Tropics: CO2 sling instead of sink?

Current net carbon footprint of the tropics contradicts current assumptions

So far, the tropics have been considered a reliable carbon sink, but there are apparently tropical regions that emit far more CO2 than absorb. © TG Trade / iStock
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Contradiction to theory: Apparently, the tropics are by no means always and everywhere a carbon sink, as commonly assumed. For parts of tropical Africa have at times released significantly more CO2 than recorded, as satellite data reveal. In 2015 and 2016, the magnitude of these emissions was so high that the tropics as a whole became a greenhouse gas spinner in their net carbon footprint. The tropics are thus less stable CO2 sinks than thought.

The tropical ecosystems are among the most productive on earth. After all, about a third of the approximately three trillion trees on our planet grow there. They absorb large amounts of CO2 and thus remove the greenhouse gas from the atmosphere. Therefore, the net carbon footprint of the tropical regions has so far been considered negative - they absorb more CO2 than they release through deforestation, fires, decomposition of plant material or soil erosion. So anyway the common theory.

CO2 balance on the test bench

But whether this assumption is correct, was so far unclear - it lacked comprehensive data. That's why Paul Palmer of the University of Edinburgh and his colleagues have now reviewed the global net balance of the tropics using two independent satellite measurements. They evaluated data from NASA's Greenhouse Gases Observing Satellite (GOSAT) and NASA's Orbiting Carbon Observatory (OCO-2) data for 2015 and 2016. Both had measured the concentrations of CO2 in the air column over different tropical regions.

In addition, the researchers evaluated other satellite data, including vegetation development, seasonal photosynthetic activity and firing, and used a model of biosphere carbon fluxes. By combining this data, the researchers were then able to determine how much CO2 was absorbed and released in these two years for the different tropical regions of the world.

CO2 net balances of the tropical regions determined by the satellites and research groups. Mer Palmer et al / Nature Communications, CC-by-sa 4.0

More release than intake

The surprising result: Contrary to expectations, the tropical land areas released significantly more CO2 in 2015 and 2016 than was recorded. "Our analysis of GOSAT and OCO-2 data reveals that the terrestrial tropics have an annual net carbon emissions of 1.03 trillion kilograms of carbon in 2015 and 1.60 trillion kilograms of carbon per year Released in 2016, "Palmer and his team report. That was much more than assumed a priori. display

However, the regional breakdown of these values ​​showed that this does not apply to all tropical areas. While South America, tropical Australia and the Asian tropics had a negative net balance, albeit a small one, tropical Africa stood out. With a net emission of 1.25 trillion kilograms of carbon, it was this region that accounted for much of the greenhouse gas emissions.

CO2 hotspots in West Africa and Ethiopia

But why? The spatial breakdown also revealed striking differences within the African tropics: "The largest seasonal intake takes place as expected over the Congo Basin, " Palmer and his team report. The dense rainforests of this region therefore still absorb more CO2 than they release. But this was different in the west and east of this area: "We found the largest emissions over the west of Ethiopia and over western tropical Africa, " the researchers say.

Could there have been more forest fires there? The combustion of biomass could explain the high CO2 emissions. In fact, the data showed that the CO2 emissions from these regions were particularly high in March and April, at a time when it is the hottest and driest. However, a comparison with the data from satellites for global fire surveillance revealed that fire alone can not explain the high CO2 emissions of tropical Africa, the researchers report.

Land use and villages as cause?

But what is it? Palmer and his team suspect that enhanced drought, combined with land-use changes, could be responsible for the net release of CO2 from tropical West Africa and Ethiopia. Accompanying data shows that there have been more dangers in these regions since 2002. Together with an overuse of the earth and the associated greater degradation, the researchers believe that this could explain why these tropical regions have changed from CO2 sinks to CO2 slings.

However, these are still speculations. It is equally unclear whether this change took place only on a temporary basis or whether it continues to this day this requires further analysis. However, the results demonstrate that the role of the tropics as a global CO2 sink may be less stable and reliable than previously thought. (Nature Communications, 2019; doi: 10.1038 / s41467-019-11097-w)

Source: Nature Communications

- Nadja Podbregar