View source for Astronomy:CLAW hypothesis

{{short description| A hypothesised negative feedback loop connecting the marine biota and the climate }}

The '''CLAW hypothesis''' proposes a [[Negative feedback|negative feedback loop]] that operates between ocean [[Earth:Ecosystem|ecosystem]]s and the Earth's [[Earth:Climate|climate]].<ref name="CLAW87">{{cite journal |doi=10.1038/326655a0 |author=Charlson, R. J., [[Biography:James Lovelock|Lovelock, J. E.]], Andreae, M. O. and Warren, S. G. |title=Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate |journal=Nature |volume=326 |issue=6114 |pages=655–661 |year=1987 |bibcode=1987Natur.326..655C|s2cid=4321239 }}</ref>  The [[Hypothesis|hypothesis]] specifically proposes that particular [[Earth:Phytoplankton|phytoplankton]] that produce [[Chemistry:Dimethyl sulfide|dimethyl sulfide]] are responsive to variations in climate forcing, and that these responses act to stabilise the temperature of the Earth's atmosphere.  The CLAW hypothesis was originally proposed by Robert Jay Charlson, [[Biography:James Lovelock|James Lovelock]], Meinrat Andreae and Stephen G. Warren, and takes its acronym from the first letter of their surnames.<ref name="andreae95">{{cite journal |doi=10.1029/94JD02828 |author=Andreae, M. O., Elbert, W. and Demora, S. J. |title=Biogenic sulfur emissions and aerosols over the tropical South Atlantic, 3. Atmospheric dimethylsulfide, aerosols and cloud condensation nuclei |journal=J. Geophys. Res. |volume=100 |issue=D6 |pages=11335–56 |year=1995 |url=http://www.agu.org/pubs/crossref/1995/94JD02828.shtml |bibcode=1995JGR...10011335A}}</ref>

==CLAW hypothesis==
[[File:CLAW hypothesis graphic 1 AYool.png|thumb|upright=1.2|right|{{center|Schematic diagram of the CLAW hypothesis (Charlson ''et al.'', 1987)<ref name="CLAW87"/>}}]]
[[File:CLAW hypothesis graphic 2 AYool.png|thumb|upright=1.2|right|{{center|Schematic diagram of the anti-CLAW hypothesis (Lovelock, 2006)<ref name="lovelock06"/>}}]]

The hypothesis describes a feedback loop that begins with an increase in the available [[Physics:Energy|energy]] from the sun acting to increase the growth rates of phytoplankton by either a [[Chemistry:Q10 (temperature coefficient)|physiological effect]] (due to elevated temperature) or enhanced [[Earth:Photosynthesis|photosynthesis]] (due to increased [[Physics:Irradiance|irradiance]]).  Certain phytoplankton, such as [[Biology:Coccolithophore|coccolithophorids]], synthesise [[Chemistry:Dimethylsulfoniopropionate|dimethylsulfoniopropionate]] (DMSP), and their enhanced growth increases the production of this [[Physics:Osmolyte|osmolyte]].  In turn, this leads to an increase in the concentration of its breakdown product, [[Chemistry:Dimethyl sulfide|dimethyl sulfide]] (DMS), first in seawater, and then in the atmosphere.  DMS is oxidised in the atmosphere to form [[Chemistry:Sulfur dioxide|sulfur dioxide]], and this leads to the production of sulfate aerosols.  These aerosols act as [[Physics:Cloud condensation nuclei|cloud condensation nuclei]] and increase cloud droplet number, which in turn elevate the liquid water content of clouds and cloud area.  This acts to increase [[Astronomy:Cloud albedo|cloud albedo]], leading to greater [[Physics:Reflection|reflection]] of incident [[Astronomy:Sunlight|sunlight]], and a decrease in the forcing that initiated this chain of events.  The figure to the right shows a summarising [[Schematic|schematic diagram]].  Note that the feedback loop can operate in the reverse direction, 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 [[Damping ratio|damped]] by the loop.  As such, the CLAW hypothesis posits an example of planetary-scale [[Homeostasis|homeostasis]] or [[Complex adaptive system|complex adaptive system]], consistent with the [[Astronomy:Gaia hypothesis|Gaia hypothesis]] framed by one of the original authors of the CLAW hypothesis, James Lovelock.<ref>{{cite book
 | last = Lovelock | first =  J.E.
 | title = Gaia: A New Look at Life on Earth
 | orig-year =1979
 | edition = 3rd
 | year = 2000
 | publisher = [[Organization:Oxford University Press|Oxford University Press]]
 | isbn = 0-19-286218-9
 }}</ref>

