Recent publications

Nature - March 2022

The land-to-ocean loops of the global carbon cycle

Regnier P., Resplandy L., Najjar R.G. and Ciais P.

Carbon storage by the ocean and by the land is usually quantified separately, and does not fully take into account the land-to-ocean transport of carbon through inland waters, estuaries, tidal wetlands and continental shelf waters—the ‘land-to-ocean aquatic continuum’ (LOAC). Here we assess LOAC carbon cycling before the industrial period and perturbed by direct human interventions, including climate change. In our view of the global carbon cycle, the traditional ‘long-range loop’, which carries carbon from terrestrial ecosystems to the open ocean through rivers, is reinforced by two ‘short-range loops’ that carry carbon from terrestrial ecosystems to inland waters and from tidal wetlands to the open ocean. Using a mass-balance approach, we find that the pre-industrial uptake of atmospheric carbon dioxide by terrestrial ecosystems transferred to the ocean and outgassed back to the atmosphere amounts to 0.65 ± 0.30 petagrams of carbon per year (±2 sigma). Humans have accelerated the cycling of carbon between terrestrial ecosystems, inland waters and the atmosphere, and decreased the uptake of atmospheric carbon dioxide from tidal wetlands and submerged vegetation. Ignoring these changing LOAC carbon fluxes results in an overestimation of carbon storage in terrestrial ecosystems by 0.6 ± 0.4 petagrams of carbon per year, and an underestimation of sedimentary and oceanic carbon storage. We identify knowledge gaps that are key to reduce uncertainties in future assessments of LOAC fluxes.

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Nature Climate Change - February 2022

Deciphering the multiple effects of climate warming on the temporal shift of leaf unfolding

Haicheng Zhang, Isabelle Chuine, Pierre Regnier, Philippe Ciais and Wenping Yuan

Changes in winter and spring temperatures have been widely used to explain the diverse responses of spring phenology to climate change. However, few studies have quantified their respective effects. Using 386,320 in situ observations of leaf unfolding date (LUD) of six tree species in Europe, we show that accelerated spring thermal accumulation and changes in winter chilling explain, on average, 61% and 39%, respectively, of the advancement in LUD for the period 1951–2019. We find that winter warming may not have delayed bud dormancy release, but rather it has increased the thermal requirement in reaching leaf unfolding. This increase in thermal requirement and the decreased efficiency of spring warming for thermal accumulation partly explain the weakening response of leaf unfolding to warming. Our study stresses the need to better assess the antagonistic and heterogeneous effects of winter and spring warming on leaf phenology, which is key to projecting future vegetation–climate feedbacks.

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Ocean Science - January 2022

A framework to evaluate and elucidate the driving mechanisms of coastal sea surface pCO2 seasonality using an ocean general circulation model (MOM6-COBALT)

Roobaert A., Resplandy L., Laruelle G. G., Liao E. and Regnier P.

The temporal variability of the sea surface partial pressure of CO2 (pCO2) and the underlying processes driving this variability are poorly understood in the coastal ocean. In this study, we tailor an existing method that quantifies the effects of thermal changes, biological activity, ocean circulation and freshwater fluxes to examine seasonal pCO2 changes in highly variable coastal environments. We first use the Modular Ocean Model version 6 (MOM6) and biogeochemical module Carbon Ocean Biogeochemistry And Lower Trophics version 2 (COBALTv2) at a half-degree resolution to simulate coastal CO2 dynamics and evaluate them against pCO2 from the Surface Ocean CO2 Atlas database (SOCAT) and from the continuous coastal pCO2 product generated from SOCAT by a two-step neuronal network interpolation method (coastal Self-Organizing Map Feed-Forward neural Network SOM-FFN, Laruelle et al., 2017). The MOM6-COBALT model reproduces the observed spatiotemporal variability not only in pCO2 but also in sea surface temperature, salinity and nutrients in most coastal environments, except in a few specific regions such as marginal seas. Based on this evaluation, we identify coastal regions of “high” and “medium” agreement between model and coastal SOM-FFN where the drivers of coastal pCO2 seasonal changes can be examined with reasonable confidence. Second, we apply our decomposition method in three contrasted coastal regions: an eastern (US East Coast) and a western (the Californian Current) boundary current and a polar coastal region (the Norwegian Basin). Results show that differences in pCO2 seasonality in the three regions are controlled by the balance between ocean circulation and biological and thermal changes. Circulation controls the pCO2 seasonality in the Californian Current; biological activity controls pCO2 in the Norwegian Basin; and the interplay between biological processes and thermal and circulation changes is key on the US East Coast. The refined approach presented here allows the attribution of pCO2 changes with small residual biases in the coastal ocean, allowing for future work on the mechanisms controlling coastal air–sea CO2 exchanges and how they are likely to be affected by future changes in sea surface temperature, hydrodynamics and biological dynamics.

