Climate change is a change in the statistical distribution of weather over periods of time that range from
decades to millions of years. It can be a change in the average weather or a change in the distribution of
weather events around an average (for example, greater or fewer extreme weather events). Climate
change may be limited to a specific region, or may occur across the whole Earth. It can be caused by
recurring, often cyclical climate patterns such as El Niño-Southern Oscillation, or come in the form of
more singular events such as the Dust Bowl.
In recent usage, especially in the context of environmental policy, climate change usually refers to
changes in modern climate. It may be qualified as anthropogenic climate change, more generally known
as "global warming" or "anthropogenic global warming" (AGW).
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall,
atmospheric particle count and numerous other meteorological elements in a given region over long
periods of time. Climate can be contrasted to weather, which is the present condition of these same
elements over periods up to two weeks.
The climate of a location is affected by its latitude, terrain, altitude, ice or snow cover, as well as nearby
water bodies and their currents. Climates can be classified according to the average and typical ranges of
different variables, most commonly temperature and precipitation. The most commonly used
classification scheme is the one originally developed by Wladimir Köppen. The Thornthwaite system,
in use since 1948, incorporates evapotranspiration in addition to temperature and precipitation
information and is used in studying animal species diversity and potential impacts of climate changes.
The Bergeron and Spatial Synoptic Classification systems focus on the origin of air masses defining the
climate for certain areas.
Paleoclimatology is the study and description of ancient climates. Since direct observations of climate are
not available before the 19th century, paleoclimates are inferred from proxy variables that include non-
biotic evidence such as sediments found in lake beds and ice cores, and biotic evidence such as tree rings
and coral. Climate models are mathematical models of past, present and future climates.
The Earth's climate has changed many times during the planet's history, with events ranging from ice ages
to long periods of warmth. Historically, natural factors such as volcanic eruptions, changes in the Earth's
orbit, and the amount of energy released from the Sun have affected the Earth's climate.
Beginning late in
the 18th century, human activities associated with the Industrial Revolution have also changed the
composition of the atmosphere and therefore very likely are influencing the Earth's climate.