Missing carbon mystery: Case solved?

Despite rapid deforestation, Stephen's team also showed that tropical forests are the net source of a mere 100 million tonnes of carbon annually, contrary to previous estimates of 1.8 billion tonnes. This suggests that carbon sequestration in the tropics is substantial enough to almost counterbalance the effects of deforestation. Stephens says "tropical forests are essentially in balance, absorbing as much carbon dioxide as they give off". One reason tropical forests could be absorbing substantially more carbon dioxide than accounted for in the models is the phenomenon known as CO₂ fertilization. Trees take in carbon dioxide to grow and when there is more of it they tend to grow faster, being fertilized by carbon dioxide. Also, as temperatures increase with climate change, soil organic matter decomposes more quickly, freeing up nutrients in the ground for forest growth.

Temperate and boreal forests are also being fertilized indirectly by nitrogen, largely from farming and fuel use, according to a study recently published a study in Nature, led by Federico Magnani from the University of Bologna. The study suggests that this is contributing to the carbon sink in northern latitudes, but Magnani says the same could not be true for tropical forests, where phosphorous and not nitrogen determines growth. He says that to understand what is happening in the tropics "we need to know how much of the carbon sink is the result of vegetation regrowth following deforestation, and how much of it comes from substantial carbon sequestration by primary forests". Manuel Gloor of the University of Leeds, UK, also argues that we need more information before any assertions on the whereabouts of the missing carbon sink can be confirmed. "To really settle the question regarding tropical versus northern hemisphere carbon sinks, a substantial amount of atmospheric concentration data over tropical land will be needed," he says.

Tropical forests are, however, rapidly disappearing. Forests in South America, Central Africa and South-East Asia are being cleared for cropland or cattle pasture, and reduced by the expansion of logging and changing patterns of cultivation. The latest IPCC report on mitigating climate change found that during 2004, the contribution of deforestation — primarily in the tropics — and the decay of biomass to global warming was 17.3% of total global greenhouse-gas emissions. "Cutting down tropical forests not only increases carbon emissions but it also removes a strong sink and its potential for offsetting future emissions," says Stephens.

Whether tropical or northern forests store more carbon might ultimately be academic, though, when it comes to mitigating climate change. Stephens believes that "relying on trees to mitigate climate change is not a good long-term strategy, because the carbon they store gets returned to the atmosphere on a timescale of around 30 years when they die and decompose. Afforestation and reforestation can provide short-term sinks to slow warming and possibly give us more time to find solutions, but ultimately we need to get the carbon into the ocean or geologic reservoirs, or not emit it in the first place".

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