Reducing uncertainties in decadal variability of the global carbon budget with multiple datasets

Conventional calculations of the global carbon budget infer the land sink as a residual between emissions, atmospheric accumulation, and the ocean sink. Thus, the land sink accumulates the errors from the other flux terms and bears the largest uncertainty. Here, we present a Bayesian fusion approach that combines multiple observations in different carbon reservoirs to optimize the land (B) and ocean (O) carbon sinks, land use change emissions (L), and indirectly fossil fuel emissions (F) from 1980 to 2014. Compared with the conventional approach, Bayesian optimization decreases the uncertainties in B by 41% and in O by 46%. The L uncertainty decreases by 47%, whereas F uncertainty is marginally improved through the knowledge of natural fluxes. Both ocean and net land uptake (B + L) rates have positive trends of 29 ± 8 and 37 ± 17 Tg C⋅y−2 since 1980, respectively. Our Bayesian fusion of multiple observations reduces uncertainties, thereby allowing us to isolate important variability in global carbon cycle processes

Aquatic carbon fluxes dampen the overall variation of net ecosystem productivity in the Amazon basin: An analysis of the interannual variability in the boundless carbon cycle

International audienc

Global perturbation of organic carbon cycling by river damming

The damming of rivers represents one of the most far-reaching human modifications of the flows of water and associated matter from land to sea. Dam reservoirs are hotspots of sediment accumulation, primary productivity (P) and carbon mineralization (R) along the river continuum. Here we show that for the period 1970-2030, global carbon mineralization in reservoirs exceeds carbon fixation (

Historical and future contributions of inland waters to the Congo Basin carbon balance

International audienceAs the second largest area of contiguous tropical rainforest and second largest river basin in the world, the Congo Basin has a significant role to play in the global carbon (C) cycle. For the present day, it has been shown that a significant proportion of global terrestrial net primary productivity (NPP) is transferred laterally to the land-ocean aquatic continuum (LOAC) as dissolved CO 2 , dissolved organic carbon (DOC), and particulate organic carbon (POC). Whilst the importance of LOAC fluxes in the Congo Basin has been demonstrated for the present day, it is not known to what extent these fluxes have been perturbed historically, how they are likely to change under future climate change and land use scenarios, and in turn what impact these changes might have on the overall C cycle of the basin. Here we apply the ORCHILEAK model to the Congo Basin and estimate that 4 % of terrestrial NPP (NPP = 5800 ± 166 Tg C yr −1) is currently exported from soils and vegetation to inland waters. Further, our results suggest that aquatic C fluxes may have undergone considerable perturbation since 1861 to the present day, with aquatic CO 2 evasion and C export to the coast increasing by 26 % (186±41 to 235 ± 54 Tg C yr −1) and 25 % (12 ± 3 to 15 ± 4 Tg C yr −1), respectively, largely because of rising atmospheric CO 2 concentrations. Moreover, under climate scenario RCP6.0 we predict that this perturbation could continue; over the full simulation period (1861-2099), we estimate that aquatic CO 2 evasion and C export to the coast could increase by 79 % and 67 %, respectively. Finally, we show that the proportion of terrestrial NPP lost to the LOAC could increase from approximately 3 % to 5 % from 1861-2099 as a result of increasing atmospheric CO 2 concentrations and climate change. However, our future projections of the Congo Basin C fluxes in particular need to be interpreted with some caution due to model limitations. We discuss these limitations, including the wider challenges associated with applying the current generation of land surface models which ignore nutrient dynamics to make future projections of the tropical C cycle, along with potential next steps

Modelling estuarine biogeochemical dynamics: from the local to the global scale

Estuaries act as strong carbon and nutrient filters and are relevant contributors to the atmospheric CO2 budget. They thus play an important, yet poorly constrained, role for global biogeochemical cycles and climate. This manuscript reviews recent developments in the modelling of estuarine biogeochemical dynamics. The first part provides an overview of the dominant physical and biogeochemical processes that control the transformations and fluxes of carbon and nutrients along the estuarine gradient. It highlights the tight links between estuarine geometry, hydrodynamics and scalar transport, as well as the role of transient and nonlinear dynamics. The most important biogeochemical processes are then discussed in the context of key biogeochemical indicators such as the net ecosystem metabolism (NEM), air–water CO2 fluxes, nutrient-filtering capacities and element budgets. In the second part of the paper, we illustrate, on the basis of local estuarine modelling studies, the power of reaction-transport models (RTMs) in understanding and quantifying estuarine biogeochemical dynamics. We show how a combination of RTM and high-resolution data can help disentangle the complex process interplay, which underlies the estuarine NEM, carbon and nutrient fluxes, and how such approaches can provide integrated assessments of the air–water CO2 fluxes along river–estuary–coastal zone continua. In addition, trends in estuarine biogeochemical dynamics across estuarine geometries and environmental scenario are explored, and the results are discussed in the context of improving the modelling of estuarine carbon and CO2 dynamics at regional and global scales

