CLAW hypothesis
Encyclopedia
The CLAW hypothesis proposes a feedback loop
that operates between ocean
ecosystem
s and the Earth
's climate
. The hypothesis
specifically proposes that particular phytoplankton
that produce dimethyl sulfide
are responsive to variations in climate forcing, and that these responses lead to a negative feedback loop
that acts to stabilise the temperature
of the Earth's atmosphere
. The CLAW hypothesis was originally proposed by Robert Charlson, James Lovelock
, Meinrat Andreae and Stephen Warren, and takes its acronym from the first letter of their surname
s.
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 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 nuclei
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. The figure to the right shows a summarising 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 damped
by the loop. As such, the CLAW hypothesis posits an example of planetary-scale
homeostasis
or complex adaptive system
, consistent with the Gaia hypothesis
framed by one of the original authors of the CLAW hypothesis, James Lovelock.
Some subsequent studies of the CLAW hypothesis have uncovered some 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
without the requirement of an active biological component.
In his 2006 book, 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 loop
.
Under future global warming
, increasing temperature may stratify
the world ocean
, decreasing the supply of nutrient
s 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 stratification.
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
that operates between ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
s and the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
's climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
. The hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
specifically proposes that particular phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
that produce dimethyl sulfide
Dimethyl sulfide
Dimethyl sulfide or methylthiomethane is an organosulfur compound with the formula 2S. Dimethyl sulfide is a water-insoluble flammable liquid that boils at and has a characteristic disagreeable odor. It is a component of the smell produced from cooking of certain vegetables, notably maize,...
are responsive to variations in climate forcing, and that these responses lead to a negative feedback loop
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
that acts to stabilise the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
of the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
. The CLAW hypothesis was originally proposed by Robert Charlson, James Lovelock
James Lovelock
James Lovelock, CH, CBE, FRS is an independent scientist, environmentalist and futurologist who lives in Devon, England. He is best known for proposing the Gaia hypothesis, which postulates that the biosphere is a self-regulating entity with the capacity to keep our planet healthy by controlling...
, Meinrat Andreae and Stephen Warren, and takes its acronym from the first letter of their surname
Surname
A surname is a name added to a given name and is part of a personal name. In many cases, a surname is a family name. Many dictionaries define "surname" as a synonym of "family name"...
s.
The CLAW hypothesis
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
from the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
acting to increase the growth rates of phytoplankton by either a physiological effect
Q10 (temperature coefficient)
The Q10 temperature coefficient is a measure of the rate of change of a biological or chemical system as a consequence of increasing the temperature by 10 °C. There are many examples where the Q10 is used, one being the calculation of the nerve conduction velocity and another being calculating the...
(due to elevated temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
) or enhanced photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
(due to increased irradiance
Irradiance
Irradiance is the power of electromagnetic radiation per unit area incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter , while the cgs units are ergs per square centimeter...
). Certain phytoplankton, such as coccolithophorids
Coccolithophore
Coccolithophores are single-celled algae, protists, and phytoplankton belonging to the division of haptophytes. They are distinguished by special calcium carbonate plates of uncertain function called coccoliths , which are important microfossils...
, synthesise dimethylsulfoniopropionate
Dimethylsulfoniopropionate
Dimethylsulfoniopropionate , is an organosulfur compound with the formula 2S+CH2CH2COO−. This zwitterionic metabolite 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
Osmolyte
Osmolytes are compounds affecting osmosis. They are soluble in the solution within a cell, or in the surrounding fluid, e.g. as plasma osmolytes. They play a role in maintaining cell volume and fluid balance. For example, when a cell swells due to external osmotic pressure, membrane channels open...
. In turn, this leads to an increase in the concentration of its breakdown product, dimethyl sulfide
Dimethyl sulfide
Dimethyl sulfide or methylthiomethane is an organosulfur compound with the formula 2S. Dimethyl sulfide is a water-insoluble flammable liquid that boils at and has a characteristic disagreeable odor. It is a component of the smell produced from cooking of certain vegetables, notably maize,...
(DMS), in first seawater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...
, and then the atmosphere. DMS is oxidised in the atmosphere to form sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
, and this leads to the production of sulfate aerosols
Sulfate aerosols
The term sulfate aerosols is used for a suspension of fine solid particles of a sulfate or tiny droplets of a solution of a sulfate or of sulfuric acid . They are produced by chemical reactions in the atmosphere from gaseous precursors...
