Climate

Climate is the average and variations of weather in a region over long periods of time. The climate of a location is affected by the presence of mountains, ice caps, as well nearby oceans and their associated currents. Locations on the western side of oceans exposed to warm water currents are relatively warm and humid, while deserts tend to exist on the east side of oceans where cold water currents contribute less warmth and moisture, leading to a more stable atmospheric profile. Mountain chains can cut off moisture streaming off an ocean or bay from reaching more inland locations. Climate zones can be defined using parameters such as temperature and rainfall to define desert, steppe, rain forest and polar ice cap regimes.

Paleoclimatology is the study and description of ancient climates using information from both non-biotic factors such as sediments found in lake beds and ice cores, and biotic factors such as tree rings and coral, and can be used to extend back the temperature or rainfall information for particular locations to a time before various weather instruments were used to monitor weather conditions. Climate models are mathematical models of past, present and future climates and can therefore be used to describe the likely patterns of future changes.

Definition

Climate (from Ancient Greek klima) is commonly defined as the weather averaged over a long period of time.^[1] The standard averaging period is 30 years,^[2] 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:

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."^[4] 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 leads to a 5 °C (9 °F) warming of the northern Atlantic ocean compared to other ocean basins.^[5] Other ocean currents redistribute heat between land and water on a more regional scale. Degree of vegetation coverage affects solar heat absorption,^[6] 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.^[7]

Zones

There are a few different ways climate zones have been defined. Originally, climes were defined within Ancient Greece to describe the weather either within the Northern Hemisphere, or globally, depending upon a location's latitude. More recently, climate zones can be defined by plant hardiness,^[8] or more generally under the Köppen climate classification. There is usually a gradual transition from zone to zone.

Köppen classification

Tropical rain forest

Rain forests are characterized by high rainfall, with definitions setting minimum normal annual rainfall between 1,750 millimetres (69 in) and 2,000 millimetres (79 in). Mean monthly temperatures exceed 18 °C (64 °F) during all months of the year.^[9] Rainforests are home to half of all the living animal and plant species on the planet.^[10] Tropical rain forests are called the "world's largest pharmacy" because over one-quarter of modern medicines originate from its plants.^[11] The undergrowth in a rainforest is restricted in many areas by the lack of sunlight at ground level.^[12] This makes it possible for people and other animals to walk through the forest. If the leaf canopy is destroyed or thinned for any reason, the ground beneath is soon colonized by a dense tangled growth of vines, shrubs and small trees called a jungle.^[13]

Tropical monsoon

A monsoon is a seasonal prevailing wind which lasts for several months, ushering in a region's rainy season. The term was first used in English in India, Bangladesh, Pakistan and neighboring countries to refer to the big seasonal winds blowing from the Indian Ocean and Arabian Sea in the southwest bringing heavy rainfall to the region.^[14] In hydrology, monsoon rainfall is considered to be that which occurs in any region that receives the majority of its rain during a particular season. This allows other regions of world such as within North America, South America. Sub-Saharan Africa, Australia and East Asia to qualify as monsoon regimes.^[15] In terms of total precipitation and total area covered, the monsoons affecting the Indian subcontinent dwarf the North American monsoon. The South Asian monsoon affects larger number of people due to the high density of population within that part of the world.

Tropical savanna

Rainfall in tropical and subtropical grasslands, savannas, and shrublands is between 750 millimetres (30 in) and 1,270 millimetres (50 in) a year, and can be highly seasonal, with the entire year's rainfall sometimes occurring within a couple of weeks. Much of the plant life on savannas is adapted to seasonal aridity. Adaptations to the dry climate include herbaceous annuals, which die in the dry season and regrow from seed each year, and perennial plants which grow long tap roots to reach groundwater, or bulbs to store water.^[17]

Humid subtropical

The Humid subtropical climate zone is characterized by hot, humid summers and chilly to mild winters. This climate type covers a broad category of climates, and the term "subtropical" may be a misnomer for the winter climate in the cooler areas within this category. Significant amounts of precipitation occur in all seasons in most areas. Winter rainfall (and sometimes snowfall) is associated with large storms that the westerlies steer from west to east. Most summer rainfall occurs during thunderstorms and from occasional tropical cyclones.^[18]

Humid subtropical climates lie on the east side continents, roughly between latitudes 20° and 40° degress away from the equator.^[19] This climate zone reaches up to latitude 46° North, are the Po Valley and the Toulouse regions in Europe. The Köppen definition of this climate is for the coldest month to be above a −3 °C (26.6 °F) mean temperature (Köppen: C), and the warmest month to be above 22 °C (72 °F) (Köppen: a); and to be constantly moist with the driest month receiving more than 60 millimetres (2.4 in) rain (Köppen: f).

