Source: The Conversation – Canada – By Knut von Salzen, Senior Research Scientist, Marine Cloud Brightening Research Program, University of Washington
Winter is setting in across the Northern Hemisphere, and with it, cold and cloudy winter days. Clouds play a vital role in the environment, providing rain but also reflecting sunlight before it reaches the Earth’s surface.
But between 2003 and 2022, clouds over the North Atlantic and Northeast Pacific became less reflective, allowing more sunlight to reach the ocean surface and causing sea surface temperatures to rise.
My colleagues and I recent conducted research that shows global efforts to improve air quality have unintentionally accelerated climate warming by modifying clouds.
While cleaner air has major health benefits, decreasing the amount of particulate pollution has also reduced the cooling effect of clouds, accelerating climate warming.
Dimming clouds and rising temperatures
Our study relied on two decades of satellite data to analyze the impacts of changes in particulate pollution and climate warming on the clouds. The data shows that low-level clouds in the Northern Hemisphere have dimmed rapidly since 2003.
In particular, cloud reflectivity over the North Atlantic and Northeast Pacific has fallen by nearly three per cent per decade. During the same period, sea surface temperatures rose about 0.4 C, intensifying marine heatwaves that have damaged ecosystems and fisheries.
We expected that climate warming from greenhouse gas increases would lead to a decrease in low clouds over the ocean. However, the observed changes were too large to be explained by this process or by natural climate variability, pointing to an additional cause of warming that many climate models underestimated.
The key factor turned out to be aerosols — tiny particles that act as seeds for cloud droplets. When there are fewer aerosols, clouds contain fewer but larger droplets. Those droplets reflect less sunlight and are more likely to rain out quickly, producing shorter-lived, darker clouds. This process weakens the cooling influence that low clouds have over marine areas.
The effect stems from two known mechanisms: the Twomey effect, where fewer aerosols make clouds less reflective, and the Albrecht effect, where larger droplets shorten cloud lifetime. Together, these changes reduce the planet’s overall reflectivity.
(Unsplash/Logan Hughes)
A cleaner atmosphere, a warmer planet
Ultimately, our study exposes a paradox: cleaner air benefits human health while also revealing the full force of greenhouse-gas warming, which has historically been “masked” by the cooling effect of particulate pollution.
Sulfur dioxide (SO₂) emissions — the main source of sulfate aerosols — have fallen sharply as countries adopted stricter air-quality regulations. China’s SO₂ emissions alone dropped by about 16 million metric tonnes per decade since 2003, with similar reductions in the United States and Europe. Cleaner air means fewer aerosol particles available to form bright, reflective clouds.
Our study showed five to 10 per cent declines in cloud droplet concentrations, especially in regions where cloud brightness fell most. The close correspondence between reduced aerosols, larger droplet size and cloud dimming confirmed that cleaner air was driving regional warming.
We analyzed 24 Earth system models and found that most underestimated the magnitude and extent of observed cloud changes. Only models that accurately represented how aerosols affect clouds matched real-world observations, highlighting a major modelling weakness.
In our study, we separated the effects of particulate air-pollution cuts from cloud changes driven by general warming. The results showed that declining aerosols accounted for 69 per cent of the cloud reflectivity loss, while warming explained 31 per cent. Our simulations indicate that changes in cloud lifetime in response to having larger droplets (the Albrecht effect) proved more influential in the change in cloud droplet size itself (the Twomey effect).
Reduced cloud brightness in these ocean regions added about 0.15 watts per square metre (W/m²) per decade to Earth’s global energy imbalance, even though the regions cover only 14 per cent of the planet’s surface. Rising global CO₂ levels added roughly 0.31 W/m² per decade during the same time, meaning cleaner air produced nearly half as much additional warming as CO₂ itself in those areas.
This finding creates a policy challenge: air-quality improvements that save lives also remove a cooling shield that has been masking a significant portion of greenhouse-gas warming. Because aerosol emissions are projected to keep falling through mid-century, this “unmasking” could continue to contribute to faster rates of warming for decades.
Importance of continued observation
The satellites observing clouds and aerosols are nearing the end of their mission, with a phaseout expected in 2026. Long-term satellite monitoring proved essential for revealing the link between cleaner air, dimmer clouds and regional warming, and will continue to be essential for understanding future warming.
Our results suggest that many climate models may underestimate near-term regional warming as air particulate pollution declines. Improving the representation in models of how aerosols affect clouds and continuing global observations will be critical for more accurate projections.
Addressing the paradox of cleaner air uncovering hidden warming demands integrating air-quality and climate policy and accelerating the reduction of greenhouse gases — the only lasting way to cool the planet.
Knut von Salzen receives funding from the University of Washington’s Marine Cloud Brightening Research Program, which is funded by a consortium of individual and foundation donors. He is affiliated with the Climate Research Division of Environment and Climate Change Canada and the University of Victoria.
– ref. Reduced air pollution is making clouds reflect less sunlight – https://theconversation.com/reduced-air-pollution-is-making-clouds-reflect-less-sunlight-269805
