RESEARCH HIGHLIGHT

Dimethylsulfide (DMS): A natural link between marine biology and climate

CONTACT: Dr. Clara Deal

September 14 , 2006

The atmospheric trace gas dimethylsulfide (DMS) and its major biological precursor, marine algal dimethylsulfoniopropionate (DMSP), are closely linked to processes occurring at all levels of the marine food web (Figure 1).  Because ocean surface waters are saturated in DMS with respect to the air above, S is transferred from the ocean to the atmosphere, via DMS.  Unlike carbon dioxide, DMS is not a greenhouse gas because it does not absorb outgoing long-wavelength energy radiated from the earth.  Instead, DMS plays a role in climate through the generation of particles in the atmosphere (aerosols) that scatter and absorb incoming solar energy and may also serve as cloud condensation nuclei (CCN), the “seeds” on which clouds form.  It has been hypothesized that increased DMS emissions due to warming, may have a cooling impact on climate by increasing the number of CCN and hence cloud reflectivity (Figure 1). 

The Arctic is an good place to look for a potential DMS-cloud albedo-climate feedback.  The impact of DMS on cloud albedo and climate is likely pronounced here since aerosol number concentrations are often very low in the north.  Therefore, changes in CCN may have a relatively strong effect on arctic clouds.  Also, because the Arctic is remote and its landscape is predominately covered by ice and snow, natural sulfur emissions from the ocean are a major component of aerosols.  Contributing to this source, are arctic marine microalgae such as sea ice algae and the phytoplankton spp. Phaeocystis, common in polar regions.  They are among the strongest producers of DMSP.  DMSP and DMS may help these organisms survive the freezing temperatures and strong salinity gradients characteristic of sea ice environments.  Considering the trend in arctic warming and diminishing arctic sea ice, resulting in decreased surface albedo, it is possible that an increase in arctic DMS emissions leading to increased cloud albedo may act to counter this trend.  

At this point in time, it is not known if a feedback loop between DMS and climate exists.  To investigate this, we are developing and utilizing computer ecosystem models, in which each variable (e.g. phytoplankton, nutrient, DMS, etc.) is described by an equation or set of equations representing the physical and biological forces acting on that variable.  Data from field and laboratory studies are crucial for constructing and testing these models.  At IARC, we work towards synthesizing and integrating all available relevant data for the models we use.  In the past several years, IARC researchers have made measurements of DMS and DMSP in the Bering and Chukchi Seas (Figure 2) and in land-fast ice off the coast of Barrow, Alaska.  Figure 3 shows DMSP accumulates near the bottom of the ice core, confirming efficient production of DMSP by ice algae.  These kinds of data are used to evaluate model performance.  An IARC led study of seawater DMS photolysis (light-mediated decomposition), in collaboration with scientists at the State University of New York in Syracuse, Woods Hole Oceanographic Institute, and Stevens Institute of Technology, New Jersey, has revealed the wavelength dependence and rates of DMS photolysis in the Bering Sea.  These results are being used to derive a more defensible equation for this important DMS loss process in our high latitude DMS ecosystem model.  In collaboration with other UAF departments, we are pursuing a complementary modeling approach for predicting DMS seawater concentrations on large spatial scales (regional to global) through statistical modeling using Geographical Information System (GIS).  By making use of large databases that are freely available on the web, we are teasing out the relationships between DMS seawater concentrations and environmental variables in order to predict DMS seawater concentrations and, hence, air-sea DMS flux.  Our ultimate goal is a better understanding of the controls on marine S cycling and the role DMS plays in climate.

To successfully provide a quantitative assessment of the predicted impacts of climate warming, physical, chemical and biological parameters need to be monitored and tied together in ecosystem models.  IARC efforts in marine ecosystem modeling on many scales, and conducting observations and process studies support and complement the IARC mission of reducing uncertainty in arctic climate change prediction and providing analyses of anthropogenic and natural effects on arctic climate change.

Acknowledgements: The 1-D ecosystem model development for the Bering Sea is supported by North Pacific Research Board (NPRB) grant 607 awarded to Jin, Deal and Wang.

Figure 1.  A “simple view” of the DMS biogeochemical cycle showing hypothesized DMS-cloud albedo-climate feedback loop (Charlson et al., Nature, 1987; Shaw, Climatic Change, 1983).  Figure courtesy Ron Keine. (click on image for larger view)

Figure 2.   DMS sampling locations in the Bering and Chukchi Seas (•). (click on image for larger view)

Figure 3.  DMSP accumulates toward the bottom of the ice core where >90% of the ice algal biomass (chlorophyll a) is observed (Uzuka, Tohuoku Geophyical Journal, 2003). (click on image for larger view)