Global climate

The global climate system is complex and dynamic, greatly complicating attempts to evaluate or predict long-term alterations to Earth’s climate. However, physical measurements of specific parameters can be made, rendering climate change quantifiable.

Background

Climate is a general characterization of long-term weather and environment conditions for a specific location. Several major factors influence climate in a given region, including latitudinal position, the distribution of land and water, and elevation. Ocean currents, prevailing winds, and the positions of high- and low-pressure areas also have significant climatic effects.

Heterogeneous distributions of heat and water result in rich and varied climates. In particular, the tropical regions receive more energy from solar radiation than they emit in the form of infrared heat. The polar regions, by contrast, receive less energy from the Sun than they emit as heat. As a result, the tropics are regions of heat surplus, while polar regions are deficient in heat. Moreover, because tropical regions include large expanses of ocean, there is more water stored in the tropical atmosphere than is stored in the atmosphere at high latitudes.

The imbalance in the heat and water budget in the tropical and polar regions often leads to circulations that transport heat and water from and to these regions. These transports are typically carried out by both atmosphere and oceans. The weakening or strengthening of heat and water transports is an important signal for climate change.

Several methods have been developed to classify global climate. The most widely used method is based on the Köppen classification system. Designed by the German climatologist Wladimir Köppen (1846-1940), this method uses the average annual and monthly temperature and precipitation to describe a global climate for various climate zones. In this method, global climate is divided into the following six major groups, and each group is divided into subgroups.

Tropical Moist Climate (Group A)

Tropical moist climate is typical of most of Earth’s tropical regions (from the equator to about 20° latitude into each hemisphere). The climate of these regions is characterized by year-round warm temperatures and abundant rainfall. In the tropics, the annual mean temperature is typically above 18° Celsius, and typical annual average rainfall exceeds 150 centimeters. Tropical moist climate is divided based on rainfall characteristics into three subtypes of climate: tropical wet, or tropical rain-forest climate (Af); tropical monsoon climate (Am); and tropical wet-and-dry climate (Aw).

The tropical rain-forest climate exhibits constant high temperatures and abundant year-round rainfall. As a result, it is marked by dense vegetation, typically composed of broadleaf trees, jungles, and evergreen forests. A large number of diversified plants, insects, birds, and animals inhabit the tropical rain forests. Many lowlands near the equator are in this type of climate, which includes the Amazon River Basin of South America, the Congo River Basin of Africa, and the East Indies, from Sumatra to New Guinea.

Unlike the tropical wet climate, the tropical wet-and-dry climate has distinctive wet and dry seasons. Although the annual precipitation usually exceeds 100 centimeters, during the dry season the average monthly rainfall can be less than 6 centimeters. The dry season lasts more than two months. The tropical wet-and-dry climate dominates most of tropical Africa, tropical South America, and South Asia. The variations of dry and wet seasons in these regions are closely associated with the migration of the Inter-Tropical Convergence Zone (ITCZ) in the tropics.

The tropical monsoon climate exists between the tropical rain-forest and wet-dry climates: It has abundant rainfall, in excess of 150 centimeters per year, but the rains do stop briefly, typically for one or two months. Tropical monsoon climate can be seen along the coasts of Southeast Asia and India and in northeastern South America. In contrast to the wet-dry climate, the rain and the pause of rain in these areas are related to monsoonal circulation.

Dry Climate (Group B)

Just outside the tropics, most of the continental land located between approximately 20° and 30° latitude in both the Northern and the Southern Hemispheres is in arid or semiarid climates. Precipitation in these areas is scarce most of the year, and evaporation and exceed precipitation.

The (BW) is the true desert climate and can be found in the Sahara Desert in Africa, a large portion of the Middle East, much of the interior of Australia, Central Asia, and the west coasts of South America and Africa. These areas are located in the subtropical high belt, which is caused by descending air from the Hadley circulation.

Around the margins of the arid regions, semiarid (BS) areas enjoy a slightly greater rainfall. The light rains of semiarid climes support the growth of short bunch grass, scattered low bushes, trees, and sagebrush. This climate can be found in the western United States, southern Africa, and the Sahel.

The Moist Subtropical Midlatitude Climate (Group C)

Most subtropical midlatitude regions are farther poleward from the major dry-climate latitudes. These areas extend approximately from 25° to 40° latitudes in both the Northern and the Southern Hemispheres. This climate has distinct summer and winter seasons. Winter is mild, with average temperatures for the coldest month of between -3° and 18° Celsius. The regions in this climate belt are typically humid and have ample precipitation.

There are three major subtypes in the group C climate: humid subtropical (Cfa); west coast marine (Cfb); and dry-summer subtropical, or Mediterranean (Cs). The humid subtropical climate typically presents hot and muggy summers, but mild winters. Summers experience heavy rains, while winters are slightly drier. This climate type can be found principally along the east coasts of continents, such as the southeastern United States, eastern China, southeastern South America, and the southeastern coasts of Africa and Australia.

