STRUCTURE AND COMPOSITION OF THE ATMOSPHERE THE TROPOSPHERE The atmosphere is divided into several layers characterized by abrupt changes in temperature due to differences in the absorption of solar energy. About 75% of the mass of Earth’s air is found in the atmosphere’s innermost layer, the troposphere, extending about 11 miles above sea level at the equator and about 5 miles over the poles. This layer of rising and falling air currents and winds is the planets weather breeder. About 99% of the volume of air in the troposphere consists of two gases: nitrogen (78%) and oxygen (21%). The remainder has slightly less than 1% argon (Ar), 0.036% carbon dioxide (CO2), and trace amounts of neon (Ne), helium (He), methane (CH4) an other gases including chlorofluorocarbons (CFCs, put there by human activities). The trace ampounts of Ozone that are found in the troposphere as a component of urban smog damage plants, the respitory systems of humans and animals, and materials such as rubber. Temperature drops with altitude in the troposphere. At the top of this zone, however, the temperature starts to rise. The boundary where this occurs is called the tropopause. THE STRATOSPHERE The tropopause marks the end of the troposphere and the beginning of the stratosphere, the atmosphere’s second layer, which extends from about 11-30 miles above Earth’s surface. The stratosphere contains less matter than the troposphere but its composition is similar with two exceptions: Its volume of water vapor is about 1,000 times less and its volume of ozone (O3) about 1,000 times greater. Stratospheric ozone is produced when some of its oxygen molecules interact with lightening and solar radiation. This thin screen of ozone keeps about 99% of the harmful ultraviolet radiation (especially ultraviolet-B, or UV-B) given off by the sun from reaching Earth’s surface. This filtering action protects us from increased sunburn, skin and eye cancer, cataracts, and damage to our immune system. Furthermore, it keeps much of the oxygen in the troposphere from being converted to ozone a component of urban smog which damages plants, the respiratory systems of humans and other animals, and materials such as rubber. Again unlike in the troposphere, temperature rises with altitude in the stratosphere, until there is another reversal at the stratopause, which marks the end of the stratosphere and the beginning of the atmosphere’s next layer. MESOSPHERE AND THERMOSPHERE Above the stratosphere the temperature begins falling with altitude in a layer called the mesosphere or middle layer. At the top of this layer there is another reversal of temperature called the mesopause. This marks the beginning of the thermosphere, where temperatures rise again. Temperatures are very high because it is bombarded by high-energy solar radiation and cosmic rays. This input of high-energy electromagnetic radiation also converts the gaseous molecules in this layer to ions. WEATHER AND CLIMATE Every moment at any spot on Earth the troposphere has a particular set of physical properties such as temperature, pressure, humidity, precipitation, sunshine, cloud cover, and wind direction and speed. These short-term properties of the troposphere at a given place and time are what we call weather. Climate is the average weather of an area over a long period--at least 30 years. The two most important factors determining the climate of an area are temperature and precipitation. CLIMATE AND GLOBAL AIR CIRCULATION The temperature and precipitation patterns that lead to different climates are caused mostly by the way air circulates over the earth’s surface. Several factors determine these patterns of global air circulation: 1) Long-term variations in the amount of solar energy striking the earth. 2) Uneven heating of the earth’s surface. Air is heated more at the equator where the sun’s rays strike directly. This is why tropical regions near the equator are hot and polar regions are cold, and regions in between generally have intermediate temperatures. 3) The tilt of the earth’s axis. Because of this tilt, various regions are tipped toward or away from the sun as the earth revolves around the sun. This creates opposite seasons in the Northern and Southern Hemispheres. There is a misconception that we are closer to the sun during our summer months when in fact we are closer to the sun during our winter months. 4) Rotation of the earth. Earth’s rotation on its axis prevents air currents from moving due north and south, in what is called the Coriolis effect. The result is six convection cells of air masses--three north and three south of the equator. 