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Nature Geoscience - October 2021

End-permian marine extinction due to temperature driven nutrient recycling and euxinia

Hülse D., Lau K.V., van de Velde S.J., Arndt S., Meyer K.M. and Ridgwell A.

Extreme warming at the end-Permian induced profound changes in marine biogeochemical cycling and animal habitability, leading to the largest metazoan extinction in Earth’s history. However, a causal mechanism for the extinction that is consistent with various proxy records of geochemical conditions through the interval has yet to be determined. Here we combine an Earth system model with global and local redox interpretations from the Permian/Triassic in an attempt to identify this causal mechanism. Our results show that a temperature-driven increase in microbial respiration can reconcile reconstructions of the spatial distribution of euxinia and seafloor anoxia spanning the Permian–Triassic transition. We illustrate how enhanced metabolic rates would have strengthened upper-ocean nutrient (phosphate) recycling, and thus shoaled and intensified the oxygen minimum zones, eventually causing euxinic waters to expand onto continental shelves and poison benthic habitats. Taken together, our findings demonstrate the sensitive interconnections between temperature, microbial metabolism, ocean redox state and carbon cycling during the end-Permian mass extinction. As enhanced microbial activity in the ocean interior also lowers subsurface dissolved inorganic carbon isotopic values, the carbon release as inferred from isotope changes in shallow subsurface carbonates is likely overestimated, not only for this event, but perhaps for many other carbon cycle and climate perturbations through Earth’s history.

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Global Biogeochemical Cycles - July 2021

Global uptake of atmospheric methane by soil from 1900 to 2100.

Murguia-Flores F., Ganesan A.L., Arndt S. and Hornibrook R.C.

Soil methanotrophy is the only biological process that removes CH4 from the atmosphere. There is good agreement about the size of the global sink but great uncertainty about its interannual variability and regional responses to changes in key environmental drivers. We used the process-based methanotrophy model MeMo v1.0 and output from global climate models to simulate regional and global changes in soil uptake of atmospheric CH4 from 1900 to 2100. The annual global uptake doubled from 17.1±2.4 to 37.2±3.3 Tg yr-1 from 1900-2015 and could increase further to 82.7±4.4 Tg yr-1 by 2100 (RCP8.5), primarily due to enhanced diffusion of CH4 into soil as a result of increases in atmospheric CH4 mole fraction. We show that during the period 1980-2015 temperature became an important influence on the increasing rates of soil methanotrophy, particularly in the Northern Hemisphere. In RCP-forced simulations the relative influence of temperature on changes in the uptake continues to increase, enhancing the soil sink through higher rates of methanotrophic metabolic activity, increases in the global area of active soil methanotrophy and length of active season. During the late 21st century under RCP6.0, temperature is predicted to become the dominant driver of changes in global mean soil uptake rates for the first time. Regionally, in Europe and Asia, nitrogen inputs dominate changes in soil methanotrophy, while soil moisture is the most important influence in tropical South America. These findings highlight that the soil sink could change in response to drivers other than atmospheric CH4 mole fraction.

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Global Change Biology - March 2021

Leaching of dissolved organic carbon from mineral soils plays a significant role in the terrestrial carbon balance.

