CLAW hypothesis

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According to the CLAW hypothesis, there is a negative feedback ( negative feedback ) in the earth's climate system via a sulfur- containing substance that is released by certain types of phytoplankton and forms aerosols in the atmosphere , which in turn promote shadow-giving clouds. The latter is very efficient in relation to the required amounts of sulfur. This is generally accepted doctrine. It is unclear whether the reduced radiation and temperature promotes or inhibits the emission of the substance. In the event of an inhibition, a thermostat would result, which would stabilize the temperature, against the slow increase in luminosity of the sun and fluctuations in the carbon cycle , see paradox of the weak young sun and paleoclimatology . The Gaia hypothesis , to which the CLAW hypothesis is intended to contribute, is generally rejected .

The hypothesis was voiced by Glenn Shaw (1983) and Robert Charlson, James Lovelock , Meinrat O. Andreae and Stephen Warren worked out ( "CLAW" are the initials , C harlson / L ovelock / A ndreae / W arren- hypothesis ).

Details of the knitting chain

The issuers include B. Coccolithophores , which produce dimethylsulfonium propionate (DMSP) to increase their osmotic pressure . When they die or are eaten, DMSP is released into methanethiol and dimethyl sulfide (CH 3 SCH 3 ; DMS). While methanethiol is quickly converted into bacteria (partly to sulfur-containing amino acids), DMS is released into the atmosphere. There it is broken down photochemically .

The first stable intermediate is dimethyl sulfoxide (CH 3 S (= O) CH 3 )  : 

DMS + OH ↔ DMS-OH, + O2 → HO2 + DMSO.

The further routes are varied, sometimes heterogeneous , but the main end product is sulfuric acid (H 2 SO 4 ). This and also the fairly stable intermediate methanesulfonic acid (CH 3 SO 3 H) have a very low saturation vapor pressure , so that the molecules combine to form numerous condensation nuclei . This causes clouds to form when there is less oversaturation or, when there is greater humidity, consist of numerous, smaller drops that rain down less quickly. The hygroscopic effect of sulphate aerosols creates extensive haze below 100% RH , which, like clouds, reduces radiation , see Earth's radiation balance . This mechanism can only be significant in clean air areas, otherwise there are enough other condensation nuclei. At higher, more stormy latitudes , salt particles formed from spray dominate the haze.

Various effects contribute to the retroactive effect on the DMS emission . A higher irradiation and temperature leads to a multiplication of the plankton in middle latitudes, in higher latitudes even to a longer growth season. In low latitudes, on the other hand, primary production is often limited by the nutrient supply, which becomes scarcer as the surface temperature rises, because then the stratification of the upper 100 to 200 meters of water column is more stable, which reduces the rise of nutrient-rich deep water. The plankton reacts to food stress with higher release of DMS, the population could shift to DMSP-producing species or it could decrease significantly. Last but not least, DMS will outgas faster with rising water temperature.

Scheme of dimethyl sulfide (DMS) in the oceans. Explanations, legend about "Dimethyl sulfide (DMS)" in the oceans: 1. Sea algae ( phytoplankton ) produce " Dimethyl sulfonium propionate (DMSP)" (yellow) which is in osmotic equilibrium with the sea water. It is broken down by the action of bacteria in "DMSP d" ( d stands for "degraded") (orange). 2. In sea water, the "DMSP" dissolves chemically; " Dimethyl sulfide (DMS)" (red) is then formed. 3. A fraction of the "DMS" enters the earthly atmosphere through evaporation . 4. The rest is metabolized by the bacteria or destroyed ( photochemically ) by the radiation of the sun ; converted to dimethyl sulfoxide (DMSO) (pink).

literature

  • Glenn E. Shaw (1983): Bio-controlled thermostasis involving the sulfur cycle. Climate Change 5, pp. 297-303, doi: 10.1007 / BF02423524 .
  • R. Charlson, J. Lovelock, M. Andreae and S. Warren (1987): Oceanic phytoplankton, atmospheric sulfur, cloud albedo and climate. Nature 326, pp. 655-661.
  • D. Harvey: Climate and global environmental change. Understanding global environmental change. Prentice Hall, 2000, ISBN 9780582322615 , pp. 28ff.
  • S. Rahmstorf and K. Richardson: How threatened are the oceans? Biological and physical aspects. Fischer, 2007, ISBN 9783104000695 , pp. 100ff.
  • Albert J. Gabric et al .: Global simulations of the impact on contemporary climate of a perturbation to the sea-to-air flux of dimethylsulfide. Australian Meteorological and Oceanographic Journal 63, 2013, pp. 365-376, ( online ).
  • M Galí, R Simó: A meta ‐ analysis of oceanic DMS and DMSP cycling processes: Disentangling the summer paradox. Global Biogeochemical Cycles, 2015, doi: 10.1002 / 2014GB004940 ( additional information free ).

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