5) Properties of air and water. Cold air is denser than hot air so it sinks through less dense, warmer air. Hot air, being less dense, rises. Hot air can also hold more water vapor than cold air. CLIMATE AND OCEAN CURRENTS Generally cold currents flow from the polar areas toward the equator, and warm currents, driven by the wind and the earth’s rotation, flow away from the equator. The deeper currents are driven partly by cooling and partly by increased salinity. Ocean currents, like air currents, redistribute heat and thus influence climate and vegetation, especially near coastal areas. Currents also help mix ocean waters and distribute nutrients and dissolved oxygen needed by aquatic organisms. Along some continents, upwelling brings plant nutrients from the deeper parts of the ocean to the surface and support large populations of phytoplankton, zooplankton, fish, and fish-eating seabirds. An exampleof this is off the western coast of South America. Periodically in the Pacific Ocean, normal coastal upwelling is affected by changes in climate patterns called the El Nino-Southern Oscillation (ENSO). In an ENSO event the prevailing westerly winds weaken or stop blowing and surface water along the South and North American coasts become warmer. The upwelling of cold, nutrient-rich water is suppressed, which reduces primary productivity and causes a sharp decline in the populations of some fish species. A strong ENSO can trigger extreme weather changes over at least two-thirds of the globe, especially in the countries along the Pacific and Indian Ocean. Some areas receive abnormally high rainfall; others suffer severe droughts. CLIMATE AND ATMOSPHERIC COMPOSITION: THE GREENHOUSE EFFECT AND THE OZONE LAYER Small amounts of carbon dioxide and water vapor and trace amounts of ozone, methane, nitrous oxide, chlorofluorocarbons, and others gases in the troposphere pla a key role in determining Earth’s averages temperatures and thus its climates. These gases known as greenhouse gases, allow light, infrared radiation, and some ultraviolet radiation from the sun to pass through the troposphere. Earth’s surface absorbs much of this solar energy and degrades it into heat. Some of this heat escapes into space, and some is absorbed by molecules of greenhouse gases, warming the air and radiating it back towards the Earth’s surface. This trapping of heat in the troposphere is called the greenhouse effect. If there were no greenhouse gases, especially water vapor, Earth would be a cold and lifeless planet with an average surface temperature of 0 degrees F. Everyone needs to be aware that the greenhouse effect is good but a runaway greenhouse effect is bad. We call this phenomenon Global Warming. MICROCLIMATES Mountains, valleys, and other topographical features of Earth’s surface create local climatic conditions, or microclimates, that differ from the general climate of an area. When moist air blowing inland from an ocean reaches a mountain range, it cools and causes the air to lose most of its moisture in the form of rain and snow on the windward side. As the drier air mass flows down the leeward (away). The lower precipitation and arid conditions on the leeward side of high mountains is called the rain shadow effect. For example the Mojave desert east of the Sierra Nevadas in California. People living near an ocean or lake enjoy cooler summers and warmer winters. Large clusters of plants also create microclimates because of their uptake and release of water, and their windbreak effect. Forests, for example, are warmer in winter and cooler in summer, and have lower wind speeds and a higher humidity, than nearby open land. Cities also create distinct microclimates. WEATHER EXTREMES: TORNADOES AND TROPICAL CYCLONES Two weather extremes are tornadoes which form over land and tropical cyclones, which form over warm oceans waters and sometimes pass over coastal land. Tornadoes, occur only when a mass of cool dry air overruns warm, humid air. This leads to thunderstorms and extremely turbulent conditions, and in some cases to the formation of a tornado, moving at a speed up to 300 miles per hour. An average of 124 tornadoes hit the United States each year, most of them in Florida and in the Midwest- the world’s most tornado-prone area. This area is known as “tornado alley”. Tropical cyclones are giant circular storm systems that develop over warm tropical waters, usually in the late summer or early fall. Tropical cyclones are called hurricanes in the Atlantic and typhoons in the Pacific. Their winds may blow up to 200 miles per hour and can cover an area as much as 1,000 miles in