Scientists show African forests are major carbon sinks
18 February 2009, by Tom Marshall
Trees in African tropical forest trees are getting bigger and this means they are net absorbers of carbon dioxide, according to new research.
The findings, published in Nature, confirm that Africa's forests are second only to those of South America, which cover a bigger area, in locking away CO2 that would otherwise stay in the atmosphere and disrupt the climate.
Based on long-term monitoring of around 70,000 trees in 79 one-hectare plots across ten African countries, these are the first direct observations on biomass change and carbon absorption in African forests that are of comparable rigour to previous Amazonian studies.
The researchers analysed the African results in combination with existing data on South American and Asian forests. The results, based on monitoring over time of some 250,000 trees, show that tropical forests remove about 4.8 billion tonnes of CO2 from the atmosphere every year. African forests account for 1.2 billion of these.
'We cannot rely on this sink forever. Even if we preserve all remaining tropical forests, these trees will not continue getting bigger indefinitely.'
Dr Simon Lewis
This cushions the effect of human emissions on the climate. But it won't do so for ever. 'We are receiving a free subsidy from nature,' says lead author Dr Simon Lewis, a Royal Society research fellow in the Earth & Biosphere Institute at the University of Leeds. 'Tropical forest trees are absorbing about 18% of the CO2 added to the atmosphere from burning fossil fuels, substantially buffering the rate of climate change.'
Limits to growth
So far this capacity even seems to be increasing - it's not certain why, but researchers think the extra CO2 in the atmosphere could be acting like a fertiliser and letting plants grow more quickly. But trees can only grow bigger for so long, before other factors such as soil nutrients limit growth. The scientists warn that action is urgently needed to curb the deforestation that threatens forests across the tropics.
'Whatever the cause of trees' net increase in CO2 absorption, we cannot rely on this sink forever,' Lewis adds. 'Even if we preserve all remaining tropical forests, these trees will not continue getting bigger indefinitely.'
The United Nations Intergovernmental Panel on Climate Change reports that humans emit 32 billion tonnes of CO2 worldwide, but that only around 15 billion tonnes stays in the atmosphere. Around half of the other 17 billion is absorbed by the oceans. The rest goes somewhere into land-based carbon sinks.
The new research for the first time lets scientists quantify just how much of this land-based absorption is down to tropical forest trees, and the answer seems to be around half of it.
The research comes at a vital time for protecting tropical forests - the idea is gaining widespread support, and is likely to an important theme at the negotiations to limit carbon emissions in Copenhagen later in 2009.
'To get an idea of the value of the sink, the removal of nearly five billion tonnes of carbon dioxide from the atmosphere by intact tropical forests, based on realistic prices for a tonne of carbon, should be valued at around £13 billion per year,' says Dr Lee White, Chief Climate Change Scientist for the government of Gabon and co-author of the study. 'This is a compelling argument for conserving tropical forests,' he adds.
Lewis argues that rich polluting countries should effectively pay countries with tropical forests to reduce deforestation rates and provide alternative pathways to development. 'I hope some measures to protect forests will come out of Copenhagen,' he says. 'The key thing is to reduce deforestation - once a forest is cut down, the carbon it contains returns to the atmosphere, and it would take hundreds of years to recover the carbon it once stored,' he explains.
'We have to stop the kinds of land use change that begin the chain leading to deforestation - primarily industrial logging and big agricultural projects,' Lewis adds. 'But we also need to help replace the revenues that the governments in question would have got from these projects, as well as providing alternative livelihoods for local people.'
One possibility to help developing nations pay to protect their forests would be to give them credits for doing so that they could sell to polluting countries under an emissions-trading scheme.
Another approach is to solicit donations from the international community to help poorer tropical nations protect their tropical forests. Brazil took this route last year, gaining a £100 million contribution from Norway. Likewise, June 2008 saw the launch of the Congo Basin Fund, with the British and Norwegian governments chipping in another £100 million.
Since the late 1990s the scientific consensus has shifted to the view that mature tropical forests continue to take in CO2, in contrast to the earlier opinion that forests stop absorbing significant volumes of carbon once they were mature.
The change began with a paper in 1998 by Professor Oliver Phillips of Leeds University, which surprised forest scientists by revealing trees in the Amazon were continuing to act as carbon sinks.
Forests in a warming world
Forests' capacity to absorb carbon is under threat, not just from rampant deforestation but also ultimately from a hotter, drier globe. In these conditions more trees are expected to die, and devastating forest fires to become more common. The drought that gripped the Amazon basin in 2005 could be a taste of things to come.
When fire sweeps through the landscape, large amounts of the carbon it contains - whether it's stored in tree stems and leaves, in root systems or in dead matter on the forest floor - gets returned to the atmosphere. This is likely in turn to raise temperatures and make fires still more likely in future.
'It's not at all clear how tropical forests will respond to rising temperatures,' explains Lewis. 'It could be that they will shut down photosynthesis, and that this could offset the current carbon sink. It's a real concern - some models show carbon absorption reaching a plateau, others predict slow decline, and others show the forests declining steeply and turning into a major carbon source.'
The research was carried out by a large number of scientists involved with the African Tropical Rainforest Observation Network, or Afritron. Funding came from the Royal Society and the Natural Environment Research Council, as well as from numerous institutions and donors who provide funding for Afritron.
Lewis 2006 Tropical Forests and the Changing Earth System. Phil Trans B 261, 195-210.
Phillips et al. 2008 The Changing Amazon Forest Phil Trans B 363, 1819-1827.
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