Cumulus clouds amplify boreal forest photosynthesis

Boreal forests, covering 14% of Earth’s land, are key players in the climate system. They store carbon, regulate energy and water cycles, and release aerosols that interact with the atmosphere.

A new study by Ezhova et al., based on long-term observations at the SMEAR II station in Finland, shows that organic aerosols enhance photosynthesis under cumulus clouds.
When aerosol concentrations are high, they scatter sunlight into more diffuse radiation. This boosts photosynthesis in boreal forests, especially in the morning, evening, or at intermediate cloud cover when leaves’ stomata stay open.
The effect is significant: daily carbon uptake under cumulus clouds rises by about 12% of the maximum possible diffuse light enhancement.

These findings provide the observational evidence that atmosphere-biosphere feedbacks strengthening photosynthesis occur not only under clear skies but also under the cumulus clouds typical of boreal summers.