Eugene S. Takle
© 2003
The eruption of Mt. Pinatubo on 15 June 1991 injected 14-20 Tg (1 Tg = 1012 g) of
SO2 into the
earth's stratosphere, where it became globally distributed within about six months. The global
impact of this event is widely known to have led to a decrease in total solar radiation reaching the
ground, warming in winter, cooling in summer, drying of the atmosphere, and modification of global
cloudiness (Gu et al, 2003). Gu, et al. (2003) also point out that total solar radiation reaching
the ground consists of direct (reaching a point on earth directly the disk of the sun) and diffuse
(reaching the same point from a direction other than the disk of the sun because of having been
"bounced" off a liquid or solid particle in the atmosphere). Aerosol particles created from the
injected SO2 reduce the amount of direct solar radiation reaching a given point on the
ground and increase the amount of diffuse radiation.
On cloud-free days, most plants receive more direct solar radiation than they can use for
photosynthesis on leaves receiving direct radiation, and leaves that are shaded do not fully use
their photosynthetic capability. Therefore, when direct radiation is reduced by increased radiative
scattering by aerosol (increased diffuse radiation), photosynthesis is not reduced for the leaves in
"full sun" but is increased for shaded leaves. The net result is that the plant takes up more
atmospheric CO2 because of increased photosynthesis under such conditions even when total
solar
radiation (direct plus diffuse) is slightly reduced. Gu et al. (2003) describe measurements showing
that the aerosol put into the atmosphere by the eruption of Mt. Pinatubo caused a global increase in
the draw-down of atmospheric CO2 by plants. Farquhar and Roderick (2003) point out that
atmospheric
pollution does the same thing and that the "dimming" of the sun over the last 50 years due to air
pollution likely also contributes to increased efficiency of the global biospheric use of solar
radiation.
References
Farquhar, G., and M. L. Roderick, 2003: Pinatubo, diffuse light, and the carbon cycle.
Science, 299, 1997-1998
Gu, L., D. D. Baldocchi, S. C. Wofsy, J. W. Munger, J. J. Michalsky, S. P. Urbanski, and T. A.
Boden, 2003: Response of a deciduous forest to the Mount Pinatubo eruption: Enhanced
photosynthesis. Science, 299, 2035-2038.