Climate-relevant trace gases

Besides its contribution to the cycling of long-lived trace gases (e.g. CO2, N2O, CH4), the ocean also plays a critical role for the atmospheric budget of climate-relevant short-lived traces gases, such as dimethylsulphide (DMS) or iodinated and brominated hydrocarbons. The climate relevance of these substances arises from their influence on formation of aerosol particles that potentially alter cloud properties (DMS) and on the transmissivity of the atmosphere for solar radiation (for DMS via changes in the suphur inventory, for halocarbons via stratospheric ozone depletion), which both alter the radiative budget of the atmosphere.   
In our group, the marine cycling and climate-relevance of natural short-life trace gases is studied by application of the marine biogeochemistry model HAMOCC; either as part of an Earth System Model and in stand-alone ocean setups.

DMS

Dimethylsulphide (DMS) is a byproduct of marine primary producers and serves an energy source for bacteria.  Oceanic DMS  emissions are  the largest source of natural sulphur and are thus an important component in the global sulphur budget. Furthermore, there is evidence for a link between oceanic DMS emissions and the heat budget of the atmosphere. In the atmosphere, DMS might alter the production of aerosol particles that can serve as cloud condensation nuclei, which exert an indirect effect on climate via changing cloud albedo and lifetime. Atmospheric sulphur also directly affects climate by scattering solar radiation.  

Recent work in our group focused on the role of ocean acidification in the production of DMS and the response of the climate system. Several mesocosm studies carried out in polar and temperate seas (S. Archer, Bigelow/USA) showed a relation of DMS concentrations and seawater pH with reduced DMS concentrations in acidified water. These findings were implemented into the DMS cycle of the biogeochemistry model HAMOCC using parametrizations that reflect different magnitudes of the response of DMS to ocean acidification ("High","Low".. in Fig. 3 below). The resulting DMS emissions for each parametrization were used to estimate the climate impact by running an atmosphere model including sulphur chemistry and aerosol-cloud microphysics. Compared to a reference run without pH-dependence of DMS  production  the reduced DMS emissions induce a global radiative forcing of approx 0.2-0.6 W m-2, which can be translated into an equilibrium temperature response of approx. +0.1-0.7K. The study thus reveals a so far overlooked key linkage between different  components of the climate system. Ocean acidification, which is considered first and foremost as a passive response of the marine environment to the anthropogenic CO2 pertubation, may feed back onto the physical environment (climate) via biologically induced mechanisms. 

 

 

 

Contact: Katharina Six 

Projects: EU FP7 project EPOCA

References:

Six K.D., Kloster S., Ilyina T., Archer S.D., Zhang.K, Maier-Reimer.E (2013): Global warming amplified by reduced sulphur fluxes as a result of ocean acidification. Nature Climate Change, doi:10.1038/nclimate1981.

Le Clainche, Y., A. Vezina, M. Levasseur, R. A. Cropp, J. R. Gunson, S. M. Vallina, M. Vogt, C. Lancelot, J. I. Allen, S. D. Archer, L. Bopp, C. Deal, S. Elliott, M. Jin, G. Malin, V. Schoemann, R. Simo, K. D. Six and J. Stefels (2010): A first appraisal of prognostic ocean DMS models and prospects for their use in climate models. Global Biogeochemical Cycles 24, Art. No. GB3021, doi: 10.1029/2009GB003721. 

Kloster, S., K.D. Six, J. Feichter, E.Maier-Reimer, E. Roeckner, P. Wetzel, P. Stier, and M. Esch (2007): Response of dimethylsulfide (DMS) in the ocean and atmosphere to global warming, Journal of Geophysical Research-Biogeosciences 112, Seq. No.: G03005. doi: 10.1029/2006JG000224. 

Six, K.D. and E. Maier-Reimer (2006): What controls the oceanic dimethylsulfide (DMS) cycle? A modelling approach, Global Biogeochem.Cycles , 20, doi:10.1029/2005GB002674.

Kloster, S., J. Feichter, E. Maier-Reimer, K. D. Six, P. Stier and P. Wetzel (2006): DMS cycle in the marine ocean-atmosphere system a global model study. Biogeosciences 3, 1, 29-51. doi: 1726-4189/bg/2006-3-29.  

 

 

Halocarbons

Natural short-lived halocarbons (e.g. bromoform, methyl ioide) are important precursors of reactive halogen species in the atmosphere, which are climate-active via the radiative effects of stratospheric ozone depletion. The ocean is assumed to be net source of these substances, however production pathways, decay, and emissions are not fully understood. The biogeochemistry model HAMOCC is used to test the relevance of different source and sink mechanisms for spatial emission patterns and its dynamics in the open ocean. Improving the knowledge on oceanic cycling and emission is of particular importance considering the short life time of these substance in air: only strong emissions co-located with rapid uplift will lead to injection of these gases into the stratosphere. 

The work focuses on marine production and sink processes of bromoform, dibromomethane, methyl iodide, and diiodomethane. 

Key findings include: 

  • Only the consideration of microbial decay in addition to abiotic decay processes can explain observed vertical profiles of bromoform.   
  • Large scale patterns of bromoform concentrations can be reproduced by linking its production to primary production . At the regional scale ecosystem dynamics and different production rates of different plankton species have to be considered to realistically capture spatial and temporal patterns of concentrations/emissions.   
  • Large scale patterns of methyl iodide can be explained by photochemical production that is not limited by semi-labile DOM availability or by biological production with higher production rates in oliotrophic waters (assuming high production under nutrient stress).  
  • The ocean is not a permanent source of short-lived halocarbons; model simulations indicate localized seasonal reversal of the gas flux from outgassing from the ocean to uptake from the atmosphere. This needs to be taken into account when extrapolating flux measurements /calculations from ship cruises, and also in atmospheric chemistry studies.

Contact: Irene Stemmler

Project:  SOPRAN: Surface Ocean Processes in the Anthropocene

References:

Stemmler, I., Hense, I., and Quack, B. (2015): Marine sources of bromoform in the global open ocean - global patterns and emissions, Biogeosciences, 12, 1967-1981, doi:10.5194/bg-12-1967-2015.

Stemmler, I, Hense, I., Quack, B, Maier-Reimer, E. (2014): Methyl iodide production in the open ocean, Biogeosciences, 11, 4459-4476, doi:10.5194/bg-11-4459-2014.

Stemmler, I., Rothe, M., Hense, I., and Hepach, H. (2013): Numerical modelling of methyl iodide in the eastern tropical Atlantic, Biogeosciences,10, 4211-4225, doi:10.5194/bg-10-4211-2013.

Bell, N, L Hsu, DJ Jacob, MG Schultz, DR Blake, JH Butler, DB King, JM Lobert, E Maier-Reimer, 2001, Methyl iodide: atmospheric budget and use as a tracer of marine convection in global models, J. Geophys. Res., 10.1029/2001JD001151.