Laurent Bopp

Climate-induced oceanic oxygen fluxes: Implications for the contemporary carbon budget

[1]xa0Atmospheric O2 concentrations have been used to estimate the ocean and land sinks of fossil fuel CO2. In previous work, it has been assumed that the oceans have no long-term influence on atmospheric O2. We address the validity of this assumption using model results and observations. Oceanic O2 fluxes for the 1860–2100 period are simulated using a coupled climate model in which is nested an ocean biogeochemistry model. Simulated oceanic O2 fluxes exhibit large interannual (±40 Tmol yr−1) and decadal (±13 Tmol yr−1) variability, as well as a net outgassing to the atmosphere caused by climate change (up to 125 Tmol yr−1 by 2100). Roughly one quarter of this outgassing is caused by warming of the ocean surface, and the remainder is caused by ocean stratification. The global oceanic O2 and heat fluxes are strongly correlated for both the decadal variations and the climate trend. Using the observed heat fluxes and the modeled O2 flux/heat flux relationship, we infer the contribution of the oceans to atmospheric O2 and infer a correction to the partitioning of the ocean and land CO2 sinks. After considering this correction, the ocean and land sinks are 1.8 ± 0.8 Pg C yr−1 and 0.3 ± 0.9 Pg C yr−1, respectively, for the 1980s (a correction of 0.1 from ocean to land) and are 2.3 ± 0.7 Pg C yr−1 and 1.2 ± 0.9 Pg C yr−1, respectively, in the 1990–1996 period (a correction of 0.5 from land to ocean). This correction reconciles the 1990s ocean sink estimated by the Intergovernmental Panel on Climate Change Third Assessment Report with ocean models.

Dust impact on marine biota and atmospheric CO2 during glacial periods

We assess the impact of high dust deposition rates on marine biota and atmospheric CO2 using a state-of-the-art ocean biogeochemistry model and observations. Our model includes an explicit representation of two groups of phytoplankton and colimitation by iron, silicate, and phosphate. When high dust deposition rates from the Last Glacial Maximum (LGM) are used as input, our model shows an increase in the relative abundance of diatoms in todays iron-limited regions, causing a global increase in export production by 6% and an atmospheric CO2 drawdown of 15 ppm. When the combined effects of changes in dust, temperature, ice cover, and circulation are included, the model reproduces roughly our reconstruction of regional changes in export production during the LGM based on several paleoceanographic indicators. In particular, the model reproduces the latitudinal dipole in the Southern Ocean, driven in our simulations by the conjunction of dust, sea ice, and circulation changes. In the North Pacific the limited open ocean data suggest that we correctly simulate the eastwest gradient in the open ocean, but more data are needed to confirm this result. From our model-data comparison and from the timing of the dust record at Vostok, we argue that our model estimate of the role of dust is realistic and that the maximum impact of high dust deposition on atmospheric CO2 must be

Comparison of global climatological maps of sea surface dimethyl sulfide

We have examined differences in regional and seasonal variability among sevenglobal climatologies of sea-surface dimethyl sulfide (DMS) concentrations. We foundlarge differences between recent climatologies and that typically used by mostatmospheric sulfur models. The relative uncertainty (1s/mean) in the latitudinaldistribution of the annual mean DMS concentration increases from about 50% in tropicaland temperate regions to nearly 100% in the high latitudes. We also compared theseclimatologies to new measurements in the North Atlantic Ocean taken during the 2001Programme Oce´an Multidisciplinaire Me´so Echelle (POMME) expeditions.

Antarctic circumpolar wave impact on marine biology: A natural laboratory for climate change study

We use the observed variations in ocean surface chlorophyll, temperature and height caused by the Antarctic Circumpolar Wave (ACW) as a natural laboratory to determine how marine biology responds to changes in ocean stratification in the Southern Ocean. Interannual variations of surface chlorophyll (±5%) observed by SeaWiFS satellite during 1997-2001 vary in phase over the entire Southern Ocean in spite of large east-west dipoles in ocean dynamics. We suggest that this behavior is due to the regional predominance of light versus nutrient limitation over the most productive regions of the Southern Ocean.

A satellite-based method for estimating global oceanic DMS and its application in a 3-D atmospheric GCM

In order to assess in three-dimensional atmospheric models the climate effects of anthropogenic sulphate aerosols, it is necessary not only to compute spatial and temporal distributions of anthropogenic sulphate, but also to simulate spatially and temporally the emission, transport and transformation of natural sulphur gases and aerosols emitted at the Earth’s surface. Jones et al. (2001) recently obtained a value of -1.9 W m-2 for the effect of anthropogenic sulphate aerosol on cloud albedo and on precipitation efficiency (the ‘indirect’ sulphate aerosol forcing effect), and demonstrated in a sensitivity test that doubling oceanic dimethylsulphide (DMS) emission fluxes reduced the indirect effect by over 25%. Thus, changes in marine DMS emissions appear to significantly affect estimates of the magnitude of anthropogenic sulphate forcing.

Assessment of the marine biota-DMS-cloud-climate hypothesis using remotely sensed data and an ocean general circulation model (OGCM)

DMS is a sulfur-compound produced naturally in the sea. Once in the atmosphere, it is photo oxidized to form a non-sea- salt sulfate aerosols which affect the radiative budget indirectly as cloud condensation nuclei (CCN). It has been postulated that the planets climate may be modulated by variations in DMS production form changes of the sea surface temperature (SST). It is shown that surface seawater DMS concentrations can be diagnosed at a near global scale using (1) satellite observations of Chl a, (2) climatological SSTs, (3) a temperature dependent index of the marine community structure and (4), observational relationships based on cruise data. This procedure is used to characterize the spatio-temporal variations in marine DMS emissions to the atmosphere in the Southern Hemisphere for the spring and summer months. Some coherence is obtained between DMS emissions and the polarization measurements from the space borne POLDER instrument used to estimate the effective radius of liquid-phase clouds; higher are the DMS emissions lower is the cloud droplet radius. An inspection of the ISCCP-D products of low-cloud optical depth a d DMS emissions at Amsterdam Island and Cape Grim indicates that higher are the DMS emissions higher is the cloud optical depth. The IPSL ocean-atmosphere coupled model, which includes an explicitly representation of plankton dynamics and Chl a, predicts significant large-scale changes of DMS fluxes in response to a doubling of atmospheric CO2. We envision that the decreased mass of sulfur in the tropical upper troposphere will lead to lower numbers of CCNs in the air subsiding back into the subtropical marine boundary layer. Hence, it is suggested that the sensitivity of mid- latitude clouds to local changes in DMS emissions will increase in the future.

Projection des changements climatiques futurs

Les modeles de climat offrent la possibilite danticiper la facon dont le systeme climatique risque detre modifie par lactivite humaine au cours du XXIe siecle. Les etudes sappuient sur des simulations numeriques qui explorent levolution du climat moyen et de sa variabilite en fonction de differents scenarios socio-economiques. Nous presentons une selection de resultats du projet CMIP5 dintercomparaison demodeles de climat avec un focus illustratif sur les deux modeles francais ayant participe a cet exercice. On y decrit les effets attendus des perturbations humaines sur la temperature de surface, les precipitations, la cryosphere mais aussi les extremes meteorologiques et le cycle du carbone. Les resultats font ressortir de nombreux aspects fiables, que ce soit sur lamplitude et la repartition geographique des changements attendus ou sur les processus mis en jeu dans ces changements. Ils montrent aussi les limites de lexercice prospectif et des incertitudes persistantes sur certains points cles.