting measurements of sunlight reaching the surface and intercepted by the atmosphere (compared with 2000-2015 averages shown as horizontal lines) over Europe and China (based on Schwarz et al. 2020 Nature Geosci. Fig. 2).

The extent of regional changes in particle pollution are also evident from satellite imagery since 2010 showing declining pollution in parts of China yet increases across India (Figure 3). And continued rapid cuts to air pollution in Europe and Asia can potentially lead to a short-term acceleration in climate change. Computer simulations show a more rapid increase in European and Asian heatwaves by 2050 associated with sharp cuts in air pollution across parts of Asia. The worst-case-scenario indicates that the hottest day of the year may be up to 4°C hotter by 2050, compared to the present day, with 30-40% of this increase due to air pollution cuts. Research under review also predicts more rapid increases in rainfall during the tropical monsoon seasons in response to dramatic cuts in Asian air pollution. However, it confirms that acceleration of climate change is limited to the next few decades, after which the effect is swamped by the dominant response to greenhouse gas increases. Continuing to emit greenhouse gases into the atmosphere at the current rate will drive far larger and more sustained temperature rises that along with changes in the global water cycle will cause serious impacts for our societies and the ecosystems upon which we depend. This underlines the importance of rapidly reducing greenhouse gas emissions and expanding upon the Paris Agreement targets in the postponed Glasgow COP26 meeting.

Figure 3: Satellite data suggests that aerosol pollution was already declining in China but increasing in India (from Samset et al. 2019 Nature Geosci. Fig. 1)

The recent improvements in air quality and temporary slowing of CO2 emissions resulting from the COVID-19 lockdowns is no silver lining to the awful damage from the pandemic on families and societies. Yet there is an opportunity to re-evaluate what is important for life and society, changing how we work and live to follow a less dangerous and healthier lower carbon path. It may even offer a unique natural laboratory to better understand how pollutant particles affect weather systems and climate. Although reducing particle pollution can temporarily accelerate climate change, moving rapidly to a cleaner, low carbon future by reducing greenhouse gas emissions and other air pollution is essential in avoiding dangerous climate change and improving quality of life. And how will populations cope when the next pandemic strikes if countries are crippled by the effects of climate change?

References

Allan RP, M Barlow, MP Byrne, A Cherchi, H Douville, HJ Fowler, TY Gan, AG Pendergrass, D Rosenfeld, ALS Swann, LJ Wilcox and O Zolina (2020) Advances in understanding large-scale responses of the water cycle to climate change, Annals of the New York Academy of Sciences, in press, doi: 10.1111/nyas.14337

Luo, F., Wilcox, L. J., Dong, B., Su, Q., Chen, W., Dunstone, N., Li, S. and Gao, Y. (2020) Projected near-term changes of temperature extremes in Europe and China under different aerosol emissions. Environmental Research Letters, 15 (3). 034013. doi: 10.1088/1748-9326/ab6b34

Samset, B. H., Lund, M. T., Bollasina, M. A., Myhre, G. and Wilcox, L. (2019) Emerging Asian aerosol patterns. Nature Geoscience, 12. pp. 582-584. doi: 10.1038/s41561-019-0424-5

Schwarz M, D Folini, S Yang, RP Allan, M Wild (2020) Changes in atmospheric shortwave absorption as important driver of dimming and brightening, Nature Geoscience, 13, 110-115, doi: 10.1038/s41561-019-0528-y

Wilcox, L. J., Liu, Z., Samset, B. H., Hawkins, E., Lund, M. T., Nordling, K., Undorf, S., Bollasina, M., Ekman, A. M. L., Krishnan, S., Merikanto, J. and Turner, A. G. (2020) Accelerated increases in global and Asian summer monsoon precipitation from future aerosol reductions. Atmospheric Chemistry and Physics Discussions. ISSN 1680-7375 doi: 10.5194/acp-2019-1188

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