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A study that used direct measurements of the Congo and the Amazon basins suggests that they contribute to climate change differently, with the Congo emitting three to four times more methane (CH4) than the Amazon.
Scientists from Belgium, Brazil and France say that global estimates of emissions of methane and carbon dioxide (CO2) tend to result from use of adjacent basins rather than actual measurements of the tropical basins, with underrepresented tropical inland waters.

“Getting the numbers right is not trivial for those countries and also for humankind.”

Alberto V. Borges, Université de Liège, Belgium

Therefore, the scientists used actual measurements to assess the degree to which the patterns of carbon dioxide and methane are similar or different in the two tropical giant water bodies.

The scientists discovered that the Amazon and the Congo emit similar distribution of CO2, although the Congo emits three to four times more CH4 than the Amazon, according the study published in the Nature on 23 October.

Alberto V. Borges, lead author and senior research associate of the Chemical Oceanography Unit at the Université de Liège in Belgium, says the findings alter the long-held perception that the Amazon is a model for functioning in ecology and chemistry for all tropical rivers in the world because the Congo behaves so differently.  

The findings imply that more research is still needed to correctly quantify the emissions of CH4 from rivers and wetlands, he adds.  
John Omara, a biochemist and lecturer at Uganda-based Makerere University, tells SciDev.Net: “The high levels of methane in River Congo may be explained by the presence of wetlands containing a lot of peat that join the rivers either directly or indirectly, coupled by higher temperatures in the tropics, which encourage high microbial activities.”

Data were collected on cruises of rivers and their tributaries, covering different stages of the annual flood cycle by using infra-red analyser to measure CO2 and a flame ionisation detector to measure the CH4 concentrations.
So far no country accounts when budgeting the sources and sinks of carbon for the carbon market for the emissions of the two gases  from rivers and wetlands; they usually only account for the carbon sink in forests, according to Borges

“The Amazon and River Congo correspond to the largest rivers in the world, but also to the largest tropical forests in the world. So getting the numbers right is not trivial for those countries and also for humankind,” Borges says.
According to the US-based World Wildlife Fund, the Congo basin affects the livelihoods of people living in six countries: Cameroon, Central African Republic, Congo, Democratic Republic of the Congo, Equatorial Guinea and Gabon.

Omara agrees that the explanation given by study’s authors are clear and convincing, but argues that they seem not to have considered detailed land uses along the Congo River which have a big bearing on delivering upland carbon into the water bodies. River Nile may actually be worse considering the land uses and numerous dams for generating electricity constructed or being constructed on it. The lakes are not any better considering the land uses around them which allow many allochthonous (imported) organic matter, Omara tells SciDev.Net.
However, Borges explains that so far the Congo basin is relatively pristine and unaffected by human activities. But in the next 25 years with increased population growth, pressure and a probable change of agriculture from traditional to intensive practice, water diversion for agriculture, deforestation and probable hydropower plant construction will be implied, he said. 

“All of these activities now have been documented to change the emissions of CO2 and CH4 from rivers. To track these changes we need a baseline description of present-day functioning and our study fulfils that,” Borges explains.
This piece was produced by SciDev.Nets Sub-Saharan Africa (English) desk.


Alberto V. Borges and others Divergent biophysical controls of aquatic CO2 and CH4 in the World’s two largest rivers (Nature, 23 October 2015)