The Amazon ‘carbon sink’ theory is as follows: a higher concentration of carbon dioxide in the atmosphere encourages photosynthesis, and therefore causes a growth spurt amongst trees and other vegetation in the Amazon rainforest. Their increased uptake of greenhouse gas allows them to grow larger in a shorter period of time, and therefore also sees larger quantities of carbon dioxide being stored.
A new study now reveals that, initially, the theory was valid. From the 1980s onwards, tree growth in the Amazon increased, and consequently the carbon sink which the rainforest created grew larger too, helping to restrict the overall amount of extra carbon dioxide entering the atmosphere.
However, the study also found signs that the ability of the trees to continue growing – and to continue limiting the effects of the increased concentrations of carbon dioxide – has peaked, as shorter lifespans and consequently earlier mortality impacts the more rapidly-growing trees. ‘What we think is happening is there’s essentially a cap on the sink,’ Oliver Phillips, professor at the School of Geography at the University of Leeds, and co-author of the study, tells Geographical. ‘There’s only so much biomass the forest can hold.’
The study, led by the University of Leeds, was unveiled in the journal Nature under the name ‘Long-term decline of the Amazon carbon sink’. Coordinated by the research organisation RAINFOR, 321 plots within the Amazon rainforest were analysed, and more than 200,000 trees identified and measured over a thirty-year period.
The results reveal that the ‘productivity’ of Amazonian vegetation – i.e. the growth rate of all the trees – has increased since the late 1980s. While this has helped the rainforest as a whole to store more carbon (acting as a net 'carbon sink'), a long-term increase in biomass mortality rates is now reducing the sink.
‘But the growth has appeared to saturate over the last ten years or so,’ says Phillips. ‘Mortality has been catching up and as a result the sink has been diminishing, so the net uptake to the forest has declined.’
Consequently, the flatlining of tree growth, and increasing mortality, means that the overall net amount of carbon dioxide being sequestered by the forest has been on a steady decline over the past two decades. ‘In the 1990s, we estimate the sink was about two billion tonnes of carbon dioxide per year,’ he continues. ‘That intact sink has declined to around one billion tonnes of CO2 in the last few years. To put that in perspective, the UK’s emissions are somewhat under half a billion tonnes of CO2 each year.’
Making reference to ongoing deforestation as well, Phillips adds that for the first time, the net global impact of the South American continent is now no longer removing more carbon dioxide from the atmosphere than is being created by human activity. ‘South American fuel emissions are approaching a billion tonnes of carbon dioxide each year,’ he says. ‘There is also at least as much coming from deforestation. So therefore this huge tropical continent is no longer acting as a carbon sink overall.’
As for the future, he warns that continuing trends, as well as the impact of drought and climate change, could lead to a situation where South American emissions exceed the amount of carbon dioxide the rainforest carbon sink can take in. ‘We’ve seen that in a couple of recent drought years – 2005 and 2010,’ he says. ‘It was actually a reversal, so mortality was greater than growth in both those years. We know it can happen, but those losses haven’t been sustained. I think the evidence suggests that drought would have to get more intense and more frequent if we were going to flip into a decline of biomass.’
This study included the work of hundreds of scientists over its three-decade lifespan, and Phillips also emphasises the importance of these kinds of studies in order to track changes in the wider environment. ‘If you don’t look for these sorts of patterns and changes and you don’t actually spend the effort on the ground tracking trees, we’d have no idea it was going on,’ he says. ‘There are other ways of looking at forests, which are also valuable different ways, but there’s nothing that can give us this degree of precision about what’s happening to individual trees. It’s fairly low-tech, it’s fairly easy to do, but it does require a lot of manpower and commitment.’