2001 Vol. 1, No. 14
Biological sequestration of carbon
long has been considered by climate change activists as a potential
way to slow the rate at which the atmosphere’s carbon dioxide (CO2)
concentration rises. Fear of possibly catastrophic changes in climate
suggested by computer simulations of what happens once the CO2 concentration
doubles caused carbon sequestration to be incorporated into climate
treaty negotiations. If one accepts the validity of the computer
model-generated global warming scenarios produced by the Intergovernmental
Panel on Climate Change, some such strategy seems warranted. The
problem with this way of thinking, however, is the notion that the
concentration will double unless something new is done. But, as
noted by Wofsy (2001), the rate of CO2 accumulation
in the atmosphere over the past two decades already "has stayed
the same or even declined slightly," in spite of the fact that
emissions of CO2 from the combustion of fossil fuels have increased
almost 40 percent over the same time period.
Wofsy entitled his article, "Where
has all the carbon gone?" If humans are not removing it from
the air, something else must be. That something only can
be a natural process of biological carbon sequestration.
Earth’s vegetation already has, automatically, risen to the challenge
(or opportunity) and grown ever more vigorously in response to the
aerial fertilization effect of atmospheric CO2 enrichment. This
biological response is the greening of planet earth.
While some contend this makes for a
nice story and admit the concept does have solid experimental backing,
they ask if there is any real-world evidence that it truly is occurring
where it really counts – in nature. Data from a number of meticulous
scientific studies suggest that it is.
In a 1994 paper published in Science,
Phillips and Gentry report results from tree biomass assessments
at forty of the world’s tropical forests. They demonstrated that
these highly productive ecosystems have become even more productive,
at least since 1960. They became still more productive after 1980.
This phenomenon – because of its "consistency and simultaneity
of the changes on several continents" – is attributed to "enhanced
productivity induced by increased CO2" (Pimm and Sugden, 1994).
Four years later, Phillips et al.
(1998) likewise published a paper in Science describing results
from similar measurements made "at fifty sites scattered throughout
the Amazon and Central America … of more than a hundred thousand
trees over the past thirty years." They also see an ongoing
acceleration of tree growth. At the time, they calculated that it
was large enough to account for approximately 40 percent of the
so-called missing carbon of the entire globe! In fact, Laurance
(1999) concludes, "In the Amazon basin alone, intact rainforests
could be absorbing over one billion tons of carbon dioxide each
If we fast-forward to the present,
we find a news item published on the National Geographic Society’s
website on December 13, 2001. Entitled "Adult Amazon Trees
Gain Mass, Puzzle Scientists," the article reports how a scientific
team – again headed by Phillips – marvels at the increasing growth
rates of large trees in the Amazon over the past two decades. They
describe their finding as "unexpected." This is because
"mature forests were thought to be ‘carbon neutral,’ giving
off as much carbon as they absorb each day." As a consequence
the scientists now contend, "The increased mass of Amazonian
forests suggests they are acting as a huge ‘carbon sink,’ absorbing
more carbon from the atmosphere than they release." Phillips
also suggests, "The Amazon forests have become more massive
as a result of increased amounts of carbon dioxide in the atmosphere."
But what about the northern boreal
forests? In a paper published September 16, 2001, in the Journal
of Geophysical Research, Zhou et al. report changes in
a satellite-sensed parameter called the normalized difference
vegetation index (NDVI) for the period July 1981 to December
1999. What did they find? Much of the vegetated area between 40°
and 70° N latitude in Eurasia shows "a persistent increase
in growing season NDVI over a broad contiguous swath of land from
central Europe through Siberia to the Aldan plateau, where almost
58% is forests and woodlands." They also note increases in
season-long NDVI in forests of the southeastern United States. Over
the two-decade period they studied, the NDVI data – which the authors
say "are surrogates of plant photosynthetic activity"
– increased in magnitude by 12 percent in Eurasia and by eight percent
in North America.
Laurance, W.F. 1999. Gaia's lungs:
Are rainforests inhaling earth's excess carbon dioxide? Natural
History (April), p. 96.
Phillips, O.L. and Gentry, A.H.
1994. Increasing turnover through time in tropical forests.
Science 263: 954-958.
Phillips, O.L., Malhi, Y., Higuchi,
N., Laurance, W.F., Nunez, P.V., Vasquez, R.M. Laurance, S.G.,
Ferreira, L.V., Stern, M., Brown, S. and Grace, J. 1998. Changes
in the carbon balance of tropical forests: Evidence from long-term
plots. Science 282: 439-442.
Pimm, S.L. and Sugden, A.M. 1994.
Tropical diversity and global change. Science 263:
Wofsy, S.C. 2001. Where has all
the carbon gone? Science 292: 2261-2263.
Zhou, L., Tucker, C.J., Kaufmann,
R.K., Slayback, D., Shabanov, N.V. and Myneni, R.B. 2001. Variations
in northern vegetation activity inferred from satellite data
of vegetation index during 1981 to 1999. Journal of Geophysical
Research 106: 20,069-20,083.
* * * * *
This continues a
series of "greening alerts" that Greening Earth Society
periodically publishes online in response to research concerning
the impact of carbon dioxide emissions on earth’s biosphere, especially
as it relates to plant life’s ability to sequester carbon. The alerts
are prepared by Drs. Sherwood B. Idso and Keith E. Idso of the Center
for the Study of Carbon Dioxide and Global Change in Tempe, Arizona