diameter. Among the lower 48 states those areas at greatest risk from hurricanes lie on the east Atlantic and Gulf coasts. CLIMATE AND VEGETATION Biomes are regions with characteristic types of natural plant communities. The world’s major biomes vary with climate. For plants, precipitation is generally the limiting factor that determines whether a land area is desert, grassland, or forest. Average annual precipitation and temperature, along with soil type, are the most important factors determining the type of desert, grassland, or forest in a particular area. Climate and vegetation both vary with latitude (distance from the equator) and altitude. A desert is an area where the average precipitation is less than 25 centimeters (10 inches) per year and evaporation is rapid. Deserts cover 30% of Earth’s land. This lack of moisture and vegetation allows the ground to radiate heat rapidly after the sun goes down, explaining why desert nights are often cold. Tropical deserts, such as the southern Sahara and the Namib in Africa, are the driest places on Earth and typically have few plants and a hard, windblown surface strewn with rocks and some sand. In temperate deserts such as the Mojave in southern California, daytime temperatures are hot in summer and cool in winter. GRASSLANDS Grasslands occur where the average annual precipitation allows grass, and in some cases a few trees and shrubs, to grow, but is so erratic that drought and fire prevent large stands of trees from growing. Tropical grasslands--such as most savannas with scattered shrubs and stunted trees are found in areas with high average temperatures and low-to-moderate average precipitation. Tropical savannas have warm temperatures year-round, with two dry seasons and much rain the rest of the year. African tropical savannas contain enormous herds of grazing (grass-and herb-eating) and browsing (twig-and leaf-nibbling) hoofed animals, including gazelle, zebra, giraffe, and antelope. Tropical savannas are efficient at converting carbon dioxide into carbohydrates through photosynthesis, equaling or even exceeding the net primary productivity of tropical rain forests. Thus deliberately burning savanna, plowing up its grasses, and converting it into cropland releases large quantities of carbon dioxide into the atmosphere. This may contribute as much to a projected enhanced greenhouse effect as (if not more than) the more publicized clearing and burning of tropical rain forests. Temperate grasslands cover vast expanses of flat and gently rolling hills in the interiors of North and South America, Europe, and Asia. Types of temperate grasslands are the tall-grass prairies and short-grass prairies of the midwestern and western United States and Canada, the South American pampas, African veld, and the steppes of central Europe and Asia. Because of their fertile soils many of the world’s temperate grasslands have been cleared of their native grasses and used for growing crops. Overgrazing, mismanagement, and occasional prolonged droughts sometimes lead to severe wind erosion and loss of topsoil, which can convert these grasslands into desert or semidesert. Polar grasslands, or arctic tundra, occur just south of the arctic polar ice cap. This biome is carpeted with a thick, spongy mat of low-growing plants. Permafrost is a thick layer of ice beneath the soil surface that remains frozen year-round. The permafrost and the icy winter weather prevent the establishment of trees. In summer, water near the surface thaws, but the permafrost layer below stays frozen and keeps the water from seeping into the ground. During this period the tundra is dotted with shallow lakes, bogs, marshes, and ponds. Mosquitoes, and other insects thrive in the pools, and feed large colonies of migratory birds, especially waterfowl, which migrate from the south to nest and breed in the bogs and ponds. Alpine tundra occurs above the limit of tree growth but below the snow line on high mountains. The vegetation there is similar to that found in arctic tundra, but there is no permafrost layer. FORESTS Areas with moderate-to-high average precipitation tend to be covered with forest, containing various species of trees and smaller forms of vegetation. Tropical rain forests are found near the equator where hot moisture-laden air rises and dumps its moisture. They have a warm annual mean temperature that varies little daily or seasonally, high humidity, and heavy rainfall almost daily--ideal for trees. A mature rain forest has more plant and animal species per unit of area than any other biome. The tallest broadleaf evergreen trees emerge above the surrounding