Humid continental

Humid continental climate is found over large areas of land masses in the temperate regions of the mid-latitudes where there is a zone of conflict between polar and tropical air masses. The humid continental climate is marked by variable weather patterns and a large seasonal temperature variance. The seasonal temperature variance can be as great as 33°C (59°F), but is typically 15-22°C (27-40°F). The temperature difference between the warmest and coldest months increases as one moves further inland and away from the moderating influence of the ocean. Places with a hottest monthly temperature above 10 °C (50 °F) and a coldest month temperature below −3 °C (26.6 °F) and which do not meet the criteria for an arid climate, are classified as continental.^[20]

Oceanic climate

An oceanic climate (also called marine west coast climate and maritime climate) is typically found along the west coasts at the middle latitudes of all the world's continents, and in southeastern Australia. Oceanic climates are characterized by a narrower annual range of temperatures than are encountered in other places at a comparable latitude, and do not have the extremely dry summers of Mediterranean climates. Precipitation is plentiful throughout the year in this climate regime.^[21]

Similar climates, at least in thermal range, are also found in tropical highlands even at considerable distance from any coastline. Generally, they fall into Köppen climate classification Cfb or Cwb. The narrow range of temperatures results not from proximity to a coastline but instead to the slight thermal range of temperatures between seasons characteristic of tropical lowlands; altitudes are high enough that such places have at least one month cooler than 14 °C (57 °F) and do not qualify for grouping in the true tropical climates. Unlike the norm in true oceanic climates, these moist highland tropical climates may have a marked winter drought, as in Mexico City. As with oceanic climates, winters are relatively warm and summers are comparatively cool, so the agricultural potential in both oceanic climates and moist tropical highland climates is practically identical.

Mediterranean climate

The Mediterranean climate climate regime resembles the climate of the lands in the Mediterranean Basin, which includes over half of the area with this climate type worldwide. Elsewhere, this climate type prevails in parts of western North America, in parts of Western and South Australia, in southwestern South Africa and in parts of central Chile. The climate is characterized by hot, dry summers and cool, wet winters.^[22] For example, the city of Perth, Australia, in the southern hemisphere winter months of June-August, experiences 450 millimetres (18 in) of rainfall and an average daily minimum of 8 °C (46 °F). Meanwhile, during the summer months of December to February, the city only averages 32 millimetres (1.3 in).^[23]

Continental steppe

A steppe is a dry forest, but not dry enough to be a desert. The term steppe originally comes from the Russian word /stɛp/ which means a flat and arid land. The climates of all steppes are summarized by a continental climate. Peaks can be recorded in the summer of up to 40 °C (104 °F) and in winter down to −40 °C (−40.0 °F).^[24] Besides this huge temperature variation, the diurnal temperature differences are also very great. In the highlands of Mongolia, 30 °C (86 °F) can be reached during the day with temperatures below 0 °C (32 °F) at night. The mid-latitude steppes average 250 millimetres (9.8 in) to 500 millimetres (20 in) of precipitation per year. In tropical locations, necessary rainfall to separate steppes from true deserts may be double this criteria due to greater evapotranspiration.

Subarctic

A subarctic climate has monthly temperatures which are above 10 °C (50 °F) for one to three months of the year, and experiences some of the largest annual temperature ranges on the planet.^[25] Except for those areas adjacent to warm ocean currents, there is usually continuous permafrost due to the very cold winters. This means that building in most subarctic regions is very difficult and expensive: cities are very few (Murmansk being the largest) and generally small, whilst roads are few and railways non-existent. An important consequence is that transportation tends to be restricted to "bush" planes, helicopters and, in summer, river boats.

Precipitation tends to be low due to the low moisture content of the cold air, with no more than 15 inches (380 mm) to 20 inches (510 mm) of precipitation per year, on average.^[25] Typically, there is a summer maximum in precipitation ranging from moderate in North America to extreme in the Russian Far East.^[26] In drier areas, glaciers are largely absent because of the lack of winter precipitation. In the wettest areas, glaciers tend to be abundant and Pleistocene glaciation covers even the lowest elevations. Soils of the subarctic are generally very acidic largely because of the influence of the vegetation both in the taiga and in peaty bogs, which tends to acidify the soil, as well as the extreme ease with which leaching of nutrients takes place even in the most heavily glaciated regions. The dominant orders are Spodosols and further north Gelisols. Agriculture is mainly limited to animal husbandry, though in some areas barley can be grown.

