The CLAW hypothesis proposes a

negative feedback loop Negative feedback (or balancing feedback) occurs when some function of the output of a system, process, or mechanism is fed back in a manner that tends to reduce the fluctuations in the output, whether caused by changes in the input or by other ...

that operates between ocean

ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...

s and the Earth's

climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...

. The

hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...

specifically proposes that particular

phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. ...

that produce

dimethyl sulfide Dimethyl sulfide (DMS) or methylthiomethane is an organosulfur compound with the formula (CH3)2S. Dimethyl sulfide is a flammable liquid that boils at and has a characteristic disagreeable odor. It is a component of the smell produced from co ...

are responsive to variations in

climate forcing Earth's climate system is a complex system having five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere (living things). ''C ...

, and that these responses act to stabilise the temperature of the

Earth's atmosphere The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...

. The CLAW hypothesis was originally proposed by Robert Jay Charlson,

James Lovelock James Ephraim Lovelock (26 July 1919 – 26 July 2022) was an English independent scientist, environmentalist and futurist. He is best known for proposing the Gaia hypothesis, which postulates that the Earth functions as a self-regulating sy ...

, Meinrat Andreae and Stephen G. Warren, and takes its

acronym An acronym is a word or name formed from the initial components of a longer name or phrase. Acronyms are usually formed from the initial letters of words, as in ''NATO'' (''North Atlantic Treaty Organization''), but sometimes use syllables, as ...

from the first letter of their surnames.

CLAW hypothesis

The hypothesis describes a feedback loop that begins with an increase in the available

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...

from the sun acting to increase the growth rates of phytoplankton by either a physiological effect (due to elevated temperature) or enhanced

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

(due to increased

irradiance In radiometry, irradiance is the radiant flux ''received'' by a ''surface'' per unit area. The SI unit of irradiance is the watt per square metre (W⋅m−2). The CGS unit erg per square centimetre per second (erg⋅cm−2⋅s−1) is often used ...

). Certain phytoplankton, such as

coccolithophorids Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingdo ...

, synthesise

dimethylsulfoniopropionate Dimethylsulfoniopropionate (DMSP), is an organosulfur compound with the formula (CH3)2S+CH2CH2COO−. This zwitterionic metabolite can be found in marine phytoplankton, seaweeds, and some species of terrestrial and aquatic vascular plants ...

(DMSP), and their enhanced growth increases the production of this

osmolyte Osmolytes are low-molecular weight organic compounds that influence the properties of biological fluids. Their primary role is to maintain the integrity of cells by affecting the viscosity, melting point, and ionic strength of the aqueous solution. ...

. In turn, this leads to an increase in the concentration of its breakdown product,

(DMS), first in seawater, and then in the atmosphere. DMS is

oxidised Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

in the atmosphere to form

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic a ...

, and this leads to the production of sulfate aerosols. These aerosols act as

cloud condensation nuclei Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2 µm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This c ...

and increase cloud droplet number, which in turn elevate the liquid water content of clouds and cloud area. This acts to increase

cloud albedo Cloud albedo is a measure of the albedo or reflectivity of a cloud. Clouds regulate the amount of solar radiation absorbed by a planet and its solar surface irradiance. Generally, increased cloud cover correlates to a higher albedo and a lower ab ...

, leading to greater

reflection Reflection or reflexion may refer to: Science and technology * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal reflection, in ...

of incident

sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when ...

, and a decrease in the forcing that initiated this chain of events. The figure to the right shows a summarising

schematic diagram A schematic, or schematic diagram, is a designed representation of the elements of a system using abstract, graphic symbols rather than realistic pictures. A schematic usually omits all details that are not relevant to the key information the sc ...

. 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 damped by the loop. As such, the CLAW hypothesis posits an example of planetary-scale

homeostasis In biology, homeostasis ( British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...

complex adaptive system A complex adaptive system is a system that is '' complex'' in that it is a dynamic network of interactions, but the behavior of the ensemble may not be predictable according to the behavior of the components. It is '' adaptive'' in that the indiv ...

, consistent with the

Gaia hypothesis The Gaia hypothesis (), also known as the Gaia theory, Gaia paradigm, or the Gaia principle, proposes that living organisms interact with their inorganic surroundings on Earth to form a synergistic and self-regulating, complex system that help ...

framed by one of the original authors of the CLAW hypothesis, James Lovelock. Some subsequent studies of the CLAW hypothesis have uncovered evidence to support its mechanism, although this is not unequivocal. Other researchers have suggested that a CLAW-like mechanism may operate in the Earth's

sulfur cycle The sulfur cycle is a biogeochemical cycle in which the sulfur moves between rocks, waterways and living systems. It is important in geology as it affects many minerals and in life because sulfur is an essential element (CHNOPS), being a con ...

without the requirement of an active biological component. A 2014 review article criticised the model for being an oversimplification and that the effect might be much weaker than proposed.

Anti-CLAW hypothesis

In his 2006 book ''

The Revenge of Gaia ''The Revenge of Gaia: Why the Earth is Fighting Back – and How We Can Still Save Humanity'' (2006) is a book by James Lovelock. Some editions of the book have a different, less optimistic subtitle: ''Earth's Climate Crisis and the Fate of Hu ...

'', 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 loop Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the ...

. Under future

global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...

, increasing temperature may stratify the

world ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the worl ...

, decreasing the supply of nutrients from the deep ocean to its

productive Productivity is the efficiency of production of goods or services expressed by some measure. Measurements of productivity are often expressed as a ratio of an aggregate output to a single input or an aggregate input used in a production proces ...

euphotic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...

. 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

albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...

. 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 stratification, leaving open the possibility that such a mechanism may exist.

References

External links

Gaia and CLAW
Max Planck Institute for Chemistry, Mainz
DMS and climate
Pacific Marine Environmental Laboratory, Seattle {{plankton Atmospheric radiation Climate change feedbacks Particulates Satellite meteorology

CLAW hypothesis

Anti-CLAW hypothesis

See also

References

External links