Carbon fluxes from land to ocean: IPCC estimates are below reality !
A new ensemble assessment improves estimates of land-to-ocean carbon fluxes
A re-evaluation of global land-to-ocean carbon exports using a multi-model ensemble and a database of observations reveals that the export of carbon by rivers is 20% higher than that reported in the 2021 Intergovernmental Panel on Climate Change (IPCC) assessment. These findings published in Nature Geoscience underscore the important contribution of riverine carbon to the carbon budget.
Why is it important ?
Accurate estimates of the amount of carbon globally transferred from terrestrial ecosystems to oceans are needed to assess the Earth’s carbon budget and project future climate change. “Rivers make a key contribution to the land-to-ocean flux but previous estimates vary substantially, posing challenges not only for scientists attempting to reconcile the global carbon budget but also for policymakers who rely on these estimates to make informed environmental decisions” said the first author of this study, Maodian Liu, Assistant Professor at Peking University.
What was discovered ?
To obtain an improved estimate of global riverine carbon fluxes we compiled an unprecedent observational dataset of river carbon measurements containing nearly 700,000 samples. This observational dataset was then combined with modeling approaches and machine learning methods relying on state-of-the-art Earth observations to develop a novel high-resolution ensemble mean estimate, which increases the accuracy and reduce the uncertainty of carbon flux estimates. We also estimated the global carbon export from fresh groundwater to oceans based on observations, which was overlooked in previous assessments.
We found that the global riverine carbon export calculated by this framework (1 billion ton of carbon/year) is around 20% higher than that reported in the 2021 IPCC assessment. Additionally, we found that groundwater carbon export makes a minor contribution to the global carbon budget. First author Liu added “In total, our approach calculated that the current IPCC assessment underestimates the land-to-ocean carbon flux by 240 million tons of carbon per year. In addition, the different types of riverine carbon differ significantly from previous assessments".
What does it imply?
The increased land-to-ocean carbon flux estimates obtained here suggest that oceanic CO2 outgassing induced by river carbon discharge is more substantial than previously recognized. “Therefore, our findings reveal that the estimation of the ocean carbon sink (the uptake of fossil-fuel carbon by the ocean) as derived by millions of ocean CO2 observations is likely underestimated. Indeed, constraining the uptake of anthropogenic carbon by the ocean requires a correction for the natural CO2 outgassing induced by the river carbon route, which our study reveals is larger than reported in the last IPCC report”, said Pierre Regnier, Full Professor at the Université Libre de Brussels who conceptualized and coordinated the research (https://loac-netwk.ulb.be/) on behalf of the Global Carbon Project (https://www.globalcarbonproject.org/). Therefore, accurately quantifying land-to-ocean carbon fluxes will help to refine Earth system models of the coupled carbon cycle-climate system, improve predictions of climate change impacts, and inform more effective and targeted carbon management and mitigation strategies.
