Extreme floods increase CO₂ outgassing from a large Amazonian river

Large Amazonian rivers are characteristically subject to seasonal floods. We examine how inundation extent affects the partial pressure of carbon dioxide (pCO₂) and CO₂ outgassing in the Madeira River, a large tributary to the Amazon River. We show data from nine field campaigns performed between 2009 and 2011, complemented with data from one additional campaign in April 2014 that is representative of the largest flood on record. Both the pCO₂ (range, 835–9694 μatm) and CO₂ outgassing (range, 641–12,253 mg C m−2 d−1) had large seasonal variability (with peaks during high water), and fit exponential relationships with water level. CO₂ outgassing per unit area in 2014 was 50% higher than the other highest rate in our dataset. Reconstruction of CO₂ fluxes since 1968 indicates that extreme-flood years outgas 20% more CO₂ per unit area than years without reported occurrence of extreme floods. Our findings indicate a positive feedback between climate change, extreme flooding, and CO₂ outgassing from river water.