Average weather and its variability over a period of time, ranging from months to millions of years. The World Meteorological Organization standard is a 30-year average.

A change in the climate's mean and variability for an extended period of decades, or more.

A mathematical representation of the climate system based on its physical, chemical and biological components, in the form of a computer programme. The computer climate models used at the Met Office Hadley Centre are detailed three-dimensional representations of major components of the climate system. They are run on ICPAC's supercomputer.

Coupled climate models combine representations of various components of the earth system such as atmosphere, ocean, sea ice and land surface. In coupled models each 'coupled' area of the earth system is influenced by other areas, as well as evolving independently.