The ability of Amazon forest to absorb carbon dioxide has halved in the past two decades, warn researchers
Trees are growing faster but dying younger, scientists found
(Pic: Flickr/CIFOR/Neil Palmer)
By Tim Radford
The Amazon rainforest, for so long one of the vital “green lungs” of the planet, is losing its capacity to absorb carbon from the atmosphere, according to new research.
Two decades ago, the forest drew down a peak of two billion tonnes of carbon dioxide each year from the atmosphere. Now, according to a study in Nature journal by more than 90 scientists, the rate of withdrawal has fallen to around half that total.
Fossil fuel emissions from Latin American countries are now running at more than a billion tonnes of CO2. So the region is putting more greenhouse gas into the atmosphere than it is taking out.
The finding is ominous. The Amazon rainforest has always been a big item in the climate modellers’ carbon budget − the calculation of carbon dioxide from fossil fuels burned, set against the natural absorption of the same trace gases by the biosphere.
Unique research
The implication now is that the forest is no longer a carbon “sink” that that can be relied upon to take up a predictable proportion of fossil fuel emissions.
The conclusion is the outcome of a unique international research network’s 30-year study of 189,000 individual trees in 321 plots of forest dotted across six million square kilometres in eight South American countries. And it has revealed a huge surge in the rate of tree deaths across the Amazon basin.
The Amazon is one of the wonders of the planet: its 300 billion trees, and 15,000 species, store one-fifth of all the carbon in the planet’s biomass.
The RAINFOR collaboration involves 57 organisations in 15 nations and is dedicated to detailed study of the forest and the ecosystems that depend upon it.
Researchers have repeatedly warned of the consequences of destroying rainforest through logging, land-clearance for plantation, and other assaults upon the gigantic rainforest, but this finding is of a different magnitude altogether.
Roel Brienen, a geographer at the University of Leeds in England, and the lead author of the study, puts it bluntly: “Tree mortality rates have increased by more than a third since the mid-1980s, and this is affecting the Amazon’s capacity to store carbon.”
Carbon dioxide is not, of itself, the problem. Plants capture it with photosynthesis to build tissues and feed the animal kingdom. And, in a greenhouse world, more carbon dioxide should make vegetation more fertile, and more likely to soak up at least a proportion of the extra carbon dioxide from car exhausts and power station chimneys.
Spurt of growth
But the message from this latest study is that it doesn’t necessarily work out like that. The extra carbon dioxide stimulated a spurt of extra growth, and the trees lived faster, only to die sooner.
The problem has been compounded by drought and unusually high temperatures in the Amazon. The spurt in arboreal death rates began well before 2005, but droughts since then have killed millions of additional trees.
“Regardless of the causes behind the increase in tree mortality, this study shows that predictions of a continuing increase of carbon storage in tropical forests may be too optimistic,” Dr Brienen says.
“Climate change models that include vegetation responses assume that as long as carbon dioxide levels keep increasing, then the Amazon will continue to accumulate carbon. Our study shows that this may not be the case, and that tree mortality processes are critical in this system.”
This story was produced by the Climate News Network