005 | The Hidden Cost of Cleaner Air? Earth System Warming
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The Hidden Cost of Cleaner Air? Earth System Warming
📘 Summary of: "Reduced Aerosol Pollution Diminished Cloud Reflectivity Over the North Atlantic and Northeast Pacific" by Knut von Salzen, Ayodeji Akingunola, Jason N. S. Cole, Ruth A. R. Digby, Sarah J. Doherty, Luke Fraser-Leach, Edward Gryspeerdt, Michael Sigmond and Robert Wood.
Pollution Cuts Are Reducing Cloud Reflectivity
Between 2003 and 2022, clouds over the North Atlantic and Northeast Pacific became notably less reflective, allowing more sunlight to reach the ocean and accelerating warming in these regions. While most climate models failed to predict this trend, new research reveals a surprising culprit: efforts to improve air quality by reducing sulfur dioxide emissions.
A new paper by von Salzen et al. finds that reducing air pollution, while essential for public health, has inadvertently made cloud droplets bigger which makes clouds less bright and diminishes their ability to cool the planet. This reduction in cooling by clouds is contributing to a recent observed acceleration in climate warming. This means that ongoing reductions in particulate air pollution could continue to drive an acceleration in warming in coming decades.
A Mystery in the Satellite Data
Scientists have been puzzled by an accelerating decline in Earth's reflectivity, particularly the dimming of low-level clouds in the Northern Hemisphere. This change has been especially pronounced over the North Atlantic and Northeast Pacific, where satellite observations show cloud reflectivity decreased by close to 3% per decade over the past 20 years. During the same period, sea surface temperatures in these regions rose rapidly, by about 0.4°C, with particularly severe marine heatwaves devastating ecosystems and fisheries in the Northeast Pacific.
Scientific studies have consistently shown that we expect the coverage of low marine clouds to decline as the climate warms (a warming-induced “feedback”), but that effect alone could not explain the changes observed in these two regions.
The timing raised questions: Could this simply be natural climate variability? The answer appears to be no. The observed changes exceed what models predict from internal climate fluctuations alone, pointing to external influences.
Fewer Aerosols, Bigger Droplets
The breakthrough came from examining multiple lines of satellite evidence together with model simulations. Researchers found that declining cloud reflectivity closely tracked reductions in the amount of aerosols in the atmosphere, as well as the size of cloud droplets. Changes in the concentration of droplets in clouds accompany the change in droplet size.
This figure highlights the Twomey effect, where an increase in aerosol particles, such as from pollution, leads to more, smaller cloud droplets.
Simulations conducted as part of the study linked the change in aerosol concentrations with wide-spread reductions of 5-10% in cloud droplet concentrations, with the most rapid losses occurring where cloud reflectivity dropped the most.
The connection traces back to basic cloud physics: aerosol particles serve as seeds for cloud droplets. When countries like China and the United States drastically reduced sulfur dioxide emissions from power plants (China's emissions alone dropped by 16 million metric tons per decade) there were fewer particles available to form cloud droplets. Fewer droplets mean larger individual droplets, which make clouds less reflective (known as the Twomey effect) and more prone to precipitation, shortening cloud lifetimes (known as the Albrecht effect).
Most Models Missed the Mark
When the authors examined 24 Earth System Models from the latest international climate modeling effort, they found a striking discrepancy: most models significantly underestimated both the spatial extent and magnitude of the cloud changes.
However, the 10 models that best simulated the aerosol-cloud interactions came much closer to observations. This divergence highlights a critical weakness: many current climate models don't adequately represent how aerosols influence clouds.
Isolating the Aerosol Contribution
To isolate the specific contributions to the observed cloud changes of different factors, the research team developed an improved version of Canada's Atmospheric Model (CanAM) with more sophisticated aerosol physics. By running the model with and without changing emissions, and with constant versus changing sea surface temperatures, they could determine the relative importance of each driver.
The results revealed that declining aerosol emissions accounted for 69% of the cloud reflectivity decrease by altering clouds, while warming-induced cloud feedbacks contributed the remaining 31%. In the model simulations, the Albrecht effect proved particularly important, contributing more to the trend than the better-known Twomey effect.
Regional Warming in a Cleaner World
These findings carry weighty implications. The cloud reflectivity changes in the North Atlantic and Northeast Pacific contributed 0.15 W/m² per decade to Earth's global energy imbalance, a substantial forcing that took place over just 14% of Earth's surface. To put this in perspective, global CO₂ increases during the same period contributed 0.31 W/m² per decade on average across the whole globe.
The research presents a difficult reality: air quality improvements that save lives by reducing particulate pollution simultaneously reduce the cooling that aerosols and clouds provide. With aerosol precursor emissions projected to continue declining through mid-century under most scenarios, this warming acceleration may persist.
A Critical Role for Sustained Observations
This study also underscores the irreplaceable value of sustained observations, specifically satellite observations. Only by combining multiple satellite datasets – CERES for radiation, MODIS for cloud properties, MISR for aerosol depth, and HadCRUT5 for sea surface temperature – could researchers piece together the full story. The satellite records spanning two decades provided the extended baseline needed to distinguish forced changes from natural variability, and the drivers of the forced changes.
The Path Forward
The path forward requires improvement of climate models to more accurately represent aerosol-cloud interactions, reassessment of near-term climate projections and continued monitoring of cloud and aerosol trends as clean air policies continue to reduce emissions globally. The research suggests current models may, on average, be systematically underestimating near-term regional warming in areas affected by declining aerosol pollution.
The Final Cut: This paper by von Salzen and colleagues shows how recent declines in particulate air pollution have inadvertently also reduced the amount of sunlight reflected by clouds over two broad regions of the ocean, helping resolve a previously unexplained mystery of what was driving reductions in sunlight reflection from these regions. Importantly, this paper also shows that many of the climate models used to attribute recent climate changes and project future climate underestimate this effect of particulate pollution on clouds, meaning they are likely to also underestimate the rate of climate warming to expect in the coming decades as more of the world moves to less-polluting energy sources in the interest of public health
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👉 Want to dive deeper? Check out:
Loeb, N.G., Ham, SH., Allan, R.P. et al. Observational Assessment of Changes in Earth’s Energy Imbalance Since 2000. Surv Geophys 45, 1757–1783 (2024). https://doi.org/10.1007/s10712-024-09838-8
Goessling et al, Recent global temperature surge intensified by record-low planetary albedo. Science, 387 , 68-73(2025). DOI:10.1126/science.adq7280
James E Hansen, Makiko Sato, Leon Simons, Larissa S Nazarenko, Isabelle Sangha, Pushker Kharecha, James C Zachos, Karina von Schuckmann, Norman G Loeb, Matthew B Osman, Qinjian Jin, George Tselioudis, Eunbi Jeong, Andrew Lacis, Reto Ruedy, Gary Russell, Junji Cao, Jing Li, Global warming in the pipeline, Oxford Open Climate Change, Volume 3, Issue 1, 2023, kgad008, https://doi.org/10.1093/oxfclm/kgad008
Cheung, W.W.L., Frölicher, T.L. Marine heatwaves exacerbate climate change impacts for fisheries in the northeast Pacific. Sci Rep 10, 6678 (2020). https://doi.org/10.1038/s41598-020-63650-z