Many people think of weather and climate as concepts that are very nearly synonymous, but this is far from the truth. Whereas weather is a term referring to the atmospheric conditions for a particular place at a particular time, climate describes the overall weather pattern of a region over an extended period. A spot on the South Carolina coast, for instance, is likely to be humid and prone to hurricanes; another place, in the Rocky Mountains, might be dry and windy; while yet another locale, in Hawaii, most likely is inclined to extremely mild and equable or unvarying temperatures. In each case, what has been described is climate; for the weather in any of those particular places, one would need to check the weather report on a specific day. Despite the fact that climate is a concept that encompasses the weather in a region over an extended period, it is still possible for climate itself to vary.
HOW IT WORKS
All weather takes place in the atmosphere, the uppermost of the four earth systems. A blanket of gases created in large part by the expulsion of elements from the geosphere through volcanic eruptions, the atmosphere sustains the biosphere through its components of oxygen and carbon dioxide. But it also recirculates water from the hydrosphere, a key function in its role as a weather-generating system. (See Earth Systems for more about the interactions among geosphere, biosphere, hydrosphere, and atmosphere.)
Composition of the atmosphere
Though we think of the atmosphere as being composed primarily of oxygen, the vast majority of it (78%) is nitrogen. The latter is a highly unreactive gas, meaning that it does not tend to bond chemically with other elements. (Nitrogen itself is discussed in considerably greater detail within the essay Nitrogen Cycle.) Therefore, the nitrogen in our air is really just “filler,” the result of volcanic eruptions billions of years ago: unlike carbon dioxide, which dissolved in the water of Earth’s oceans, the nitrogen simply stuck around in the atmosphere.
Air is not a chemical compound but a mixture, of which nitrogen and oxygen together account for 99%. Another 0.93% is argon, which is a noble gas, meaning that it, too, is highly unreactive. The last 0.07% comes from trace gases (i.e., gases in very small quantities), including two that are extremely important to the operation of Earth: carbon dioxide and water vapor. The first of these is a key component in the biosphere and in biogeochemical cycles (see Carbon Cycle), while the second is of enormous importance in weather and climate.
The Troposphere and Air
It may be surprising to learn that water vapor is such a small portion of the atmosphere; even more shocking is the tiny fraction that this amount of water (equivalent to many billions of gallons) constitutes in proportion to Earth’s entire water supply. Furthermore, all our weather, which plays such an integral role in our lives, is generated in a very small portion of the atmosphere.
For every climate region on Earth, there are particular types of plants and animals that have adapted to the prevailing conditions .Animals that live in polar regions. like this bear, typically have very small ears, giving them lees surface area to expose to the cold:they also tend to hibernate as a means of en during the winter.
This is the troposphere, the lowest layer, which extends to about 10 mi. (16 km) above the surface of Earth. The height of the troposphere is about one-fifth the combined height of the troposphere, stratosphere, and mesosphere, the three atmospheric layers that contain air. (Beyond these layers are the thermosphere, which eventually dissolves into the exosphere and the emptiness of space, which is characterized precisely by its lack of an atmosphere.)
A high Concentration of air
Small as the troposphere is within the larger atmosphere, however, it contains 80% of the atmosphere’s mass. The reason is that at higher altitudes, Earth exerts less gravitational attraction on the gas molecules that make up air. Thus, if one were to travel up through the outer layers, the amount of air in the atmosphere would simply shrink to nothingness because at that height, Earth does not exert enough gravitational force to hold in place those ultralight particles.
By contrast, in the troposphere, the gravitational force—a function of the distance between two objects, in this case, a gas molecule and Earth—is extremely strong. This results in a high concentration of gas near the surface. (Note the term gas: remember that air is a mixture, not a compound, so there is no such thing as an “air molecule.” Instead, there are only molecules of oxygen , ozone [Oj], nitrogen [N2], carbon dioxide, and water, as well as atoms of argon and other noble gases.)
In any case, the high concentration of gas creates pressure, which, along with several other factors, causes air to move. The result is weather, and weather patterns over a long enough period of time yield what we call climate. (See Weather for more about the components that make up weather as well as their interactions.)
The Atmospheric Sciences
Weather is like a daily newspaper, whereas climate is like a topic of history: whereas weather reflects the atmospheric conditions for a particular place at a particular time, climate is the overall pattern of weather over an extended period. This difference is reflected in the organization of the atmospheric sciences, the area of earth sciences concerned with atmospheric phenomena.
Principal among the atmospheric sciences are meteorology, which is the study of weather,and climatology, or the study of climate. Meteorology focuses on daily or even hourly changes in conditions within the troposphere and the lower stratosphere, the region just above it. By contrast, climatology is concerned with analyzing the weather in a particular area over periods as short as a month or as long as several million years.
