Volcanism, soft light and photosynthesis

Massive amounts of gas are emitted during volcanic eruptions, including H₂O, CO₂ and SO₂ - in that order of abundance – plus many others.  The H₂O has very little impact on climate, while the SO₂ has a significant net cooling effect because it is converted to liquid and solid sulphate aerosols in the atmosphere – and the aerosols reflect some of the incoming sunlight back into space.  The CO₂ has the potential to promote climate warming because of the greenhouse effect, but research on historical eruptions has not shown this to be the case.

A recently completed study – led by Lianghong Gu of the Oak Ridge National Lab - may shed some light on this conundrum (Gu et al., 2003).  Following the massive 1991 eruption of Mt. Pinatubo, Gu and his colleagues studied the light conditions and photosynthesis rates in a mixed deciduous forest.  Sulphate aerosols reflect some incoming sunlight, but they also produce a strong scattering effect.  One result is global cooling (by an average of 0.5º C for around 2 years following the Pinatubo eruption).  The other result is that plants receive less light directly from the sun, and more light from other parts of the sky.  While bright sunlight is a wonderful thing, few plants have leaves that can take full advantage of the intense direct radiation.  Furthermore, under direct sunlight conditions, it is only the leaves on the outside of the canopy that “see” the bright light.  Leaves lower down tend to be in full shade. 

With diffusion by volcanic areosols, on the other hand, the light is much more evenly spread within the forest canopy, a much higher proportion of the leaves experience effective light levels, and the overall rate of photosynthesis is strongly enhanced.

Gu and colleagures collected data on cloud-free days, with and without the effects of the Pinatubo aerosols, and found that the diffusion caused by the aerosols increased the photosynthetic rate by approximately 20% in 1992 versus the period from 1995 to 1997.  This finding is consistent with measurements of global CO₂ growth rates, which fell more dramatically after the 1991 eruption than at any other time since 1950, when record-keeping started. 

Sulphate aerosols also contribute to cloudiness, and increased cloudiness also enhances the diffuse light effect. 

Although the 1991 Pinatubo eruption was the largest in a century, it was small in comparison to some other historical eruptions (eg. the 1815 Tambora eruption was roughly 10 times as large), and especially in comparison with some of the long-sustained flood basalt eruptions of the distant past, such as the Deccan Traps at the Cretaceous-Tertiary boundary (65 m.y.) or the Siberian Traps of the Permian-Triassic boundary (250 m.y.).  Land vegetation was well established by the Permian, and it is interesting to speculate whether these large volcanic events would have had correspondingly large impacts on atmospheric carbon dioxide levels.

Reference

Gu, L, Baldocchi, D, Wofsy, S, Munger, J, Michalsky, J, Urbanski, S and  Boden, T, 2003, Response of a deciduous forest to the Mount Pinatubo eruption: Enhanced photosynthesis. Science, V. 299, p. 2035-2038.

Steven Earle, Malaspina University College, Nanaimo, B.C., 2003. Return to Earth Science News