for: Global warming

Climate change is any long-term significant change in the expected patterns of average weather of a specific region (or, more relevantly to contemporary socio-political concerns, of the Earth as a whole) over an appropriately significant period of time. Climate change reflects abnormal variations to the expected climate within the Earth's atmosphere and subsequent effects on other parts of the Earth, such as in the ice caps over durations ranging from decades to millions of years.

In recent usage, especially in the context of environmental policy, climate change usually refers to changes in modern climate (see global warming). For information on temperature measurements over various periods, and the data sources available, see temperature record. For attribution of climate change over the past century, see attribution of recent climate change.

Climate change factors

Climate change is the result of a great many factors including the dynamic processes of the Earth itself, external forces including variations in sunlight intensity, and more recently by human activities. External factors that can shape climate are often called climate forcings and include such processes as variations in solar radiation, deviations in the Earth's orbit, and the level of greenhouse gas concentrations. There are a variety of climate change feedbacks that will either amplify or diminish the initial forcing.

Most forms of internal variability in the climate system can be recognized as a form of hysteresis, where the current state of climate does not immediately reflect the inputs. Because the Earth's climate system is so large, it moves slowly and has time-lags in its reaction to inputs. For example, a year of dry conditions may do no more than to cause lakes to shrink slightly or plains to dry marginally. In the following year however, these conditions may result in less rainfall, possibly leading to a drier year the next. When a critical point is reached after "x" number of years, the entire system may be altered inexorably. In this case, resulting in no rainfall at all. It is this hysteresis that has been mooted to be the possible progenitor of rapid and irreversible climate change.

Plate tectonics

On the longest time scales, plate tectonics will reposition continents, shape oceans, build and tear down mountains and generally serve to define the stage upon which climate exists. During the Carboniferous period, plate tectonics may have triggered the large-scale storage of Carbon and increased glaciation. More recently, plate motions have been implicated in the intensification of the present ice age when, approximately 3 million years ago, the North and South American plates collided to form the Isthmus of Panama and shut off direct mixing between the Atlantic and Pacific Oceans.

Solar output

main: Solar variation thumb|Variations in solar activity during the last several centuries based on observations of sunspots and beryllium isotopes.