In the context of a biogeochemical cycle, a sink is a reservoir that provides storage for a substance. For example, the process of photosynthesis in plants removes carbon dioxide from the atmosphere and the carbohydrates produced by the plants represent a storage reservoir. Plant matter is thus a sink in the carbon cycle.
Historical Background and Scientific Foundations
Biogeochemical cycles describe the manner in which a substance, such as carbon or nitrogen, cycles through the atmosphere, biological organisms, regions of water, and rocks. These cycles include sinks or reservoirs, which are storage areas where the material remains for some period of time. Examples of sinks are rocks for phosphorous, soils for nitrogen, fossil fuels for carbon, and the atmosphere for oxygen.
The flows of substances between sinks are described by transformations. These transformations can involve the breaking of chemical bonds, as in the conversion of nitrogen gas in the atmosphere to nitrate by nitrogen-fixing bacteria in the soil. They may also be state changes, as when frozen water in ice sheets melts into the ocean.
Impacts and Issues
The burning of fossil fuels to drive industry adds additional carbon dioxide, nitrous oxides, and some sulfur products to the atmosphere. These gases act as greenhouse gases, trapping heat and increasing the global temperature of Earth. In order to compensate for the growing reservoir of greenhouse gases in the atmosphere, scientists have begun researching alternative sinks for these substances.
Because carbon dioxide is the major greenhouse gas, research into carbon sinks has garnered significant attention. The ocean acts as a sink for nearly half the carbon released by the burning of fossil fuels and it currently holds twice as much carbon as the atmosphere. Climate change affects circulation patterns and ecological processes in the ocean. The effect of such changes on the cycling of carbon and the ability of the ocean to continue acting as a major carbon sink is unknown although significant research is underway to address these issues.
WORDS TO KNOW
ANOXIC: Lacking free oxygen (O2). Similar in meaning to anaerobic, but implies that oxygen has been depleted in a given organism or environment, rather than being naturally at a low level.
BIOGEOCHEMICAL CYCLE: The chemical interactions that take place among the atmosphere, biosphere, hydrosphere, and geosphere.
BIOMASS: The sum total of living and once-living matter contained within a given geographic area. Plant and animal materials that are used as fuel sources.
PHOTOSYNTHESIS: The process by which green plants use light to synthesize organic compounds from carbon dioxide and water. In the process, oxygen and water are released. Increased levels of carbon dioxide can increase net photo-synthesis in some plants. Plants create a very important reservoir for carbon dioxide.
PHYTOPLANKTON: Microscopic marine organisms (mostly algae and diatoms) that are responsible for most of the photosynthetic activity in the oceans.
On land, plants act as the major carbon sink. As they convert carbon dioxide into carbohydrates through photosynthesis, they store significant carbon. The size of the terrestrial biomass reservoir is shrinking due to deforestation, especially in the Southern Hemisphere. Numerous agencies are funding research into understanding the causes of variability in terrestrial carbon sinks.
IN CONTEXT: NATURAL “SINKS” OVERWHELMED BY ANTHROPOGENIC EMISSIONS
In 2007, Intergovernmental Panel on Climate Change (IPCC) scientists concluded that “Since the 1980s, natural processes of CO2 uptake by the terrestrial biosphere (i.e., the residual land sink)” and “by the oceans” were able to remove only about 50% of “anthropogenic emissions.”
In the robust findings section of the IPCC technical summary, assertions regarding natural processes are further clarified as “CO2 uptake by the oceans and terrestrial biosphere remove about 50 to 60% of anthropogenic emissions.”
IPCC scientists further concluded, “Observations demonstrate that dissolved CO2 concentrations in the surface ocean have been increasing nearly everywhere, roughly following the atmospheric CO2 increase but with large regional and temporal variability [a variability related to time and seasons].”
SOURCE: Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.
The nitrogen cycle has received less attention, but plays an important role in the global biogeochemical cycle. To a large degree, the amount of photosynthesis in the ocean is limited by the availability of nitrogen, which is used to create the enzymes and other proteins required for photosynthesis in phytoplankton. The atmosphere acts as a huge sink for nitrogen gas, which can only be converted to a form that plants can use for photosynthesis in anoxic conditions. Pollution in many of the coastal oceans has created large regions where oxygen levels are extremely low. These waters are proving to be a major sink for nitrogen. The effects of this novel nitrogen sink on the carbon cycle are not well understood.