Anthropogenic perturbation of the carbon fluxes from land to ocean

A substantial amount of the atmospheric carbon taken up on land through photosynthesis and chemical weathering is transported laterally along the aquatic continuum from upland terrestrial ecosystems to the ocean. So far, global carbon budget estimates have implicitly assumed that the transformation and lateral transport of carbon along this aquatic continuum has remained unchanged since pre-industrial times. A synthesis of published work reveals the magnitude of present-day lateral carbon fluxes from land to ocean, and the extent to which human activities have altered these fluxes. We show that anthropogenic perturbation may have increased the flux of carbon to inland waters by as much as 1.0 Pg C yr-1 since pre-industrial times, mainly owing to enhanced carbon export from soils. Most of this additional carbon input to upstream rivers is either emitted back to the atmosphere as carbon dioxide (~0.4 Pg C yr-1) or sequestered in sediments (~0.5 Pg C yr-1) along the continuum of freshwater bodies, estuaries and coastal waters, leaving only a perturbation carbon input of ~0.1 Pg C yr-1 to the open ocean. According to our analysis, terrestrial ecosystems store ~0.9 Pg C yr-1 at present, which is in agreement with results from forest inventories but significantly differs from the figure of 1.5 Pg C yr-1 previously estimated when ignoring changes in lateral carbon fluxes. We suggest that carbon fluxes along the land–ocean aquatic continuum need to be included in global carbon dioxide budgets.Peer reviewe

Interaction between soil type and cropping system on albedo dynamics leads to contrasted impact on climate mitigation

The biogeochemical effects of conservation agriculture (CA), such as soil organic carbon storage and greenhouse gas emissions, have been extensively studied. However, recent research has shown that management practices also have biogeophysical effects on both local and global climates by altering surface albedo and energy partitioning. We assessed the biogeophysical impacts of CA in maize fields during two successive seasons (2021/22-2022/23) at two long-term experimental sites in Zimbabwe with contrasting soil properties: Domboshawa Training Center (DTC) with a light abruptic Lixisol (sandy soil) and the University of Zimbabwe Farm (UZF) with a dark xanthic Ferralsol (clayey soil). We monitored surface albedo, longwave radiation, leaf area index (LAI), and soil moisture/temperature under three treatments: conventional tillage (CT), notillage (NT), and no-tillage with mulch (NTM). Our findings reveal that, across all treatments during the two monitored seasons, the average surface albedo of the xanthic Ferralsol at UZF was consistently lower than that of the abruptic Lixisol at DTC. It results a cooling effect in both NT and NTM treatments compared to CT in the clayey soil at UZF. During the 2021/22 season, the mean annual radiative forcing (RF) of NT and NTM were -0.83 W.m-² and -0.43 W.m-2 respectively, while during the second season (2022/23) the annual mean RF was -1.43 W.m-2 for NT and -1.03 W.m-2 for NTM.On the sandy soil at DTC, a warming effect was observed due to soil darkening induced by mulching. The mean annual RF of NT in this site was -3.34 W.m-2 during the first season and -2.78 W.m-2 during the second. In contrast, NTM showed a warming effect with an RF of 1.2 W.m-2 in 2021/22, and 2.77 W.m-2 during the 2022/23 season. The RF induced by albedo change were converted into CO2-equivalents in order to compare it with biogeochemical effects of CA through changes in soil N2O emissions and SOC storage. The results demonstrated an opposite effect on RF and of the same magnitude between albedo and soil organic carbon (SOC) in the NT and NTM treatments at DTC, suggesting that CA might not bring any mitigation benefit if mulch is applied on light coloured soils

Predicting Future Trends of Terrestrial Dissolved Organic Carbon Transport to Global River Systems

A fraction of CO2 uptake by terrestrial ecosystems is exported as organic carbon (C) through the terrestrial-aquatic continuum. This translocated C plays a significant role in the terrestrial C balance; however, obtaining global assessments remains challenging due to the predominant reliance on empirical approaches. Leaching of dissolved organic C (DOC) from soils to rivers represents an important fraction of this C export and is assumed to drive a large proportion of the net-heterotrophy of river systems and the related CO2 emissions. Using the model JULES-DOCM, we projected DOC leaching trends over the 21st century based on three scenarios with high (RCP 2.6), intermediate (RCP 4.5), and low (RCP 8.5) climate mitigation efforts. The RCP 8.5 scenario led to the largest DOC leaching increase of +42% to 395 Tg C yr−1 by 2100. In comparison, RCP 2.6 and RCP 4.5 led to increases of 10% and 21%, respectively. Under RCP 8.5, the sub-tropical zone showed the highest relative increase of 50% above current levels. In the boreal and tropical zones, the simulations revealed similar increases of 48% and 41%, respectively. However, given the pre-eminence of the tropics in DOC leaching, the absolute increment is markedly substantial from this region (+59 Tg C yr−1). The temperate zone displayed the lowest relative increase with 35%. Our analysis identified the rising atmospheric CO2 concentration and its fertilizing effect on terrestrial NPP as the main reason for the future increase in DOC leaching