The climate metrics and their origin are reviewed. The emphasis has been to clearly connect the climate metrics to the fundamental physical chemistry principles and parameters. Without greenhouse gases (GHGs) in the atmosphere, the IR radiation emitted by the surface would escape to space and cool the planet. At equilibrium, the incoming energy would be equal to the outgoing energy. In addition to greenhouse gases, condensed matter aerosols, and clouds, also influence the radiation balance. Aerosols are condensed matter suspended in air; however, cloud droplets or ice particles are not considered aerosols. Changes in the abundance of greenhouse gases and aerosols create a forcing on climate, called radiative forcing which is balanced by a change in the Earth's surface temperature. Atmospheric lifetime is the key parameter for determining how much of an emission accumulates in the atmosphere and the rate of decay of the radiative forcing from a pulsed injection of a greenhouse gas. Lifetimes of short-lived chemicals depend on where and when they are emitted, and their forcing will be spatially inhomogeneous.
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