Science for Brazil (Reino Unido)

Source or Sink? Unravelling Amazonia’s confused carbon balance

Publicado em 01 março 2014

There’s no more “evergreen” a scientific teaser than the role that the Amazon rainforest plays in climate change, and how to preserve it as a global “sink” for atmospheric carbon dioxide, rather than allowing it to become a “source” of the gas that exacerbates global warming

For decades, scientists have sought to understand the carbon balance in the Amazon Basin and discover how the forest affects global climate models. No surprise then, that the prestigious publication Nature in February 2014 weighed in with a cover edition to the subject, sporting a bouquet of new scientific papers.

Yet instead of offering definitive answers, the headline conclusions simply raised further questions as to why science may have been gazing at Amazonia through a distorting lens fogged by history, political sensitivities — and what turn out to have been some basic observational mistakes.

In effect, Nature in 2014 is reporting a tropical re-run of the bitter late 1980s debate over the state of the Antarctic ozone hole.

Back then, American satellites continued to report observations of a stable ozone hole at the same time as earth-based observations from Antarctic bases, often using quite crude devices, showed a dramatically enlarge hole. It was eventually shown that software on the satellites had been erroneously programmed to exclude ozone changes.

This time around, it is now revealed that satellites have for years been showing the Amazon to be in a much healthier condition than is really the case. Once again, it has taken low-level atmospheric observations by a team of Brazilian, UK and US scientists to help show up the mistake.

Research led by Luciana Vanni Gatti of Brazil’s Nuclear Energy Research Institute (IPEN) used carbon-sniffing aircraft to measure the “breath” of the Amazon forest. And in the process Gatti and her research colleagues Emanuel Gloor of Leeds University (U.K.), and John B. Miller of the National Oceanic and Atmospheric Administration (NOAA), have provided evidence that helps lay to rest some long-held and misleading observations about the real state of Amazonia.

Gatti found that the drought that hit the Amazon Basin in 2010 was so severe that it compromised the forest’s capacity to absorb excess carbon dioxide. By contrast in 2011, a year with above-average rainfall, the vegetation managed to absorb not only all the CO2 emitted through natural processes but also the emissions resulting from human activities, including fires. “Lack of rainfall modifies the dynamics of the forest and the carbon balance in the region,” she said.

The data are part of a study funded by the United Kingdom’s Natural Environment Research Council (NERC) and FAPESP (in the FAPESP Research Program on Global Climate Change), which was published in the cover of the latest issue of Nature magazine.

The other paper published by Nature was, in effect, a significant correction to the body of scientific thought about Amazonia and its role in climate change, that was based on an elementary mistake. For years, scientists had believed the Amazon rainforest continued robustly healthy during periods of drought, because the forest appeared green. Therefore, they reasoned, the forest must also be absorbing carbon dioxide as it continued to grow during dry periods.

But they were wrong. It was no more than a “trick of the light” that made the forest look green and deceived the satellites. New Scientist also reported on the research findings in the paper of D.C Morton  from NASA’s Goddard Space Flight Center,  published by Nature.

This is how Nature presented it: “In a study published on Nature’s website (D. C. Morton et al. Nature; 2014), researchers show that this is, literally, an illusion. The forest does not become greener during dry periods at all. It just looks that way when the sensor and the Sun are both in the south of the sky. It is not photosynthesis that drives the apparent greening of the forest at such times, but a lack of shadow.

The uncertainty was triggered by a surprising result from satellite images, which seemed to show that Amazon forests became greener during the dry season, and greenest of all during years of severe drought such as 2005 (S. R. Saleska et al. Science 318, 612; 2007). More green means more photosynthesis, so this result suggested that it was the availability of light, and not water, that was the controlling factor. Clear skies and sunny weather were more important than moisture in the soil.

The finding drags attention away from the importance of light in the Amazon’s photosynthesis equation, and towards the need for water. But what of the third point of the triangle, carbon dioxide? There is uncertainty there too: this time over whether in years of drought, the trees will switch from being a net carbon sink to a source, which could worsen global warming.”