See Also Biofuel Impacts; Biogeochemical Cycle; Biomass; Carbon Cycle; Carbon Dioxide (CO2); Carbon Sequestration Issues; Carbon Sinks; Forests and Deforestation; Glacier Retreat; Global Warming; Greenhouse Effect; Greenhouse Gases; Hydrologic Cycle; IPCC Climate Change 2007 Report: Mitigation of Climate Change; Melting; Nitrous Oxide; Ocean Circulation and Currents; Offsetting; Sequestration; Social Cost of Carbon (SCC).
Raven, Peter H., and Linda R. Berg. Environment. Hoboken, NJ: John Wiley and Sons, 2006.
“Anoxic Waters: Nitrogen Sink.” Nature, April 10, 2003. <http://www.nature.com/nature/links/030410/030410-9.html> (accessed October 25, 2007).
“Global Carbon Cycle.” National Oceanographic and Atmospheric Administration, September 2007. <http://www.climate.noaa.gov/cpo_pa/gcc/> (accessed October 25, 2007).
“The Ocean and the Carbon Cycle.” National Aeronautics and Space Administration (NASA), March 2, 2007. <http://science.hq.nasa.gov/oceans/system/carbon.html> (accessed October 25, 2007).
sink1 / singk/ • v. (past sank / sangk/ sunk / səngk/ ; past part. sunk / səngk/ ) 1. [intr.] go down below the surface of something, esp. of a liquid; become submerged: he saw the coffin sink below the surface of the waves. ∎ (of a ship) go to the bottom of the sea or some other body of water because of damage or a collision: the trawler sank with the loss of all six crew members. ∎ fig. disappear and not be seen or heard of again: the film sank virtually without trace. ∎ [tr.] cause (a ship) to go to the bottom of the sea or other body of water: a freak wave sank their boat near the shore. ∎ [tr.] fig. cause to fail: she apparently wishes to sink the company. ∎ [tr.] fig. conceal, keep in the background, or ignore: they agreed to sink their differences.2. [intr.] descend; drop: Sam felt the ground sinking beneath his feet | you can relax on the veranda as the sun sinks. ∎ (of a person) lower oneself or drop gently: she sank back onto her pillow. ∎ gradually penetrate the surface of something: her feet sank into the thick pile of the carpet. ∎ (sink in) fig. (of words or facts) be fully understood or realized: Peter read the letter twice before its meaning sank in. ∎ [tr.] (sink something into) cause something sharp to penetrate (a surface): the dog sank its teeth into her arm.3. [intr.] gradually decrease or decline in value, amount, quality, or intensity: their output sank to a third of the prewar figure | the reputation of the mayor sank to a very low level. ∎ lapse or fall into a particular state or condition, typically one that is unwelcome or unpleasant: he sank into a coma after suffering a brain hemorrhage. ∎ be overwhelmed by a darker mood; become depressed: her heart sank as she thought of Craig. ∎ approach death: the doctor concluded that Sanders was sinking fast.4. [tr.] insert beneath a surface by digging or hollowing out: rails attached with screws sunk below the surface of the wood. ∎ excavate (a well) or bore (a shaft) more or less vertically downward: they planned to sink a gold mine in Oklahoma. ∎ pocket (a ball) in billiards. ∎ Golf hit the ball into the hole with (a putt or other shot). ∎ [tr.] insert into something: Kelly stood watching, her hands sunk deep into her pockets. ∎ [intr.] (of eyes) appear unusually deep or receded: her eyes had sunk deep into their sockets.5. [tr.] (sink something into) put money or energy into (something); invest something in: many investors sank their life savings into the company.PHRASES: a (or that) sinking feeling an unpleasant feeling caused by the realization that something unpleasant or undesirable has happened or is about to happen.sink or swim fail or succeed entirely by one's own efforts.DERIVATIVES: sink·a·ble adj.sink·age / ˈsingkij/ n.sink2 • n. a fixed basin with a water supply and a drain. ∎ short for sinkhole. ∎ a pool or marsh in which a river's water disappears by evaporation or percolation. ∎ technical a body or process that acts to absorb or remove energy or a particular component from a system; the opposite of source: a heat sink the oceans can act as a sink for CO2. ∎ fig. a place of vice or corruption: a sink of unnatural vice, pride, and luxury.
Hence sink sb. †pit for the receipt of water, conduit XV; basin, etc. of stone, etc. having an escape pipe for water XVI (also fig.).