Scientists in the US say the world’s tropical forests may be making a much larger contribution to slowing climate change than many of their colleagues have previously recognised.
A new study − led by the space agency NASA and the US National Center for Atmospheric Research, and published in the Proceedings of the National Academy of Sciences − suggests that the forests are absorbing far more carbon dioxide from human sources than they are given credit for.
It estimates that the forests are absorbing 1.4 billion tonnes of human-derived CO2 − a sizeable slice of the total global absorption of 2.5 billion tonnes.
If the tropical forests are left undisturbed, the trees should be able to go on reducing the rate of global warming by removing CO2 from the atmosphere.
Damaging effect
Conversely, continuing destruction of the forests may prove to have an even more damaging effect on countering the rising rate of CO2 emissions, because if the rate of absorption slows down, the rate of global warming will accelerate.
Lead author David Schimel, a research scientist at the NASA Jet Propulsion Laboratory, says: “This is good news, because uptake in northern forests may already be slowing, while tropical forests may continue to take up carbon for many years.”
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Until our analysis, no one had successfully completed a global reconciliation of information about carbon dioxide effects from the atmospheric, forestry, and modeling communities
Joshua Fisher, researcher, NASA Jet Propulsion Laboratory
The question of which type of forest absorbs more carbon “is not just an accounting curiosity”, says one of the paper’s co-authors, Britton Stephens, a scientist at the National Center for Atmospheric Research’s Earth Observing Laboratory in Boulder, Colarado.
“It has big implications for our understanding of whether global terrestrial ecosystems might continue to offset our carbon dioxide emissions or might begin to exacerbate climate change.”
Forests and other land vegetation currently remove up to 30 per cent of human CO2 emissions from the atmosphere by absorbing the gas during photosynthesis.
The new study is the first to devise a way to make direct comparisons of CO2 uptake estimates from many sources at different scales, including computer models of ecosystem processes, atmospheric models used to deduce the sources of today’s concentrations (called atmospheric inverse models), satellite images, and data from routine and experimental forest plots.
Ecosystem model
The researchers reconciled these analyses and assessed the accuracy of the inverse models based on how well they reproduced independent, airborne and ground-based measurements. They obtained their new estimate of the tropical carbon absorption from the weighted average of atmospheric, ecosystem model and ground-based data.
“Until our analysis, no one had successfully completed a global reconciliation of information about carbon dioxide effects from the atmospheric, forestry, and modeling communities,” says the report’s co-author, Joshua Fisher, a researcher at the NASA Jet Propulsion Laboratory. “It is incredible that all these different types of independent data sources start to converge on an answer.”
As human-caused emissions add more CO2 to the atmosphere, forests worldwide are using it to grow faster, reducing the amount that stays airborne. This effect is called carbon dioxide fertilisation.
But climate change also decreases the amount of water available in some regions and warms the Earth, causing more frequent droughts and larger wildfires.
For about 25 years, most atmospheric inverse models have been showing that mid-latitude forests in the northern hemisphere absorb more CO2 than tropical forests. This result was based on the prevailing understanding of global air flows and limited data suggesting that deforestation was causing tropical forests to release more CO2 than they were absorbing.
Measurements of CO2
In the mid-2000s, Britton Stephens used measurements of CO2 made from aircraft to show that many atmospheric inverse models were not correctly representing flows of the gas in the air above ground level. Models that matched the aircraft measurements better showed more carbon absorption in the tropical forests.
Dr Schimel says the new paper reconciles results at every scale − from the pores of a single leaf, where photosynthesis takes place, to the whole Earth, as air moves carbon dioxide around the globe.
There is still considerable uncertainty about the part played by the tropical forests in moderating the climate. One study, for example, found that trees in the forests of Borneo absorbed much more CO2 than those in Amazonia. Another found that the southern Amazon forest was drying out far faster than had been projected.
Meanwhile, the rate of deforestation continues to increase in many vulnerable areas.
In June 2014, it was reported that Indonesia’s clearance of its forests was, for the first time, happening faster than in Brazil. Three months later, the Brazilian NGO Imazon said the rate of forest loss in the country’s Amazon region had risen by 290 per cent in the past 12 months.