That is exactly what has happened, as was demonstrated by the study led by Gatti using aircraft-based C02 monitoring above Santarém, Alta Floresta, Rio Branco and Tabatinga. As Nature reports: “The news is not good. Fire and drought can indeed make the Amazon a net source of atmospheric carbon.”

Gatti, Gloor and Miller’s Nature paper says: We find that the Amazon basin lost 0.48 ± 0.18 petagrams of carbon per year (Pg C yr−1) during the dry year but was carbon neutral (0.06 ± 0.1 Pg C yr−1) during the wet year.

Taking into account carbon losses from fire by using carbon monoxide measurements, we derived the basin net biome exchange (that is, the carbon flux between the non-burned forest and the atmosphere) revealing that during the dry year, vegetation was carbon neutral.

During the wet year, vegetation was a net carbon sink of 0.25 ± 0.14 Pg C yr−1, which is roughly consistent with the mean long-term intact-forest biomass sink of 0.39 ± 0.10 Pg C yr−1 previously estimated from forest censuses7.”

In 2010 (a dry year) it was estimated the forest fires produced some 510 billion kilograms of carbon. The forest was only able absorbed only as much carbon as produced naturally (excluding biomass fires) – around 30 billion kilograms of carbon – leaving a balance of 480 billion kilograms of carbon emitted into the atmosphere.

In 2011 (a wet year) the fires produced approximately 300 billion kilograms of carbon, and the final balance for the Amazon Basin was approximately 60 billion kilograms of carbon.

In plain language, the Amazon does act as a carbon sink in wet years and even reabsorbs much of the C02 emitted by local burning — but not in dry years.

Deforestation forecasts for 2050

So back in 2007, how could researchers and their peer reviewers have got things wrong by implying that a deceptively “green” Amazonia served as a carbon sink even in dry years?

What’s more, how could they have believed that Amazonia’s vegetation was actually contradicting one of the basic laws of botany?

We know photosynthesis requires the presence of water, atmospheric carbon and solar energy. Reduce any one of the three, and the rate of photosynthesis falls. So in the dry season, photosynthesis should fall and Amazonia is more likely to be a source of carbon than a sink.

Nature doesn’t reflect on the reasons why scientists might have gone  badly wrong.

But scientists are human too. It’s arguable that scientists were, albeit unconsciously, seeking not to exacerbate an already-sensitive situation. Researching the Brazilian Amazon’s significance in global matters has always been a highly emotive subject.

Ever since 1800 when the Prussian geographer Alexander von Humboldt first ventured into the region via the Casaquire canal and incurred the wrath of Portugal – then Brazil’s colonial master – scientists have had to tread carefully on all Amazon-related topics. Over the last two centuries there have been numerous plans to expropriate Amazonia to create a miraculous “global breadbasket” or source of other natural products.

In the 1970s, the pseudo-scientific branding of Amazonia as “the lungs of the world” stirred nationalist anger and “Amazonia is ours” cries within Brazil. In the 1980s, images of ranchers wilfully cutting and burning forest inspired a generation of European eco-activists to shun products from Amazonia. Brazilian officials during the 1990s rebutted the first attacks on Amazonia’s  putative role in global warming angrily with counter-charges that it was being driven by rich nations’ energy consumption, not by their forest burning.

So in succeeding decades, scientists have been extremely cautious when studying how the Amazon might really affect planetary wellbeing. For some researchers, this led to a conveniently “hands off” solution of studying the vast region from space using satellite imagery.

So it’s just possible that the “trick of the light” observed in satellite imagery over Amazonia has a very human origin. It also delivers a sobering reminder that sometimes our trust in data from a “spy in the sky” can deliver turn out to be “pie in the sky.”

Nature Subscribers can read its February 2014 cover article by clicking here

Non-subscribers can see a PDF  by clicking here.

You can read a full article by Brazilian journalist Karina Toledo by clicking here .