Climate

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Climate is the average and variations of weather over long periods of time. Climate zones can be defined using parameters such as temperature and rainfall. Paleoclimatology 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.

[edit] Definition

Climate (from ancient word "Clime") 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.

[edit] Classifications

In the original Greek sense, 'Clime,' was a concept used to divide the world into climatic zones sharing similar climatic attributes such as temperature, length of the solar day (sunlight), and in particular latitudinal distance from the equator.^[3] Modern data and analysis show that the latitudinal concepts of antiquity are too simplistic. Instead, the latitudinal trend is altered by such features as hills or mountain ranges, large lakes, and seas or oceans.

Consequently, today's climate regions are classified on the basis of temperature and precipitation alone. Classification schemes usually seek a close correspondence between climate regions and vegetation biomes. The most commonly used classification is the Köppen classification system, which divides Earth's climates into five (sometimes six) primary classes and numerous subclasses.^[4] Another classification system is the Thornthwaite climate classification. In broad terms, climate zones range in latitude from tropical (hot, no real seasons based on temperature) to mid-latitude (moderate; conventional winter-spring-summer-autumn seasons) to polar (cold; strong seasonality; long winters). They also divide into continental (high contrast between summer and winter) and maritime (ocean influence moderates temperature extremes, but generally contribute extra moisture to local weather patterns). The mid-latitude 'conventional four seasons' only work well in the mid-latitudes. In other regions, the more useful divide is into "wet" and "dry" seasons based on the monsoon.

For more details about specific climates, see

• Tropical climate
• Subtropical climate
• Arid climate
• Semiarid climate
• Mediterranean climate
• Temperate climate
• Oceanic climate
• Continental climate
• Alpine climate
• Subarctic climate
• Polar climate
• Climate of Antarctica
• Köppen climate classification (Widely accepted classification of five climate types)

[edit] Climate record

[edit] Modern

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.^[5]

[edit] Paleoclimatology

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.^[6]

[edit] Climate change

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.^[7]

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.^[8]

[edit] Climate models

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.

[edit] National and regional climates

• Climate of the Alps
• Climate of India
• Climate of the United Kingdom
• Climate of Norway