Tundra

Arctic tundra occurs in the far Northern Hemisphere, north of the taiga belt. It is one of the world's youngest biomes, forming 10,000 years ago.^[27] The word "tundra" usually refers only to the areas where the subsoil is permafrost, or permanently frozen soil. Permafrost tundra includes vast areas of northern Russia and Canada ^[28]. The polar tundra is home to several peoples who are mostly nomadic reindeer herders, such as the Nganasan and Nenets in the permafrost area (and the Sami in Sápmi). The soil there is frozen from 25 centimetres (9.8 in) to 90 centimetres (35 in) down, and it is impossible for trees to grow. Bare and rocky land can only support low growing plants such as moss, heath, and lichen.

There are two main seasons, winter and summer. During the winter it is very cold and dark, with the average temperature around −28 °C (−18.4 °F), sometimes dipping as low as −50 °C (−58.0 °F). However, extreme cold temperatures on the tundra do not drop as low as those experienced in taiga areas further south. During the summer, temperatures rise and the top layer of the permafrost melts. The permafrost thaws enough to let plants grow and reproduce, but because the ground below this is frozen, the water cannot sink any lower, and so the water forms the lakes and marshes found during the summer months. Although precipitation is light, evaporation is also relatively minimal. Generally daytime temperatures during the summer rise to about 12 °C (54 °F) but often drop to 3 °C (37 °F) or lower. The tundra is a very windy area, with winds often blowing upwards at 48 kilometres per hour (30 mph) to 97 kilometres per hour (60 mph).^[27] In terms of precipitation, the tundra is desert-like,^[29] with only about 15–25 cm (6–10 inches) falling per year, mainly in the summer.

Polar ice cap

A polar ice cap, or polar ice sheet, is a high-latitude region of a planet or moon that is covered in ice. There are no requirements with respect to size or composition for a body of ice to be termed a polar ice cap, nor any geological requirement for it to be over land; only that it must be a body of solid phase matter in the polar region. Earth's polar ice caps are mainly water ice, and the coldest locations on the planet. There is so little moisture in the air that it is hypothesized that the polar ice cap is as dry as most deserts. Polar ice caps form because high-latitude regions receive less energy in the form of solar radiation from the sun than equatorial regions, resulting in lower surface temperatures.^[30] Seasonal variations of the ice caps will take place due to varied solar energy absorption as the planet or moon revolves around the sun. Additionally, at the geologic time scale, the ice caps may grow or shrink due to climate variation.

Desert

A desert is a landscape form or region that receives very little precipitation. Just over 14% of the landmass of the Earth fits within this climate classification.^[16] Deserts can be defined as areas that receive an average annual precipitation of less than 250 millimetres (9.8 in),^[31]^[32] or as areas in which more water is lost than falls as precipitation.^[33]

Deserts usually have a large diurnal and seasonal temperature range, with high daytime temperatures (in summer up to 45 °C or 113 °F), and low night-time temperatures (in winter down to 0 °C; 32 °F) due to extremely low humidity. As soon as the sun sets, the desert cools quickly by radiating its heat into space. Urban areas in deserts lack large (more than 25 °F/14 °C) daily temperature ranges, partially due to the urban heat island effect. Many deserts are formed by rain shadows, as mountains block the path of moisture and precipitation to the desert.^[34] Deserts are often composed of sand and rocky surfaces. Sand dunes called ergs and stony surfaces called hamada surfaces compose a minority of desert surfaces. Exposures of rocky terrain are typical, and reflect minimal soil development and sparseness of vegetation.

Record

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.^[35]

Paleoclimatology

Paleoclimatology is the study of past climate over a great period of the Earth's history. It uses evidence from ice sheets, tree rings, sediments, coral, and rocks to determine the past state of the climate. It demonstrates periods of stability and periods of change and can indicate whether changes follow patterns such as regular cycles.^[36]

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.^[37]

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.^[38]

Climate models

Climate models use quantitative methods to simulate the interactions of the atmosphere,^[39] 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 imbalance 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 the build-up of greenhouse gases in the atmosphere, primarily 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.

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