Some subsequent studies of the CLAW hypothesis have uncovered evidence to support its mechanism,<ref name="andreae95"/><ref>{{cite journal |doi=10.1029/2004GB002436 |author=Cropp, R.A., Gabric, A.J., McTainsh, G.H., Braddock, R.D. and Tindale, N. |title=Coupling between ocean biota and atmospheric aerosols: Dust, dimethylsulphide, or artifact? |journal=Global Biogeochemical Cycles |volume=19 |issue=4 |pages=GB4002 |year=2005 |url=http://www.agu.org/pubs/crossref/2005/2004GB002436.shtml |bibcode=2005GBioC..19.4002C|hdl=10072/4405 |s2cid=13631729 |hdl-access=free }}</ref> although this is not unequivocal.<ref>{{cite journal |doi=10.1029/2006GB002787 |author=Vallina, S. M., Simo, R., Gasso, S., De Boyer-Montegut, C., del Rio, E., Jurado, E. and Dachs, J. |title=Analysis of a potential "solar radiation dose-dimethylsulfide-cloud condensation nuclei" link from globally mapped seasonal correlations |journal=Global Biogeochemical Cycles |volume=21 |issue=2 |pages=GB2004 |year=2007 |url=http://www.agu.org/pubs/crossref/2007/2006GB002787.shtml |bibcode=2007GBioC..21.2004V|hdl=10261/11184 |s2cid=128377799 |hdl-access=free }}</ref>  Other researchers have suggested that a CLAW-like mechanism may operate in the Earth's [[Biology:Sulfur cycle|sulfur cycle]] without the requirement of an active biological component.<ref>{{cite journal |doi=10.1023/A:1005341506115 |author1=Shaw, G.E.|author2=Benner, R.L.|author3=Cantrell, W.|author4=Clarke, A.D.|title=The regulation of climate: A sulfate particle feedback loop involving deep convection — An editorial essay |journal=Climate Change |volume=39 |pages=23–33 |year=1998 |s2cid=150898991 }}</ref> A 2014 review article criticised the hypothesis for being an oversimplification and that the effect might be much weaker than proposed.<ref>{{cite journal |last1=Green |first1=Tamara |last2=Hatton |first2=Angela |title=The CLAW hypothesis: a new perspective on the role of biogenic sulphur in the regulation of global climate |journal=Oceanography and Marine Biology |date=1 September 2014 |volume=52 |pages=315–336 |doi=10.1201/b17143-7 |isbn=9780429161391 |url=https://www.researchgate.net/publication/265384642}}</ref>

==Anti-CLAW hypothesis==
In his 2006 book ''[[Earth:The Revenge of Gaia|The Revenge of Gaia]]'', Lovelock proposed that instead of providing negative feedback in the climate system, the components of the CLAW hypothesis may act to create a [[Positive feedback|positive feedback loop]].<ref name="lovelock06">{{cite book | author=Lovelock, James | title=The Revenge of Gaia | publisher=Penguin | year=2007 | isbn=978-0-14-102597-1| title-link=The Revenge of Gaia }}</ref>

Under future [[Earth:Global warming|global warming]], increasing temperature may [[Earth:Ocean stratification|stratify]] the world ocean, decreasing the supply of nutrients from the deep ocean to its [[Earth:Primary production|productive]] euphotic zone. Consequently, phytoplankton activity will decline with a concomitant 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 [[Earth:Albedo|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 [[Computer simulation|simulating]] future [[Earth:Primary production#Oceanic production|oceanic primary production]] have found evidence of declining production with increasing ocean stratification,<ref name=cox00>{{cite journal |author=Cox, P. M., Betts, R. A., Jones, C. D., Spall, S. A. and Totterdell, I. J. |title=Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model |journal=Nature |volume=408 |pages=184–7 |year=2000 |pmid=11089968 |issue=6809 |doi=10.1038/35041539|bibcode=2000Natur.408..184C |s2cid=2689847 }}</ref><ref>{{cite journal |author1=Steinacher, M. |author2=Joos, F. |author3=Fr ¨ olicher, T. L. |author4=Bopp, L. |author5=Cadule, P. |author6=Cocco, V. |author7=Doney, S. C. |author8=Gehlen, M. |author9=Lindsay, K. |author10=Moore, J. K. |author11=Schneider, B. |author12=Segschneider, J. |name-list-style=amp |title=Projected 21st century decrease in marine productivity: a multi-model analysis |journal=Biogeosciences |volume=7 |issue=3 |pages=979–1005 |doi=10.5194/bg-7-979-2010 |year=2010|bibcode=2010BGeo....7..979S |doi-access=free |hdl=11858/00-001M-0000-0011-F69E-5 |hdl-access=free }}</ref> leaving open the possibility that such a mechanism may exist.

==See also==
* [[Earth:Earth science|Earth science]]
* Geophysiology
* [[Earth:Iron fertilization|Iron fertilization]]

==References==
{{reflist|2}}

==External links==
* [https://web.archive.org/web/20041229183238/http://www.atmosphere.mpg.de/enid/1w1.html Gaia and CLAW], Max Planck Institute for Chemistry, Mainz
* [https://web.archive.org/web/20150214081038/http://saga.pmel.noaa.gov/review/dms_climate.html DMS and climate], [[Organization:Pacific Marine Environmental Laboratory|Pacific Marine Environmental Laboratory, Seattle]]

{{global warming}}
{{plankton}}

[[Category:Atmospheric radiation]]
[[Category:Climate change feedbacks]]

{{Sourceattribution|CLAW hypothesis}}