Why a volcanic eruption once triggered a surprising boom in global plant growth
In short: When Mount Pinatubo erupted in 1991, it blanketed the stratosphere in haze, dimming direct sunlight by 30 percent. Scientists expected a agricultural catastrophe, but instead, global plant productivity surged. The secret lies in diffuse sky radiation, which allows light to penetrate deep into forest canopies, fueling photosynthesis everywhere.
Diffuse sky radiation is sunlight that has been scattered by atmospheric molecules or particulates, deviating from its direct path without changing wavelength. This process, governed by Rayleigh and Mie scattering, is responsible for the sky's blue color and the soft, even illumination experienced under overcast conditions. While direct sunlight creates sharp shadows that limit photosynthesis to the top layer of a canopy, diffuse light acts like frosted glass, scattering photons into the shaded understory.
The 1991 eruption of Mount Pinatubo in the Philippines provided a massive, unintended experiment in this phenomenon. The volcano injected 17 million metric tons of sulfur dioxide into the stratosphere, creating a persistent haze that reduced direct solar irradiation. While researchers initially feared this would harm global food production, the Keeling Curve revealed a mysterious drop in the rate of atmospheric carbon dioxide accumulation. This was not due to reduced soil respiration, but rather an unprecedented boost in global plant growth.
This surge in net primary production occurred because the volcanic aerosols increased the proportion of diffuse light reaching the Earth's surface. By bypassing the limitations of direct shadows, this light enabled lower leaves to contribute more effectively to whole-plant photosynthesis. This mechanism, known as the aerosol direct radiative effect, suggests that even under conditions of reduced total sunlight, the quality of light can be the deciding factor in biological productivity, a principle that also applies to other atmospheric aerosols like smoke pollution.
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- Summary by Intellect · source: Diffuse sky radiation
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