Climate—The mean state of the atmosphere over a long period of
time. This period is generally at least 30 years, to produce a
statistically significant sample and to avoid shorter term
variability. The principal elements of climate are the same as
the principal elements of weather (IPCC 2007a). Climate
change—Alteration in the mean state of the atmosphere over a
long period of time that can be detected statistically. Climate
change consists of trends in the mean and the variability of
climate that persist for an extended period, typically decades
or longer. Climate change may be due to natural internal
processes, natural external forcings, or persistent
anthropogenic changes in the composition of the atmosphere or in
land use (IPCC 2007a). Climate variability—Variations in the
mean state of the atmosphere on all spatial and temporal scales
beyond the scales of individual weather events. Climate
variability may be due to natural internal processes within the
climate system (internal variability) or to variatio