Nakhavali M., Lauerwald R., Regnier P., Guenet B., Chadburn S. and Friedlingstein P.

The leaching of dissolved organic carbon (DOC) from soils to the river network is an overlooked component of the terrestrial soil C budget. Measurements of DOC concentrations in soil, runoff and drainage are scarce and their spatial distribution highly skewed towards industrialized countries. The contribution of terrestrial DOC leaching to the global‐scale C balance of terrestrial ecosystems thus remains poorly constrained. Here, using a process based, integrative, modelling approach to upscale from existing observations, we estimate a global terrestrial DOC leaching flux of 0.28 ± 0.07 Gt C year−1 which is conservative, as it only includes the contribution of mineral soils. Our results suggest that globally about 15% of the terrestrial Net Ecosystem Productivity (NEP, calculated as the difference between Net Primary Production and soil respiration) is exported to aquatic systems as leached DOC. In the tropical rainforest, the leached fraction of terrestrial NEP even reaches 22%. Furthermore, we simulated spatial‐temporal trends in DOC leaching from soil to the river networks from 1860 to 2010. We estimated a global increase in terrestrial DOC inputs to river network of 35 Tg C year−1 (14%) from 1860 to 2010. Despite their low global contribution to the DOC leaching flux, boreal regions have the highest relative increase (28%) while tropics have the lowest relative increase (9%) over the historical period (1860s compared to 2000s). The results from our observationally constrained model approach demonstrate that DOC leaching is a significant flux in the terrestrial C budget at regional and global scales.

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Nature Communications - January 2021

Mineral phosphorus drives glacier algal blooms on the Greenland Ice Sheet.

McCutcheon J., Lutz S., Williamson C., Cook J.M., Tedstone A.J., Vanderstraeten A., Wilson S.A., Stockdale A., Bonneville S., Anesio A.M., Yallop M.L., McQuaid J.B., Tranter M. & Benning L.G.

Melting of the Greenland Ice Sheet is a leading cause of land-ice mass loss and cryosphere-attributed sea level rise. Blooms of pigmented glacier ice algae lower ice albedo and accelerate surface melting in the ice sheet’s southwest sector. Although glacier ice algae cause up to 13% of the surface melting in this region, the controls on bloom development remain poorly understood. Here we show a direct link between mineral phosphorus in surface ice and glacier ice algae biomass through the quantification of solid and fluid phase phosphorus reservoirs in surface habitats across the southwest ablation zone of the ice sheet. We demonstrate that nutrients from mineral dust likely drive glacier ice algal growth, and thereby identify mineral dust as a secondary control on ice sheet melting.

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Nature Communications - January 2021

Around one third of current Arctic Ocean primary production sustained by rivers and coastal erosion.

Terhaar J., Lauerwald R., Regnier P., Gruber N. & Bopp L.

In this study, a international team of scientists from the Institut Pierre Simon Laplace in Paris, the Université Libre de Bruxelles, and the Eidgenössische Technische Hochschule in Zürich, provides a first comprehensive estimate of nutrient inputs from rivers and coastal erosion to the Arctic Ocean, and a quantitative assessment of the importance of these two sources of nutrients for Arctic Ocean primary production. The riverine fluxes are based on observed monthly fluxes from the six largest Arctic rivers, whereas the fluxes from coastal erosion were calculated using satellite images of the Arctic coastline and measurements of the nutrient content in these eroding soils. The riverine and coastal erosion fluxes were then used to force a state-of-the-art high-resolution ocean-biogeochemical model. The results suggest that terrigenous nutrients sustain 28-51 % of the total Arctic Ocean productivity, with rivers accounting for 9-11 % of total Arctic Ocean and coastal erosion for 19-41 %.

This study suggests a much more prominent imprint of terrestrial inputs on Arctic Ocean productivity compared to previous studies and highlights the important role of coastal erosion for the Arctic Ocean ecosystem. Terrigenous nitrogen input from both rivers and coastal erosion will likely increase over the 21st century, which may further augment Arctic Ocean productivity through a reduced nutrient stress on its marine ecosystem.

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