vegetation and capture direct sunlight. Beneath this emergent layer lies the canopy, where leaves and top branches of shorter trees overlap and allow only dim light to reach the understory of smaller trees. Even less light reaches the shrub layer near the ground, and only about 2% of the incoming sunlight reaches the dark ground layer, where there is relatively little vegetation. Most animals live in the canopy layer, with its abundant shelter and food. Undisturbed tropical rain forests can sustain themselves indefinitely. Moving a little farther from the equator, we find tropical deciduous forests usually located between tropical rain forests and tropical savannas. These forests are warm year-round. Most of their plentiful rainfall occurs during a wet (monsoon) season that is followed by a long dry season. Where the dry season is even longer, we find tropical scrub forests containing mostly small deciduous trees and shrubs. In scattered temperate areas with ample rainfall or moisture from ocean fogs, we find temperate rain forests. Along the west coast of North America from Canada to northern California these biomes are dominated by large conifers such as Sitka spruce, Douglas fir, and redwood. Temperate deciduous forests grow in areas with moderate average temperatures that change significantly during four distinct seasons. These areas have long summers, cold but not severe winters, and much precipitation, often spread evenly throughout the year. This biome is dominated by a few species of broadleaf deciduous trees, such as maple, popular and sycamore. They survive the winter by dropping their leaves in the fall and becoming dormant (thats what deciduous means). Unlike rainforests, these forests contain relatively few tree species. However, the penetration of sunlight supports a richer diversity of plant life at ground level. This layering of vegetation creates niches for animals. Hawks and owls keep down populations of mice and other rodents, which would destroy much of the vegetation on the forest floor. All but about 0.1% of the original stands of temperate deciduous forests in North America have been cleared for farms, orchards, timber, and urban development. Some have been converted to intensely managed tree farms or plantations, where a single species is grown for timber, pulpwood, or Christmas trees. Evergreen coniferous forests, also called boreal forests (meaning “northern forests”) and taigas (meaning “swamp forests”), are found just south of the arctic tundra in northern regions across North America, Asia, and Europe with a subarctic climate. Winters are long, cold, and dry, and sunlight is available only 6-8 hours a day. Summers are short, with mild-to-warm temperatures, and the sun typically shines 19 hours a day. These forests are dominated by a few species of coniferous evergreens such as spruce, fir, cedar, hemlock, and pine. Plant diversity is low in these forests because few species can survive the winters. The crowded needles of evergreen trees block out much of the light. Beneath the dense stands of trees, a carpet of fallen needles and leaf litter covers the nutrient-poor soil, making the soil acidic and preventing most other plants from growing on the dim forest floor. During the brief summer the soil becomes waterlogged, forming bogs. The world’s boreal forests extract three-quarters as much carbon dioxide from the atmosphere as do the tropical forests of the Amazon. LIFE IN WATER ENVIRONMENTS OCEAN ZONES Oceans cover more than 70% of Earth’s surface. By serving as a giant reservoir for CO2, oceans help regulate the temperature of the troposphere. The oceans mix and dilute many wastes flowing or dumped into them to less harmful or harmless levels. Oceans have two major life zones: the coastal zone and the open sea. The coastal zone is the relatively warm, nutrient-rich, shallow water that extends from the high-tide mark on land to the gently sloping, shallow edge of the continental shelf, the submerged part of continents. This zone has a very high net primary productivity per unit area. Although it makes up less than 10% of the ocean’s area, the coastal zone contains 90% of all marine species and is the site of most of the large commercial marine fisheries. The open sea contains only about 10% of all ocean species. It is divided into 3 vertical zones: euphotic, bathyal and abyssal based on the penetration of sunlight. The lighted upper (euphotic) zone is where photosynthesis takes place in the open sea. The coastal zone is all euphotic. The dimly lit bathyal zone and the dark abyssal zone are found only in the open sea. The euphotic