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Climate is the average and variations of weather in a region over long periods of time. Climate zones can be defined using parameters such as temperature and rainfall. P focuses on ancient climate information derived from sediment found in lake beds, ice cores, as well as various fauna and flora including tree rings and coral. Climate models can be used to determine the amount of climate change anticipated in the future.

Annual average surface temperatures from 1961-1990. This is an example of how climate varies with location.

Climate, (from Ancient Greek klima) is commonly defined as the weather averaged over a long period of time. The standard averaging period is 30 years but other periods may be used depending on the purpose. Climate also includes statistics other than the average, such as the magnitudes of day-to-day or year-to-year variations. The Intergovernmental Panel on Climate Change (IPCC) glossary definition is:

Climate in a narrow sense is usually defined as the “average weather”, or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.^[1]

The main difference between climate and everyday weather is best summarized by the popular phrase "Climate is what you expect, weather is what you get."^[2] Over historic time spans there are a number of static variables that determine climate, including: latitude, altitude, proportion of land to water, and proximity to oceans and mountains. Other climate determinants are more dynamic: The thermohaline circulation of the ocean distributes heat energy between the equatorial and polar regions; other ocean currents do the same between land and water on a more regional scale. Degree of vegetation coverage affects solar heat absorption, water retention, and rainfall on a regional level. Alterations in the quantity of atmospheric greenhouse gases determines the amount of solar energy retained by the planet, leading to global warming or global cooling. The variables which determine climate are numerous and the interactions complex, but there is general agreement that the broad outlines are understood, at least insofar as the determinants of historical climate change are concerned.

Instrumental temperature record of the last 150 years

Details of the modern climate record are known through the taking of measurements from such weather instruments as thermometers, barometers, and anemometers during the past few centuries. The instruments used to study weather conditions over the modern time scale, their known error, their immediate environment, and their exposure have changed over the years, which must be considered when studying the climate of centuries past.^[3]

Main article: Paleoclimatology

Paleoclimatology is the study of climate change taken on a longer scale, spanning a greater section of the Earth's history. It uses records from ice sheets, tree rings, sediment, coral, and rocks to determine the past state of the climate system on Earth, and whether or not long term climate cycles exist.^[4]

See also: Climate change, Global warming, temperature record, and attribution of recent climate change

Climate change refers to the variation in the Earth's global climate or in regional climates over time. It describes changes in the variability or average state of the atmosphere over time scales ranging from decades to millions of years. These changes can be caused by processes internal to the Earth, external forces (e.g. variations in sunlight intensity) or, more recently, human activities.

In recent usage, especially in the context of environmental policy, the term "climate change" often refers only to changes in modern climate, including the rise in average surface temperature known as global warming. In some cases, the term is also used with a presumption of human causation, as in the United Nations Framework Convention on Climate Change (UNFCCC). The UNFCCC uses "climate variability" for non-human caused variations.^[5]

Earth has undergone periodic climate shifts in the past, including four major ice ages. These consisting of glacial periods where conditions are colder than normal, separated by interglacial periods. The accumulation of snow and ice during a glacial period increases the surface albedo, reflecting more of the Sun's energy into space and maintaining a lower atmospheric temperature. Increases in greenhouse gases, such as by volcanic activity, can increase the global temperature and produce an interglacial. Suggested causes of ice age periods include the positions of the continents, variations in the Earth's orbit, changes in the solar output, and vulcanism.^[6]

See also: Climate models and Climatology

Climate models use quantitative methods to simulate the interactions of the atmosphere, oceans, land surface, and ice. They are used for a variety of purposes from study of the dynamics of the weather and climate system to projections of future climate. All climate models balance, or very nearly balance, incoming energy as short wave (including visible) electromagnetic radiation to the earth with outgoing energy as long wave (infrared) electromagnetic radiation from the earth. Any unbalance results in a change in the average temperature of the earth.

The most talked-about models of recent years have been those relating temperature to emissions of carbon dioxide (see greenhouse gas). These models predict an upward trend in the surface temperature record, as well as a more rapid increase in temperature at higher altitudes.

Models can range from relatively simple to quite complex:

• A simple radiant heat transfer model that treats the earth as a single point and averages outgoing energy
• this can be expanded vertically (radiative-convective models), or horizontally
• finally, (coupled) atmosphere–ocean–sea ice global climate models discretise and solve the full equations for mass and energy transfer and radiant exchange.

Weather Portal

• Air conditioning
• Climate change
• Climateprediction.net - a distributed computing project (using, amongst others, BOINC) to try and produce a forecast of the climate in the 21st century Website
• Biome - an ecological term for a major regional group of distinctive plant and animal communities best adapted to the region's physical environment
• Climatology
• Effect of sun angle on climate
• Electronic Climate Control
• Solar variation
• Temperature extreme

The external links in this article may not comply with Wikipedia's content policies. Please improve this article by removing excessive or inappropriate external links.

• IFAS AgClimate
• Climate Prediction Project
• WorldClimate
• ESPERE Climate Encyclopaedia
• Weatherbase
• Global Climate Data
• Climate index and mode information
• Why and how do scientists study climate change in the Arctic? What are the Arctic climate indices?
• A near-realtime Arctic Change Indicator Website
• A current view of the Bering Sea Ecosystem and Climate
• Climate: Data and charts for world and US locations
• World climates list and articles
• UNEP World Conservation Monitoring Centre - Climate Programme