Whereas meteorology involves daily weather forecasts and reports, efforts in climatology are directed toward explaining differences in climate around the earth. Climatologists also investigate how these differences are related to other aspects of the natural environment. Paleoclimatology is a specialized branch of climatology devoted to the study of climatic conditions in the distant past. Such study may require investigation of fossils and other materials that provide physical or chemical clues regarding the past climate and changes it experienced.
Climates Zones and Organisms
One of the few ancient scientific thinkers whose work is still held in high regard was the Roman geographer Pomponius Mela (ft. ca. a.d. 44). In De situ orbis, Mela introduced the idea of five climate zones on Earth: northern frigid or cold, northern temperate or mild, torrid (very hot), southern temperate, and southern frigid. In a world that knew of no lands further north than Scandinavia (a semi-mythical place the Greeks had called “Thule”) or further south than the lower Nile, Mela’s designation of climate zones was extraordinarily accurate.
Only in the 1910s did the Russian climatologist Wladimir Koppen (1846-1940) improve on Mela’s system, developing his own five-part climate designation. By Koppen’s time, it was clear that there was not necessarily any climatic difference between northern and southern seasons, except inasmuch as the seasons in the Southern Hemisphere took place at opposite times of the year from those in the Northern Hemisphere. Therefore, he dispensed with all references to latitude except insofar as they related to position relative to the equator,
LARGE CLIMATE REGIONS
The Koppen system recognizes the zones of humid tropical, dry, humid mid-latitude with mild winters, humid mid-latitude with cold winters, and polar. Each of these larger categories is then subdivided into smaller climate types: for example, among the dry-climate group, there is a distinction between deserts and steppes, which are arid but not completely barren plains of a type found in Russia and central Asia.
Two factors, the average annual temperature and amount of precipitation, serve to differentiate climate types. Humid tropical, dry, and polar zones are fairly self-explanatory; as for the two humid mid-latitude types, they are distinguished by their distance from the equator. For instance, the southern United States would be an example of a humid mid-latitude region with mild winters, while the northeastern United States—New York and New England—would be considered a humid mid-latitude region with cold winters.
For every climate region on Earth, there are particular types of plants and animals that have adapted to the prevailing conditions. Animals that live in desert regions, for example, might have large ears to add a greater surface area for perspiration. On the other hand, animals in polar regions typically have very small ears, giving them less surface area to expose to the cold. Both camels and cacti are organisms well adapted to a desert climate: the camel can store large amounts of water and food in its hump, while the cactus requires little moisture to survive. Many a desert animal is nocturnal, allowing it to survive the heat, while most polar animals tend to hibernate as a means of enduring winter.
Not all climates necessarily spread across a whole desert or mountain range—or, in the case of Antarctica, with its decidedly polar climate, a whole continent. A very specific area either on Earth’s surface or just a few feet or meters above or below it (i.e., up in the trees or in the soil) likewise can have its own microclimate. The very existence of a microclimate, with all its complexity, serves to show just how complex the larger Earth system is,
A particular microclimate, such as that of a forest or even a particular spot within a forest, has its own specific weather conditions: temperature, humidity, wind patterns, dew, frost, and evaporation or transpiration. Soil is a major factor influencing the quality of the microclimate: if the soil is sandy in texture, it is likely to reflect more light and heat. The same is true if it Is light in color. Also important is topography, an example being the rain shadow created by mountains. (See Mountains for an explanation of rain shadows.)
BENI ABBES DUNES, SAHARA DESERT. SOIL IS A MAJOR FACTOR INFLUENCING THE QUALITY OF A MICROCLIMATE: SOIL THAT IS SANDY IN TEXTURE AND LIGHT IN COLOR IS LIKLELY TO REFLECT MURE LIGHT AND HEAT.
Microclimates in action
A beautiful example of microclimate In action can be found when hiking on Mount Santo Tomas outside Baguio in the northern Philippines. Nestled in the mountains, with still higher regions such as Santo Tomas nearby, Baguio has long been a popular resort owing to its cool temperatures. It is one of the few places in the Philippines where one can find evergreen conifers such as pine trees, and the semi-alpine climate makes Baguio a welcome relief from the heat of Manila and other regions further south.
Even though it lies in the northern portion of Luzon, the northernmost of the major islands in the Philippine archipelago, it Is not latitude that gives Baguio its cool climate; instead, it is altitude. The same is true of other places around the world: Quito, Ecuador, for instance, which has an extremely cool climate because it Is high in the Andes, even though it lies on the equator. (The 2001 movie Proof of Life, which portrays climatic conditions ranging from mild to cold, was filmed in and around Quito.)