zone supports large populations of phytoplankton. They are fed upon by slightly larger, more mobile primary consumers called zooplankton, ranging from single-called organisms to jellyfish. On the ocean floor near hydrothermal vents, scientists have discovered communities in which microscopic bacteria use chemosynthesis to produce food for themselves and other organisms feeding on them. The coastal zones of warm tropical oceans often contain coral reefs which are colonies of tiny coral animals, called polyps. When a polyp dies, its empty outer skeleton remains as a framework for more growth. Thus the core of a reef is dead but covered by living coral. Human activities are causing destruction and damage to these ecosystems, making them the most threatened ecosystems in the coastal zone. WETLANDS A wetland is an area of land covered all or part of the year with water. An estuary is a partially enclosed coastal area at the mouth of a river where its fresh water, carrying silt and runoff, mixes with salty seawater. About 5% of all wetlands in the United States are coastal wetlands, which extend inland from estuaries. The other 95% are inland wetlands. Estuaries and coastal wetlands are highly productive areas. Coastal wetlands dilute and filter out large amounts of nutrients and waterborne pollutants. Just 1 acre of tidal estuary provides $75,000 of free waste treatment and has an estimated value of $83,000 when production of fish for food and recreation is included. By comparison, 1 acre of Kansas farmland has a top value of $1,200 and an annual production value of $600. In temperate areas coastal wetlands usually consist of a mix of bays, lagoons, salt flats, mudflats, and salt marshes, where grasses are the dominant vegetation. These highly productive ecosystems serve as nurseries and habitats for shrimp and many other aquatic animals. Some coasts have steep rocky shores. Many organisms live in the numerous intertidal pools in the rocks. Other coasts have gently sloping beaches. If not destroyed by human activities, natural sand dunes (with the sand held in place by the roots of grasses) serve as the first line of defense against the ravages of the sea. Along some coasts (such as most of North America’s Atlantic and Gulf coasts) we find barrier islands: long, thin, low, offshore islands of sediment parallel to the shore. Many of the structures built barrier islands or beaches are damaged or destroyed by flooding, severe beach erosion, and wind from major storms (including hurricanes). Lakes are large bodies of fresh water. A lake with a large or excessive supply of nutrients (mostly nitrates and phosphates) needed by producers is called a eutrophic (well-nourished) lake. Such lakes are typically shallow and have limited transparency. They have a high net primary productivity, with large population of phytoplankton (especially cyanobacteria), zooplankton, and a diverse population of fish. A lake with a small supply of nutrients is called an oligotrophic (poorly nourished) lake. Such lakes usually have crystal-clear blue or green water. Many lakes fall somewhere between the two extremes of nutrient enrichment and are called mesotrophic lakes. Precipitation that doesn’t sink into the ground becomes runoff, The entire land area that delivers water, sediments, and dissolved substances via small streams to a major stream (river), and ultimately to the sea, is called a watershed, or a drainage basin. A river system is a system of different ecosystems. First, narrow headwater or mountain highland stream with cold, clear water rushing over waterfalls and rapids. In the second phase the headwater streams merge to form wider, deeper streams that flow down gentler slopes with fewer obstacles. In the third phase, these streams join into wider and deeper rivers that meander across broad, flat valleys. At its mouth a river may divide into many channels as it moves across a delta and coastal wetlands and estuaries, where the river water mixes with ocean water and deposits it’s sediments. Inland wetlands area areas covered with fresh water at least part of the year (excluding lakes, reservoirs, and streams) and located away from coastal areas. They include bogs, marshes, prairie potholes, swamps, mud flats, floodplains, fens, wet meadows, and the wet arctic tundra in summer. Floodplain wetlands near rivers help regulate stream flow by storing water during periods of heavy rainfall and releasing it slowly, which reduces riverbank erosion and flood damage. By storing water many seasonal and year-round wetlands allow increased infiltration, thus helping recharged groundwater supplies.