The CLAW hypothesis proposes a feedback loop that operates between
ocean ecosystems and the Earth's climateCharlson, R. J., Lovelock, J. E., Andreae, M. O. and Warren, S. G. (1987). [http://www.nature.com/nature/journal/v326/n6114/abs/326655a0.html Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate.] "Nature" 326, 655-661.] . The hypothesisspecifically proposes that particular phytoplanktonthat produce dimethyl sulfideare responsive to variations in climate forcing, and that these responses lead to a negative feedback loop that acts to stabilise the temperatureof the Earth's atmosphere. The CLAW hypothesis was originally proposed by Robert Charlson, James Lovelock, Meinrat Andreaeand Stephen Warren, and takes its acronymfrom the first letter of their surnamesAndreae, M. O., Elbert, W. and Demora, S. J. (1995). [http://www.agu.org/pubs/crossref/1995/94JD02828.shtml Biogenic sulfur emissions and aerosols over the tropical South Atlantic, 3. Atmospheric dimethylsulfide, aerosols and cloud condensation nuclei.] "J. Geophys. Res." 100, 11335-11356.] .
The CLAW hypothesis
The hypothesis describes a feedback loop that begins with an increase in the available
energyfrom the sunacting to increase the growth rates of phytoplankton by either a physiological effect (due to elevated temperature) or enhanced photosynthesis(due to increased irradiance). Certain phytoplankton, such as coccolithophorids, synthesise 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 oxidised in the atmosphere to form sulfur dioxide, and this leads to the production of sulfate aerosols. These aerosols act as cloud condensation nucleiand increase clouddroplet 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. The figure to the right shows a summarising schematic diagram. Note that the feedback loop can operate in reverse, such that a decline in solar energy leads to reduced cloud cover and thus to an increase in the amount of solar energy reaching the Earth's surface.
A significant feature of the chain of interactions described above is that it creates a negative feedback loop, whereby a change to the climate system (increased/decreased solar input) is ultimately counteracted and damped by the loop. As such, the CLAW hypothesis posits an example of planetary-scale
homeostasisor complex adaptive system, consistent with the Gaia hypothesisframed by one of the original authors of the CLAW hypothesis, James Lovelock [cite book
last = Lovelock | first = J.E.
title = Gaia: A New Look at Life on Earth
edition = 3rd ed.
year = 2000
publisher = Oxford University Press
id = ISBN 0-19-286218-9 ] .
Some subsequent studies of the CLAW hypothesis have uncovered some evidence to support its mechanism [Cropp, R.A., Gabric, A.J., McTainsh, G.H., Braddock, R.D. and Tindale, N. (2005). [http://www.agu.org/pubs/crossref/2005/2004GB002436.shtml Coupling between ocean biota and atmospheric aerosols: Dust, dimethylsulphide, or artifact?] "Global Biogeochemical Cycles" 19, GB4002.] , although this is not unequivocal [Vallina, S. M., Simo, R., Gasso, S., De Boyer-Montegut, C., del Rio, E., Jurado, E. and Dachs, J. (2007). [http://www.agu.org/pubs/crossref/2007/2006GB002787.shtml Analysis of a potential "solar radiation dose-dimethylsulfide-cloud condensation nuclei" link from globally mapped seasonal correlations.] "Global Biogeochemical Cycles" 21, GB2004.] . Other researchers have suggested that a CLAW-like mechanism may operate in the Earth's
sulfur cyclewithout the requirement of an active biological component [Shaw, G.E., Benner, R.L., Cantrell, W. and Clarke, A.D. (1998). [http://www.ingentaconnect.com/content/klu/clim/1998/00000039/00000001/00164320 The regulation of climate: A sulfate particle feedback loop involving deep convection - An editorial essay.] "Climate Change" 39, 23-33.] .
The Anti-CLAW hypothesis
In a recent book, "
The Revenge of Gaia", Lovelock has proposed that instead of providing negative feedback in the climate system, the components of the CLAW hypothesis may act to create a positive feedback loopcite book | author=Lovelock, James | title= The Revenge of Gaia| publisher=Penguin Books Ltd | year=2007 | id=ISBN 0141025972] .
global warming, increasing temperature may stratify the world ocean, decreasing the supply of nutrients from the deep ocean to its productive euphotic zone. Consequently, phytoplankton activity will decline with a concommitant fall in the production of DMS. In a reverse of the CLAW hypothesis, this decline in DMS production will lead to a decrease in cloud condensation nuclei and a fall in cloud albedo. The consequence of this will be further climate warming which may lead to even less DMS production (and further climate warming ...). The figure to the right shows a summarising schematic diagram.
Evidence for the anti-CLAW hypothesis is constrained by similar uncertainties as those of the sulfur cycle feedback loop of the CLAW hypothesis. However, researchers simulating future oceanic primary production have found evidence of declining production with increasing ocean stratificationCox, P. M., Betts, R. A., Jones, C. D., Spall, S. A. and Totterdell, I. J. (2000). [http://www.nature.com/nature/journal/v408/n6809/abs/408184a0.html Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model.] "Nature", 408, 184-187.] .
* [http://www.atmosphere.mpg.de/enid/1w1.html Gaia and CLAW] , Max Planck Institute for Chemistry, Mainz
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