On Santo Tomas, however, where the altitude is even greater than that of Baguio, It Is possible to experience both the heat that characterizes most of the Philippines and the cool conditions of the mountains in northern Luzon. Standing at a particular spot along the mountainside, one can feel the heat that falls directly on the mountainv owing to its tropical latitude— which means that the sunlight reaches the surface at a more or less perpendicular angle. At the same time, one can feel the cool breeze that blows up the mountain as the result of its altitude. It is possible, in fact, to stand In such a way that one’s back, to the mountain itself, is hot and sweaty while one’s face enjoys the cool, moist highland breeze,
CAN CLIMATE CHANGE?
Though we have established that climate is a long-term weather pattern, that does not mean that climate itself is fixed and unchanging. Any number of factors can affect it. For example, the “fog” in London was once such an established fact that it became associated with that city in the way that wind is with Chicago.
But just as Chicago’s status as the “Windy City” is something of a myth (in fact, there are more than a dozen cities more windy, though Chicago does experience powerful winds as a result of its proximity to Lake Michigan), the London fog was not actually what it seemed to be. The fog was not really fog at all but pollution produced by the burning of coal for heat. As coal gave way to gas and other types of heat during the twentieth century, the fog in London’s climate changed.
Heating up and cooling Down
A less advantageous condition in the global climate may be a result of other technological changes, according to some scientists. The burning of fossil fuels, such as coal, natural gas, and petroleum, during the past century or more has put large amounts of carbon dioxide into the atmosphere. That part of the situation is fairly cut and dried; as to the potential result, the scientific community is divided.
Some scientists believe that higher concentrations of carbon dioxide will lead to a greater retention of heat in the atmosphere, resulting in a higher annual average global temperature. This phenomenon, known as global warming, has attracted considerable media attention owing to Its promotion by environmental activists, including celebrities who embrace environmental causes. Yet the global warming position is far from the only interpretation that the facts suggest.
Other experts contend that high levels of carbon dioxide in the atmosphere will have the opposite effect. According to this view^ the concentration of carbon dioxide may heat the atmosphere in the short term, but this will result only in a larger amount of evaporation from the oceans. This evaporation, in turn, will result in the formation of much larger cloud masses, which would have the effect of reflecting sunlight back into space—thus, in fact, lowering Earth’s temperature.
The global warming position, with its media and celebrity support, has held the lead since the 1980s. For this reason, it is easy to forget that in the mid- to late 1970s, many environmentalists held an exactly opposite position. At that time, a series of extremely cold winters led to the claim that Earth was cooling and that in a few more centuries, a new ice age would ensue. In fact, this is a considerably more plausible position, given the fact that Earth has experienced countless ice ages in the past.
Blocking out the sun
Of course, it could be maintained (as advocates of global warming do) that the present situation of massive fossil-fuel burning has never occurred in Earths history. This is certainly true, but there is less credibility in the claim that never before has so much carbon dioxide been introduced to the atmosphere. In fact, billions of years ago, volcanoes belched vast quantities of the gas into the region above Earth’s surface, but because carbon dioxide is highly water-soluble, most of it dissolved Into the oceans’ waters.
This points to one of the means by which a rapid climate change, in particular, a sudden cooling, is brought about: by a volcanic eruption or other phenomenon that places enormous amounts of dust and ash in the air. In so doing, it reduces the amount of solar radiation that reaches Earth’s surface, causing temperatures to drop. An extreme example of this may have occurred about 65 million years ago, as the result of a meteorite hitting Earth and causing rapid climatic changes that brought about the mass extinction of the dinosaurs and other life-forms (see Paleontology).
A more benign example of atmospheric blockage occurred In 1991, after Mount Pinatubo in the Philippines erupted. The eruption apparently released so much ash and other material into the atmosphere that it temporarily reversed a general warming trend that had prevailed during the 1980s. By the mid-1990s, however, the materials from Mount Pinatubo appeared to have settled out of the atmosphere, thus causing a return to early climate trends.
New York City has a massive concentration of humans, machines. and concrete in a very small area. Cities without adequate green space, such as New York’s Central Park [shown here), become giant reflectors , and the presence of smog and heat from cars and other machines adds to the unhealthy environment.
Heating up a microclimate
Despite the questionable nature of some environmentalists1 claims concerning the global climate, environmentalists are absolutely correct in noting the effects of human civilization, development, and technology on microclimates. An excellent example is New York City. Though it is located in the humid temperate climate zone described earlier, with cold winters, the city has less snowfall—and hotter summers—than less populated areas of New York State or even areas in Pennsylvania that lie on the same latitude.
The difference, of course, lies in the fact that New York City is one of the planet’s greatest municipalities, a massive concentration of humans, machines, and concrete in a very small area. Heat-sensitive satellite imaging equipment routinely produces images of the United States that show great areas of heat around the major cities, particularly New York. Concrete, buildings roads, and the like constitute what developers call impervious surface, meaning that rain is not supposed to leach through these surfaces to the soil; rather, it will remain on the surface as runoff. But sunlight cannot gather in puddles or run off into storm drains: it can only reflect, and thus a big city is a huge mirror to the Sun’s rays.
