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A winning proposal for the Innovative Research Program, 2009:
How will global climate changes affect ocean productivity in the tropics?
Investigator: Brandi McCarty
Objectives:
Climate models predict that the region of low productivity in the tropics will increase by 4%
between the beginning of the industrial revolution and 2050 (Sarmiento, et al, 2004). It is
estimated that the low surface chlorophyll areas in these oceans (North and South Pacific, North
and South Atlantic, outside the equatorial zone) combined have expanded by 6.6 million km2 or
by about 15% from 1998 through 2006. While the biological productivity of these oligotrophic
ecosystems is low, their large size allows for a significant contribution on the global scale
(Polovina, et al, 2008). Several factors work together to limit productivity, and thus the uptake of
carbon dioxide by phytoplankton, in these waters. These factors include temperature, the
availability of nutrients, light and iron. It is our goal to gain insight into the relative effects of
aerosol and temperature on observed ocean productivity.
Background and Importance:
The climate-plankton link is found primarily in the tropics and mid-latitudes, where there is
limited vertical mixing because the water column is stabilized by thermal stratification (that is,
when light, warm waters overlie dense, cold waters). In these areas, the typically low levels of
surface nutrients limit phytoplankton growth. Climate warming further inhibits mixing, reducing
the upward nutrient supply and lowering productivity (Behrenfeld, et al, 2006).
The idea that an increase in global temperature would also increase phytoplankton activity and
therefore cloud condensing nuclei, (CCN), numbers was seen as a possible natural phenomenon
that would counteract climate change. This is known as the CLAW hypothesis but no conclusive
evidence to support this has yet been reported. The hypothesis describes a feedback loop that
begins with an increase in the available energy from the sun acting to increase the growth rates
of phytoplankton due either to elevated temperature or to increased irradiance. Certain
phytoplankton synthesize dimethylsulfoniopropionate, (DMSP), and their enhanced growth
increases the production of this osmolyte. In turn, this leads to an increase in the concentration of
its breakdown product, dimethyl sulfide (DMS), in first seawater, and then the atmosphere. DMS
is oxidized in the atmosphere to form sulfur dioxide, and this leads to the production of sulfate
aerosols. These aerosols act as CCN and increase cloud droplet number, which in turn elevate the
liquid water content of clouds and cloud area. This acts to increase cloud albedo, leading to
greater reflection of incident sunlight, and a decrease in the forcing that initiated this chain of
events (Charlson, et al, 1987).
A counter-hypothesis is advanced in “The Revenge of Gaia”, the book by James Lovelock.
Warming oceans are likely to become stratified, with most ocean nutrients trapped in the cold
bottom layers while most of the light needed for photosynthesis in the warm top layer. Under this
scenario, deprived of nutrients, marine phytoplankton would decline, as would sulfate cloud
condensation nuclei, and the high albedo associated with low clouds (Lovelock, 2007). As of
2007 this hypothesis remains speculative.
Research Plan:
Over ten years of satellite observations of the ocean are now available from the Sea-viewing
Wide Field-of-view Sensor mission (SeaWiFS), which provides estimates of surface chlorophyll
concentration, aerosol optical depth (AOD), cloud cover, and sea surface temperature, (SST).
This is a short record for climate studies, however we can use short term climate variability as a
proxy for long term changes. It is long enough, for example, to include variability due to El
Nino Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO).
Productivity varies with the season, and also locally and globally. Variation in primary
productivity, measured typically as the concentration of chlorophyll in water, is a primary
determinant of all biological productivity up the food web and trophic pyramid.
We plan to use the surface chlorophyll concentration to derive the temporal changes in surface
area of oligotrophic habitat in the tropical gyres in the Pacific Ocean. Using co-spectral analysis,
we will attempt to identify the temporal and spatial scales of the processes that link chlorophyll
concentration to aerosols and temperature. Specifically, we will test the null hypothesis, “The
correlation between the area of the tropical oligotrophic gyres and SST is greater than the
correlation between that area and aerosol optical depth over the last ten years”
If our hypothesis is correct, we will build on these results to propose a larger study to investigate
the specific mechanisms. In that study, we would use other data sources to separate the effects
of continental aerosols, which would be expected to deposit iron into the ocean, from aerosols
produced over the ocean, which would not be expected to fertilize the ocean.
Why is this work innovative?
Climate investigations using satellite data generally average over monthly or seasonal time scales
and spatial scales of degrees or more. By using the sea surface chlorophyll concentration,
aerosol optical depth and Sea-Surface temperature all from the same platform, interactions at a
variety of temporal and spatial scales can be investigated to identify those scales of potential
importance. This project will significantly improve our understanding of a unique oceanatmosphere
interaction and the opportunity to further investigate the specific mechanisms in
which aerosols contribute. Results of the study will likely be of interest to researchers in a range
of scientific disciplines as we link atmospheric chemistry and marine biology, two fields that are
usually considered to be non-overlapping entities.
References:
1Behrenfeld, M.J., O'Malley, R.T., Siegel, D.A., McClain, C.R., Sarmiento, J.L., Feldman, G.C.,
Milligan, A.J., Falkowski, P.G., Letelier, R.M., Boss, E.S. (2006). Climate-driven trends in
contemporary ocean productivity, Nature, Vol. 444 (7 December 2006), pp. 752-755.
2Charlson, R. J., Lovelock, J. E., Andreae, M. O. and Warren, S. G. (1987). Oceanic
phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature 326, 655-661.
3Cropp, R. A., A. J. Gabric, G. H. McTainsh, R. D. Braddock, and N. Tindale (2005), Coupling
between ocean biota and atmospheric aerosols: Dust, dimethylsulphide, or artifact?, Global
Biogeochem. Cycles, 19, GB4002, doi:10.1029/2004GB002436.
4Lovelock, James (2007). The Revenge of Gaia. Penguin Books Ltd. ISBN 0141025972.
5Polovina, J. J., E. A. Howell, and M. Abecassis (2008). Ocean's least productive waters are
expanding, Geophys. Res. Lett., 35, L03618, doi:10.1029/2007GL031745.
6Sarmiento, J. L., R. Slater, R. Barber, L. Bopp, S. C. Doney, A. C. Hirst, J. Kleypas,
R. Matear, U. Mikolajewicz, P. Monfray, V. Soldatov, S. A. Spall, and R. Stouffer, (2004).
Response of ocean ecosystems to climate warming, Global Biogeochemical Cycles, Vol. 18,
GB3003, doi:10.1029/2003GB002134.
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216 UCB