Cloudy skies have slowed global warming – but that may be changing
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- Gregory Cesana and Assia Arouf at Columbia University estimated low cloud cover back to 1979 for the first time, correlating sea surface temperature, wind speed, and atmospheric stability with modern satellite data to fill gaps in earlier measurements.
- Low cloud cover has been expanding since 1979, increasing the energy reflected from Earth's surface by 0.32 watts per square metre — a small but real fraction of the 2.72 W/m² of human-caused warming.
- Eastern Pacific low clouds have grown by 0.18% per decade because the region warmed less than models expected while the western Pacific heated strongly, producing temperature inversions that stabilize stratocumulus decks.
- Stricter sulphur pollution controls are expected to heat the eastern Pacific, drying out the low cloud deck — Cesana calls this potentially 'unmasking' warming and further accelerating climate change.
- Paulo Ceppi at Imperial College London reported in March that global low cloud cover has decreased since 2003, and a separate 2024 study linked recent warming acceleration to an emerging positive low-cloud feedback.
- The developing El Niño in the eastern Pacific is already diminishing low clouds, giving researchers 'a taste of what could happen in the future,' according to Cesana.
- The study was published in Geophysical Research Letters (DOI: 10.1029/2026GL124158).
Why it matters: If the low-cloud cooling trend reverses, decades of partly offset warming could be unleashed at once — and the trigger isn't only greenhouse gases but also sulphur pollution rules phasing out. That makes a climate-policy success (cleaner air) a potential accelerant of the warming those same policies aim to limit.
