Mots-clé
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marine carbon cycle; marine nitrogen cycle; carbon dioxide; nitrous oxide; air-sea exchange; inverse methods; Bayesian synthesis; data assimilation
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Programme de recherche
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COST-Action 735 - Tools for Assessing Global Air Sea Fluxes of Climate and Air Pollution Relevant Gases
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Description succincte
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This project aims to improve our quantitative understanding of the marine carbon and nitrogen cycles, with a particular emphasis on the global-scale estimation of the air-sea fluxes of CO2 and N2O. We address this goal by continuing the development and application of Green's function-based data assimilation methods that add oceanic CO2 and N2O data into a dynamically consistent framework. In the first part of the proposed work, we expand a recently developed inversion scheme to incorporate surface ocean observations of the partial pressure of CO2 in order to improve the ability of this scheme to represent spatio-temporal variability of the air-sea CO2 fluxes. In the second part of the proposed work, we apply the Green's function methodology to N2O, in order to estimate air-sea N2O fluxes on the basis of ocean interior and surface observations of N2O. An ancillary benefit of the inversion of N2O is an improved understanding of the mechanisms in the ocean responsible for the production of N2O. The proposed work will be undertaken in close collaboration with the group of Prof. F. Joos at the University of Bern, and with several other members of the European COST action 735, in particular Dr. H. Bange from the Leibniz Institute of Marine Sciences in Kiel, Germany, with whom we plan to exchange students.
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Autres indications
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Full name of research-institution/enterprise: ETH Zürich Environmental Physics Institute of Biogeochemistry and Pollutant Dynamics, CHN E21.1
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Partenaires et organisations internationales
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BE, CH, CY, DE, DK, ES, FI, FR, GR, HU, IE, IT, NL, NO, PL, SE, TR, UK
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Résumé des résultats (Abstract)
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The main objective of this project was to estimate the spatio-temporal pattern of the flux of CO2 across the ocean-atmosphere interface and to use these estimates to better constrain the global carbon budget. To this end, we developed an atmosphere-ocean joint inversion framework for the estimation of seasonal fluxes of CO2 both over ocean and land. The first key ingredient of this joint inversion is the information about air-sea fluxes that we obtained from the inversion of ocean carbon observations, which provided us with annual mean CO2 flux estimates for 11 oceanic regions. While the uncertainties attached to these fluxes are very low, the results remain limited to the annual mean time scale. In order to resolve fluxes on a monthly time scale, we mapped the seasonal flux pattern obtained from surface observations of the partial pressure of CO2 onto the annual mean absolute fluxes from the ocean inversion. The resulting monthly flux estimates serve as a strong prior in our joint inversion, which propagates then this ocean-derived information onto the land. This is particularly valuable over land regions that are generally under-sampled (i.e. where not many CO2 observations are available due to a very sparse observational network), resulting in large flux uncertainties when inverting atmospheric CO2 data alone. This information propagation from the ocean onto the land takes place via atmospheric transport, represented by 12 atmospheric general circulation models. These models connect surface fluxes to observed atmospheric CO2, similar to the ocean inversion, where oceanic transport models connected air-sea fluxes to ocean interior DIC. By using 12 different models we can assess the uncertainty contribution arising from imperfect transport models. The joint inversion results in monthly resolved fluxes for 22 oceanic and terrestrial regions around the globe. A key result is that the tropical land is consistently estimated to be a source of carbon for the period 1992-1996, with a magnitude of about 1.1 (± 0.9) PgC yr?1 , which is comparable to global estimates of deforestation rates in tropical forests and therefore implies an annually balanced tropical land biosphere flux. This balance is not found, however, at the regional level: for the Amazonian region we find a biospheric source of 0.6 (± 0.5) PgC yr?1 . This is at the upper range of estimates from bottom-up methods, which usually identify the region as a sink for carbon. We find only a small source over aggregated Tropical and Southern Hemisphere Land (TSL) regions, owing in large parts to the applied selection of transport models. All constraints point towards a seasonal TSL amplitude much larger than estimated by previous studies. While all constraints have a significant impact on flux estimates, the ocean constraint has the additional value to make land fluxes consistent with independent measurements taken in the ocean.
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Références bases de données
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Swiss Database: COST-DB of the State Secretariat for Education and Research Hallwylstrasse 4 CH-3003 Berne, Switzerland Tel. +41 31 322 74 82 Swiss Project-Number: C07.0077
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