While cleaner air has major health benefits<\/a>, decreasing the amount of particulate pollution has also reduced the cooling effect of clouds, accelerating climate warming.<\/p>\n Dimming clouds and rising temperatures<\/p>\n 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.<\/p>\n 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<\/a>. <\/p>\n We expected that climate warming from greenhouse gas increases would lead to a decrease in low clouds<\/a> 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.<\/p>\n The key factor turned out to be aerosols \u2014 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.<\/p>\n The effect stems from two known mechanisms: the Twomey effect<\/a>, where fewer aerosols make clouds less reflective, and the Albrecht effect<\/a>, where larger droplets shorten cloud lifetime. Together, these changes reduce the planet\u2019s overall reflectivity.<\/p>\n View of an overcast sky from the coast near Ogunquit, Maine. With fewer aerosols in the air, clouds become less reflective, allowing more sunlight to reach the ocean surface. A cleaner atmosphere, a warmer planet<\/p>\n 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 \u201cmasked\u201d by the cooling effect of particulate pollution. <\/p>\n Sulfur dioxide (SO\u2082) emissions \u2014 the main source of sulfate aerosols \u2014 have fallen sharply<\/a> as countries adopted stricter air-quality regulations. China\u2019s SO\u2082 emissions alone dropped by about 16 million metric tonnes per decade since 2003<\/a>, with similar reductions in the United States and Europe. Cleaner air means fewer aerosol particles available to form bright, reflective clouds.<\/p>\n 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.<\/p>\n 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.<\/p>\n 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).<\/p>\n Reduced cloud brightness in these ocean regions added about 0.15 watts per square metre (W\/m\u00b2) per decade to Earth\u2019s global energy imbalance, even though the regions cover only 14 per cent of the planet\u2019s surface. Rising global CO\u2082 levels added roughly 0.31 W\/m\u00b2 per decade during the same time, meaning cleaner air produced nearly half as much additional warming as CO\u2082 itself in those areas.<\/p>\n 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 \u201cunmasking\u201d<\/a> could continue to contribute to faster rates of warming for decades.<\/p>\n![\"a](\"https:\/\/www.newsbeep.com\/nz\/wp-content\/uploads\/2025\/11\/file-20251114-76-tl29yb.jpg\")
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\n (Unsplash\/Logan Hughes)<\/p>\n
![\"clouds](\"https:\/\/www.newsbeep.com\/nz\/wp-content\/uploads\/2025\/11\/file-20251114-56-ykv4l9.jpg\")
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