Climate change increases riverine carbon outgassing while export to the ocean remains uncertain.

Carbon fluxes in the Amazon Basin are considerably influenced by annual flooding during which terrigenous organic material is imported to the river. This regular interaction affects carbon pools within the riverine system, terrestrial carbon, and carbon exported to the ocean and released to the atmosphere. The processes of generation, conversion, and transport of organic carbon in this coupled terrigenous–riverine system strongly interact and are climate-sensitive, yet their response to climate change is still largely unknown. To quantify climate change effects on carbon pools and on carbon fluxes within the river and to the ocean and the atmosphere, we developed the riverine carbon model RivCM, which is directly coupled to the well-established dynamic vegetation and hydrology model LPJmL. We show here that RivCM successfully reproduces observed values in exported carbon and riverine carbon concentration. We evaluate future changes in riverine carbon by applying RivCM for climate forcing from five climate models and three CO₂ emission scenarios (SRES). We find that climate change causes a doubling of riverine organic carbon in the Southern and Western basin while reducing it by 20% in the eastern and northern parts. In contrast, the amount of riverine inorganic carbon shows a 2- to 3-fold increase in the entire basin, independent of the SRES scenario. The export of carbon to the atmosphere increases as well with an average of about 30 %. In contrast, changes in future export of organic carbon to the Atlantic Ocean depend on the SRES scenario and are projected to either decrease by about 8.9% (SRES A1B) or increase by about 9.1% (SRES A2). Such changes in the terrigenous–riverine system could have local and regional impacts on the carbon budget of the whole Amazon Basin and parts of the Atlantic Ocean. Changes in the riverine carbon could lead to a shift in the riverine nutrient supply and pH, while changes in the exported carbon to the ocean leads to changes in the supply of organic material that acts as food source in the Atlantic. On the larger scale the increased outgassing of CO₂ could turn the Amazon Basin from a sink of carbon to a considerable source. Therefore we propose that the coupling of terrestrial and riverine carbon budget should be included in subsequent analysis of the future regional carbon budget. [ABSTRACT FROM AUTHOR]