P
Parameterisation
Some processes are not explicitly included in models because of simplifications, lack of knowledge of the mechanisms, or because the spatial resolution of the model is not high enough to include them. To take the first order effects of these processes into account, they are represented by parameterisations in models. See sections 3.1.1, 3.1.2, 3.2.2 and 3.3.1.
Partial differential equations
Partial differential equations (PDEs) are equations involving an unknown function of several independent variables and its partial derivatives with respect to these variables.
Perihelion
The perihelion is the point in the Earth's orbit that is the closest to the Sun. See sections 2.1.3.1 and 5.4.1. See also aphelion.
Permafrost
The permafrost is a layer of soil or rock beneath the surface that remains below 0oC throughout the year. It occurs when the summer warming is insufficient to reach the bottom of the layer of frozen ground. Permafrost can include ground ice, or simply soil or rock at subzero temperatures (dry permafrost). See sections 1.4.1 and 5.4.2.
pH
The pH is a measure of the acidity of a solution, for instance sea water, generally estimated by the logarithm of its concentration of hydrogen ions (H+):
An acidic solution has a pH lower than 7 (at 25°C), a neutral solution a pH of 7 and an alkaline (basic) solution a pH higher than 7.
Plankton
Plankton consists of organisms (mostly microscopic plants and animals) that drift in the seas or in bodies of fresh water. See phytoplankton and zooplankton.
Photosynthesis
Photosynthesis is the process by which plant, algae and some bacteria produce complex organic compounds from carbon dioxide using energy from light.
Phytoplankton
Phytoplankton is a type of plankton that produces complex organic compounds from simple inorganic molecules. This can be achieved by using energy from light (by photosynthesis) or through inorganic chemical reactions.
Planetesimals
Planetesimals are small bodies (much smaller than a planet) in the solar system.
Plant functional type
Plant functional types (PFTs) are groups of plants that share common characteristics (e.g. tropical trees, deciduous temperate trees, needle leaf boreal trees, different types of grass). See section 3.3.4.
Polynya
A polynya is a region of open water, larger than a lead, inside the ice pack. See sections 1.4.1 and 1.4.2.
Potential temperature
The potential temperature is the temperature that a sample of seawater or air initially at some depth z would take if it were lifted adiabatically (i.e. without heat or mass exchanges with surrounding parcels) to a reference level zr.
Precession
See climatic precession.
Projection
A climate projection is a potential future state of the climate system. The main difference between predictions and projections is the additional uncertainty in projections as they depend on the scenario selected for future changes in external forcings.
Proxy data
Proxy data is indirect information on climate variability collected from various climate sensitive recorders (tree rings, corals, ice and marine cores, lake sediments, historical data, etc). See sections 5.3.2, 5.4.2 and 5.5.2.1.