Amazon Carbon Ebbs and Flows
12/16/98
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RELAYED TEXT STARTS HERE:
Title: Amazon carbon ebbs and flows
The Amazon contains immense quantities of carbon, some of
which escapes to the atmosphere
Source: BBC
Status: Copyright 1998, contact source for permission to reprint
Date: December 16, 1998
Byline: Environment Correspondent Alex Kirby
The forests of the Amazon sometimes breathe out vast
quantities of carbon dioxide (CO2) - far more than they
absorb, new research has suggested.
The region contains almost half of the world's undisturbed
tropical evergreen forest. Its vegetation had been thought
always to absorb a lot of the CO2 that is the chief human
contribution to global warming.
But a paper in the science magazine Nature says that
Amazonia may sometimes have the opposite effect. The reason,
the researchers say, is that the trees can become too hot
and dry.
Ten per cent of all the CO2 stored in terrestrial ecosystems
is locked up in the Amazon basin, when it is in storage
mode. But researchers from the Max Planck Institute for
Biogeochemistry in Germany say that this is dependent on a
number of factors.
In 1981 and 1993, for example, the Amazon stored away 0.7
petagrams (Pg) of CO2 (a petagram is a trillion kilograms).
for In 1987 and 1992, however, even after using some of the
CO2 by photosynthesis in growing vegetation, it added to the
atmosphere 0.2 Pg.
Soil moisture
For comparison, deforestation in the Amazon in the early
1990s is thought to have added 0.3 Pg of CO2 annually. One
of the main factors that decides whether the Amazon stores
In carbon or emits it appears to be the moisture of the soil.
Soil moisture holds the key to growth rates. And that in
turn is affected by temperature and rainfall, which can be
influenced by events thousands of miles from Amazonia.
The researchers say that El Nio episodes, which have been
particularly pronounced in recent decades, have a strong
effect on land ecosystems in the tropics. And in El Nio
years - times of hot, dry weather in the region - Amazonia
produces more CO2 than it absorbs.
The increasing amounts of CO2 in the atmosphere are known to
have a marked effect on the productivity of the region's
vegetation. The researchers say: "The strength of the CO2
fertilisation effect for the Amazon basin was between 0.1 to
0.4 Pg [of added carbon] per year for the 15-year period
1980-1994.
"The CO2 effect includes both the direct stimulation of
plant growth by CO2 and the indirect enhancement of plant
water use efficiency."
They were anxious to establish the credibility of the model
they used by seeing whether its results were consistent with
field measurements. They found their model-derived estimate
of what was happening to the CO2 was the same as the field
estimate for the forest in Rondonia, in western Amazonia.
Near Manaus in central Amazon basin, and at a research
station on the Brazilian savannah, the model results were
significantly lower than the field estimates.
The researchers say that the productivity of the Amazon
ecosystem is clearly sensitive to climate variability. And
they believe that changes in the amount and timing of
rainfall cause the main effects on growth rates.
They say that rainfall changes "combine with changes in
temperature to affect soil moisture, the factor we have
identified as an important controller of carbon storage in
the Amazon basin."
And they call for regional and global information on carbon
uptake and release by terrestrial ecosystems. They say this
will be needed "as we enter a post-Kyoto period in which we
aim to manage the global carbon cycle".