A major city, from the standpoint of climate, is rather like a person who places a mirrored piece of metal around his or her face to “soak up the Suns rays”—an extremely unhealthy, dangerous, and inadvisable practice. Cities without adequate green space, such as parks, become giant reflectors, and the presence of smog and heat from cars and other machines only adds to the unhealthy, artificially heated environment.
It is particularly unfortunate to see such changes in a city such as Atlanta, which until about 1980 was a fairly sleepy town noted for its abundance of trees. Atlanta has long since become a boomtown, and the trees of its metropolitan area are being massacred at an alarming rate. Not only is this denuding the city and suburbs (and in the process, some would say, stripping its last vestiges of charm), but it is raising Atlanta’s average temperature—which was already high as a result of the city’s latitude.
Earth, Space, and Ice Ages
If Earth were a horse on a carousel, with the Sun at the spinning center of the merry-go-round, the horse representing our planet would not be sitting upright, at a 90° angle to the carousel floor. Rather, it would be tilted off the perpendicular angle by 23.5°, a fact responsible for many features of the global climate and microclimates of Earth.
At certain times of the year, rays from the Sun strike either the Northern or Southern hemisphere more directly than at other times. Summer occurs in the Northern Hemisphere during the middle of the year, because, at that point in the planet’s movement around the Sun, the rays of the Sun strike the Northern Hemisphere at an angle close to the perpendicular. The same happens in the Southern Hemisphere around the end of the year, when that section of the planet receives solar radiation at a nearly perpendicular angle.
Atmosphere: In general an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon (0.07%).
Atmospheric sciences: A major division of the earth sciences, distinguished from geoscience and the hydrologic sciences by its concentration on atmospheric phenomena. Among the atmospheric sciences are meteorology and climatology.
Climates: The pattern of weather conditions :n a particular region over an extended period. Compare with weather.
Climatology: An area of the atmospheric sciences devoted to studying the weather in a particular region or regions over periods as short as a month or as long as several million years.
Meteorology: The study of the atmosphere, weather, and weather prediction.
Microclimate: The climate of a very specific region a few feet or meters above or below Earth’s surface. The size of microclimates is undefined and variable, but they are most definitely smaller than the regions (a state, a country, even a continent) over which a particular climate is said to prevail.
Troposphere: The atmospheric layer closest to the surface, extending upward approximately 10 mi. (16 km). The troposphere contains about 80% of the air in the atmosphere and is the region where weather occurs.
Weather: The condition of the atmosphere at a given time and place. Compare with climate.
Another important factor that relates Earth’s position in space to its climate and microclimates is its pattern of movement around the Sun. The carousel analogy Is a flawed one, because a carousel Is round; Earth’s orbit, in fact, describes an ellipse, or oval. The reason is that the Sun exerts a greater gravitational pull on Earth at different parts of its orbital path, and the result is that at its closest approach to the Sun, Earth receives more solar energy than it does when it is farthest away from it. This has little do with seasons: the closest approach occurs in January, winter In the Northern Hemisphere. Conversely,in July, the hottest time of year for the Northern Hemisphere, Earth is at its furthest point from the Sun.
Explaining ice ages
It is possible that Earths position relative to the Sun may explain those dramatic periods of cooling known as ice ages, when much of Earth’s water freezes, larger amounts of land are exposed, and some life-forms die off while new ones develop. There have been numerous ice ages in Earth’s history, and what we call the Ice Age, which ended about 10,000 years ago, was just the last Ice age.
That one was particularly significant, of course, not only because it was most recent, occurring as it did on the eve of civilization’s beginnings, but because it was a formative juncture in human development. During those thousands of years, human hunter-gatherer society and Paleolithic technology developed to its highest point. Also, many of the significant migrations of people took place, most notably the movement of Siberian tribes eastward across the land bridge of what is now the Bering Strait.
What causes ice ages and other large-scale climate changes? In the 1930s the Serbian astrophysicist Milutin Milankovitch (1879-1958) put forward a theory maintaining that changes in the pattern of Earth’s orbit around the sun, as well as in Earth’s inclination to the plane of its orbit, could explain these changes in climate. The planet’s axis of inclination (that is, its angle in relation to the plane of orbit) changes over a period of about 22,000 years, a cycle known as the precession of the equinox. As a result, first one hemisphere and then the other is be pointed toward the Sun.
Milankovitch evolved a complex theory that took into account precession of the equinoxes, changes in the shape of Earth’s orbit, and changes in its orbital tilt. The theory has been improved and modified numerous times since the 1930s, but, in general, Milankovitch ideas still provide climatologists with a basic understanding as to how large changes in Earth’s climate take place.