C4 photosynthetic pathway
A biochemical pathway used by certain plants to obtain carbon during photosynthesis.  Such plants possess biochemical and anatomical CO2-concentrating mechanisms that increase the intercellular CO2 concentration at the site of fixation, which greatly reduces carbon losses by photorespiration.  It is thought that the primary selective mechanism for the development of C4 photosynthesis is the low level of CO2 that has prevailed during the last 50 to 60 million years.

C4 plants
Plants that use PEP carboxylase during initial carbon fixation to make a four-carbon compound that is subsequently transferred to specialized cells where carbon dioxide is internally released and refixed using rubisco.  This phenomenon greatly reduces carbon loss by photorespiration, and in many cases, it completely inhibits it.  Less than 1% of earth's plant species can be characterized as C4 plants.

Calcification
The process by which the calcium carbonate skeletons of corals are created.

CAM plants
Plants that close their stomata during the day to reduce water loss and open them at night for carbon uptake.  PEP carboxylase nocturnally fixes carbon into a four-carbon compound that is accumulated within vacuoles.  During the day, this compound internally releases carbon dioxide, which is then refixed using rubisco.  This phenomenon also effectively inhibits carbon loss by photorespiration.  Only about 3 to 4% of earth's plant species can be characterized as CAM plants.

Carbon 14 dating
A method that uses radioactive carbon to date fossils or other biological materials.  It is based on the observation that the ratio of ¹⁴C to ¹²C in a biological sample is reflective of the ratio of ¹⁴C to ¹²C in the air at the time that the carbon was photosynthetically "fixed" by the plant that assimilated it out of the air.  Because carbon loses half of its radioactivity in 5,600 years, this decay rate can be used with the present-day ratio and a ratio from a sample of unknown age to date the unknown sample.

Carnivores
Organisms that eat animals as opposed to plants.

Carotenoid
One of the primary photosynthetic pigments involved in absorbing sunlight.  Carotenoids can absorb more energetic wavelengths of light than those absorbed by chlorophylls.

Catalase
An antioxidizing enzyme that detoxifies hydrogen peroxide.

Catalyst
A substance that either speeds up or slows down a chemical reaction but which does not experience a permanent chemical change in the process.

Cell cycle
The time that it takes a newly formed cell to grow and divide, producing thereby a second cell.

Chalk grassland swards
Extremely biodiverse assemblages of coexisting grasses and forbs that are traditionally managed by continuous summer livestock grazing.

Chemical weathering
The wearing away or breaking down of various minerals in rocks and soils due to the chemical actions of natural physical and biological processes.  This phenomenon frees up elements that are needed by plants and utilized by animals as well.

Chlorofluorocarbon
Any of a group of compounds composed of chlorine, fluorine and carbon atoms.  They figure prominently in the refrigeration industry and have been implicated in the breakdown of stratospheric ozone.

Chlorophyll
One of the primary photosynthetic pigments involved in absorbing sunlight.  It plays a key role in the light-dependent reactions of photosynthesis, which convert sunlight into usable chemical energy.

Cholera
An infectious intestinal disease, often fatal, that is characterized by violent diarrhea and vomiting accompanied by severe muscle cramps.

Climate drift
Movement away from reality in terms of temperature, precipitation, receipt of solar radiation, or some other climatic variable, as a model "run" progresses.

Climate models
Also referred to as General Circulation Models, or GCMs, they are mathematical representations of atmospheric and oceanic properties and processes that attempt to describe earth's climate system.

Climatic Optimum
Also referred to as the Holocene Maximum, the time period between 4,000 and 7,000 years ago when global temperatures reached as high as 2.0°C warmer than present.

Clones
Vegetatively propagated plants that do not differ in their genetic composition.

Clutch
A nest of eggs or brood of young birds.

Cloud condensation nuclei
Small particles in the atmosphere that provide nuclei about which water vapor may condense to form clouds.

CO2 Sequestration
The process of removing carbon dioxide from the atmosphere and making it unavailable for release back to the air.

Coniferous
Trees that possess needle-like or scale-like leaves that are not dropped during cooler winter temperatures.  Coniferous species are also known as "evergreens."

Cooling Degree Day (CDD)
A parameter based on mean daily air temperature that is used to estimate the energy needed to cool indoor air to a comfortable level.  CDDs are calculated by subtracting a threshold value (24°C, for example) from the mean daily air temperature.  On days when the mean temperature is below the threshold value, there are no CDDs and cooling is not needed.

Coppice-with-standards
A silviculture system in which trees are of two different ages: the coppice, which grows from past stumps and which is clear-cut every 25-30 years, and the standards, which, starting from seeds, grow initially within the coppice, then above the coppice, as they mature, and which are cut about every 150 years.

Coral bleaching
A whitening of corals indicative of colony stress, where symbiotic algal cells, or zooxanthellae, either leave or are ejected from the colonies.

Cortical cell
Cells found commonly in plant stems and roots.  In stems, cortical cells can be photosynthetic and often store starch.  In roots, cortical cells are typically used for storage and may also contain starch.

Crustaceans
Invertebrates characterized by a hard outer shell and jointed appendages and bodies.  They usually live in water and breathe through gills.  Higher forms of this class include lobsters, shrimp and crawfish; lower forms include barnacles and fish lice.

Cryosphere
The sum total of earth's fresh water supply that is locked up in frozen forms including polar ice, mountain glaciers, permafrost and snow.

Cuticle
A waxy hydrophobic material that is produced on the outer surface of most leaves, which functions to reduce internal water loss.

Cyanoglycoside
A defensive compound produced in the leaves of certain plants, which acts to deter against herbivory.  This secondary carbon compound usually requires large amounts of nitrogen in its synthesis.

Cyclone
An area of low-pressure often associated with stormy weather.