Wednesday, December 16, 1998 Published at 19:21 GMT
Amazon carbon ebbs and flows
The Amazon contains immense quantities of carbon, some of which escapes to the atmosphere
By 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). 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.
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 carbon or emits it appears to be the moisture of the soil.
The researchers say that El Niño episodes, which have been particularly pronounced in recent decades, have a strong effect on land ecosystems in the tropics. And in El Niño 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.
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".