. These aerosols act as cloud condensation nuclei
Cloud condensation nuclei
Cloud condensation nuclei or CCNs are small particles typically 0.2 µm, or 1/100 th the size of a cloud droplet ) about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a vapour to a liquid. In the atmosphere, this surface presents itself as tiny...
and increase cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
droplet number, which in turn elevate the liquid water content of clouds and cloud area. This acts to increase cloud albedo
Cloud albedo
Cloud albedo is a measure of the albedo of a cloud - higher values mean that the cloud reflects more solar radiation, or more radiation is transmitted....
, leading to greater reflection
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
of incident sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
, and a decrease in the forcing that initiated this chain of events. The figure to the right shows a summarising schematic diagram
Schematic
A schematic diagram represents 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 information the schematic is intended to convey, and may add unrealistic elements that aid comprehension...
. 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
Damping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
by the loop. As such, the CLAW hypothesis posits an example of planetary-scale
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
homeostasis
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...
or complex adaptive system
Complex adaptive system
Complex adaptive systems are special cases of complex systems. They are complex in that they are dynamic networks of interactions and relationships not aggregations of static entities...
, consistent with the Gaia hypothesis
Gaia hypothesis
The Gaia hypothesis, also known as Gaia theory or Gaia principle, proposes that all organisms and their inorganic surroundings on Earth are closely integrated to form a single and self-regulating complex system, maintaining the conditions for life on the planet.The scientific investigation of the...
framed by one of the original authors of the CLAW hypothesis, James Lovelock.
Some subsequent studies of the CLAW hypothesis have uncovered some 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
Sulfur cycle
The sulfur cycle are the collection of processes by which sulfur moves to and from minerals and living systems. Such biogeochemical cycles are important in geology because they affect many minerals...
without the requirement of an active biological component.
The Anti-CLAW hypothesis
The Revenge of Gaia
The Revenge of Gaia: Why the Earth is Fighting Back – and How we Can Still Save Humanity is a book by James Lovelock.- External links :* The Revenge of Gaia * , edited extract from The Guardian, 24 March 2006...
, 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
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
.
Under future global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...
, increasing temperature may stratify
Stratification (water)
Water stratification occurs when water masses with different properties - salinity , oxygenation , density , temperature - form layers that act as barriers to water mixing...
the world ocean
World Ocean
The World Ocean, world ocean, or global ocean, is the interconnected system of the Earth's oceanic waters, and comprises the bulk of the hydrosphere, covering almost 71% of the Earth's surface, with a total volume of 1.332 billion cubic kilometres.The unity and continuity of the World Ocean, with...
, decreasing the supply of nutrient
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...
s from the deep ocean to its productive
Primary production
400px|thumb|Global oceanic and terrestrial photoautotroph abundance, from September [[1997]] to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it...
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
Computer simulation
A computer simulation, a computer model, or a computational model is a computer program, or network of computers, that attempts to simulate an abstract model of a particular system...
future oceanic primary production have found evidence of declining production with increasing ocean stratification.
See also
- CloudCloudA cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
- Coccolithophorid
- Dimethyl sulfideDimethyl sulfideDimethyl sulfide or methylthiomethane is an organosulfur compound with the formula 2S. Dimethyl sulfide is a water-insoluble flammable liquid that boils at and has a characteristic disagreeable odor. It is a component of the smell produced from cooking of certain vegetables, notably maize,...
- DimethylsulfoniopropionateDimethylsulfoniopropionateDimethylsulfoniopropionate , is an organosulfur compound with the formula 2S+CH2CH2COO−. This zwitterionic metabolite found in marine phytoplankton, seaweeds, and some species of terrestrial and aquatic vascular plants...
- Earth scienceEarth scienceEarth science is an all-embracing term for the sciences related to the planet Earth. It is arguably a special case in planetary science, the Earth being the only known life-bearing planet. There are both reductionist and holistic approaches to Earth sciences...
- GeophysiologyGeophysiologyGeophysiology is the study of interaction among living organisms on the Earth operating under the hypothesis that the Earth itself acts as a single living organism ....
- PhytoplanktonPhytoplanktonPhytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
External links
- Gaia and CLAW, Max Planck Institute for Chemistry, MainzMax Planck SocietyThe Max Planck Society for the Advancement of Science is a formally independent non-governmental and non-profit association of German research institutes publicly funded by the federal and the 16 state governments of Germany....
- DMS and climate, Pacific Marine Environmental Laboratory, SeattlePacific Marine Environmental LaboratoryThe Pacific Marine Environmental Laboratory is a laboratory in the National Oceanic and Atmospheric Administration /Office of Oceanic and Atmospheric Research ....