The has cool summers and mild winters and produces more precipitation in winter than in summer. The largest area with this climate is Europe. Finally, the dry-summer or Mediterranean climate is distinctively characterized by extreme summer aridity and heavy rains in winter. Countries surrounding the Mediterranean Sea and the US West Coast, including Northern California and Oregon, are in this type of climate.

Moist Continental Climate (Group D)

The moist continental climate is located farther north of the moist subtropical midlatitude climate zone, from 40° to 50° north latitudes. This climate mostly occurs in North America and Eurasia. The general characteristics of the moist continental climate are warm-to-cool summers and cold winters. The average temperature of the warmest month exceeds 10° Celsius, and the coldest month’s average temperature generally drops below -3° Celsius. Winters are severe, with snowstorms, blustery winds, and bitter cold. The climate is controlled by a large landmass.

The group D climate is further divided by summer temperature into three major subtypes: humid continental with hot summers (Dfa), humid continental with cool summers (Dfb), and subpolar (Dfc). Both winter and summer temperatures in the Dfa climate are relatively severe. That is, winter is cold and summer is hot. Farther north is the Dfb climate, which experiences long, cool summers and long, cold, windy winters. The subpolar climate presents severely cold winters and short summers. In the subpolar region, moisture supply is limited. Therefore, precipitation is low.

Polar Climate (Group E)

The polar climate exists over the northern coastal areas of North America and Eurasia, Greenland, the Arctic, and Antarctica. It is characterized by low temperatures year-round. Even during the warmest month, the temperature is below 10° Celsius. Precipitation is scarce in these parts of the Earth.

The polar climate can be divided into two subtypes: the polar tundra (ET) climate and the polar ice-cap (EF) climate. The tundra climate occupies the coastal fringes of the Arctic Ocean, many Arctic islands, and the ice-free shores of northern Iceland and southern Greenland. In these regions, the ground is permanently frozen to depths of hundreds of meters, a condition known as permafrost. In summer, the temperature can remain above freezing, allowing tundra vegetation to grow. The monthly mean temperature under the ice-cap climate is mostly below 0° Celsius. The occupies the interior ice sheets of Greenland and Antarctica. The growth of plants is prohibited, and the landscape is perpetually covered with snow and ice. Many studies show that these regions are most sensitive to global warming and have experienced rapid snow and ice melting in recent decades.

Highland Climates (Group H)

The distribution of global mountain ranges and plateaus creates another type of climate. Climate in highland regions is unique. Highland climates are characterized by a great diversity of conditions. Because air temperature decreases with altitude, climatic changes corresponding to those from group B to group E will be experienced when ascending mountain slopes. In general, every 300 meters of mountain elevation will correspond to a change of climate type.

In addition to the drop of temperature with increased altitude, orography modifies precipitation and wind patterns in many ways. For example, a mountain’s windward slopes typically receive more precipitation than its leeward slopes. Therefore, more dense vegetation grows on the windward slopes of large mountains, such as the western slope of the Rockies, than on the leeward slopes, such as the eastern slope of the Rockies. Often, the leeward foot of a mountain receives very little precipitation. These areas are often called “rain shadows.” Leeward mountain foots are also subject to downslope mountain winds from time to time, especially during winters. These winds are called “chinook wind” in North America, or “foehn wind” in Europe.

The most prominent highland climate occurs over the Tibetan Plateau, where the average elevation is over 4,000 meters. In North America, highland climates characterize the Rockies, Sierra Nevada, and Cascades. In South America, the Andes create a continuous band of highland climate. Many of these mountains and highlands play central roles in monsoonal circulation, an important global climate system in various parts of the world.

Context

Global climate is a complex system that involves interactions among Earth’s atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere. In an even larger context, the global climate is just a part of the Sun-Earth system. For a particular place on Earth, the formation of the local climate pattern is dependent upon a set of climate controls. Despite its complexity, the global climate can be classified according to two basic physical parameters: mean temperature and precipitation. Future climate change can be measured and quantified by closely monitoring the change of these parameters in various parts of the world.

Key Concepts

• chinook/foehn wind: a warm, dry wind on the eastern side of the Rocky Mountains or the Alps
• climate controls: the relatively permanent factors that govern the general nature of the climate of a region
• evaporation: the process by which a liquid changes into a gas
• Hadley circulation: an atmospheric circulation pattern in which a warm, moist air ascends near the equator, flows poleward, descends as dry air in subtropical regions, and returns toward the equator
• Inter-Tropical Convergence Zone (ITCZ): a low-pressure belt, located near the equator, where deep convection and heavy rains occur
• Köppen climate classification system: a system for classifying climate based mainly on average temperature and precipitation
• monsoon: a seasonal climate system characterized by wind and precipitation patterns
• precipitation: liquid or solid water particles that fall from the atmosphere to the ground
• rain shadow: the region on the lee side of a mountain where precipitation is noticeably less than on the windward side
• subtropical high belt: a high-pressure belt where warm, dry air sinks closer to the surface
• transpiration: the process by which water in plants is transferred as water vapor into the atmosphere

Bibliography

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