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Global Climate Change 21 Global Climate Change begins with an examination of the climate system and the various techniques for analyzing Earth’s climate. Following a discussion of the composition of air, electromagnetic radiation is explained in detail. Climate change and the effect of humans on global climate are discussed at length. The chapter concludes with a brief look at the complexity of climatic systems. Learning Objectives After reading, studying, and discussing the chapter, students should be able to: Explain the climate system. Describe the composition of the atmosphere. Discuss the various aspects of electromagnetic radiation. Briefly discuss how Earth is heated by solar radiation. Discuss the various ideas that have been formulated to explain climate change. List and briefly discuss carbon dioxide and other gases added to the atmosphere by humans. Explain how climatic systems are altered, including climate feedback mechanisms. Briefly discuss the role of aerosols in affecting global climate change. Discuss the complexity of the climate system. Understand how such complexity may prevent the prediction of future changes in climate. Chapter Outline___________________________________________________________________ I. The shoreline: a dynamic interface A. The shoreline is a dynamic interface (common boundary) among air, land, and sea B. Constantly being modified by waves C. Today the coastal zone is experiencing intense human activity II. The coastal zone A. Clarification of terms used to describe the land-sea boundary 1. Shoreline is the line that marks the contact between land and sea 2. The shore is the area that extends between the lowest tide level and the highest elevation on land that is affected by storms 3. The coast extends inland from the shore as far as ocean related features are found 4. Coastline marks the coast’s seaward edge B. Shore is divided into the 1. Foreshore – area exposed at low tide 2. Backshore – landward of the hightide shoreline 179 180 CHAPTER 21 3. The nearshore zone lies between the low tide shoreline and the point where waves break at low tide 4. Seaward of the nearshore is the offshore zone C. Beach – an accumulation of sediment found along the landward margin of the ocean or a lake 1. The relatively flat platform composed of sand and marked by a change in slope at the seaward edge is a berm 2. Beach face is the wet sloping surface that extends from the berm to the shoreline III. Waves A. Wind-generated waves provide most of the energy that shapes and modifies shorelines B. Characteristics of waves 1. Waves derive their energy and motion from the wind 2. Parts of a wave a. Crest – top of the wave b. Trough – low area between waves 3. Measurements of a wave a. Wave height – the distance between a trough and a crest b. Wavelength – the horizontal distance between crests c. Wave period – the time interval between the passage of two successive crests 4. Height, length, and period of a wave depend on a. Wind speed b. Length of time wind has blown c. Fetch – the distance that the wind has traveled across open water C. Types of waves 1. Wave of oscillation a. Wave energy moves forward, not the water itself b. Occur in the open sea in deep water 2. Wave of translation a. Begins to form in shallower water when the water depth is about one-half the wavelength and wave begins to “feel bottom” b. As the speed and length of the wave diminish, the wave grows higher c. The steep wave front collapses and the wave breaks along the shore d. Turbulent water advances up the shore and forms surf IV. Wave Erosion A. Breaking waves exert a great force B. Wave erosion is caused by 1. Wave impact and pressure 2. Abrasion by rock fragments V. Sand movement on the beach A. Movement perpendicular to the shoreline 1. Waves seldom approach the shore straight on, but rather at an angle 2. When waves reach shallow water with a smoothly sloping bottom they are bent and tend to become parallel to the shore B. Wave refraction 1. Bending of a wave 2. Causes waves to arrive nearly parallel to the shore 3. Consequences of wave refraction a. Wave energy is concentrated against the sides and ends of headlands b. Wave energy is spread out in bays and wave attack is weakened C. Moving sand along the beach 1. Waves that reach the shoreline at an angle cause the sediment to move along a beach in a zigzag pattern called beach drift 2. Oblique waves also produce longshore currents Global Climate Change a. b. c. VI. Currents in the surf zone Flow parallel to the coast Easily moves fine suspended sand and rolls larger sand and gravel along the bottom Shoreline features A. Features vary depending on 1. The rocks of the shore 2. Currents 3. Wave intensity 4. Whether the coast is a. Stable b. Sinking c. Rising B. Features caused by wave erosion 1. Wave-cut cliffs 2. Wave-cut platform 3. Features associated with headlands a. Sea arch b. Sea stack C. Features related to beach drift and longshore currents 1. Spits a. Elongated ridges of sand extending from the land into the mouth of an adjacent bay b. Often the end of a spit hooks landward in response to wave generated currents 2. Baymouth bar – a sand bar that completely crosses a bay 3. Tombolo – a ridge of sand that connects an island to the mainland or another island D. Barrier islands 1. Mainly along the Atlantic and Gulf coasts 2. Low ridges of sand that parallel the coast 3 to 30 kilometers offshore 3. Probably form in several ways E. If the shoreline remains stable the result of shoreline erosion and deposition is to eventually produce a straighter coast VII. Stabilizing the shore A. 181 Influenced by the local factors 1. Proximity to sediment-laden rivers 2. Degree of tectonic activity 3. Topography and composition of the land 4. Prevailing wind and weather patterns 5. Configuration of the coastline and near shore areas B. Three basic responses to erosion problems 1. Building structures a. Types of structures 1. Jetties a. Usually built in pairs to develop and maintain harbors b. Extend into the ocean at the entrances to rivers and harbors 2. Groin a. Built to maintain or widen beaches b. Constructed at a right angle to the beach to trap sand 3. Breakwater a. Barrier built offshore and parallel to the coast b. Protects boats from the force of large breaking waves 4. Seawall a. Barrier parallel to shore and close to the beach to protect property b. Stops waves from reaching the beach areas behind the wall b. Often the building of structures is not an effective means of protection 2. Beach nourishment a. The addition of large quantities of sand to the beach system b. Only an economically viable longrange solution in a few areas 3. Relocation 182 CHAPTER 21 a. Instead of building structures such as groins and seawalls to hold the beach in place or adding sand for replenishment, another option is available b. Such proposals are controversial C. Contrasting the Atlantic and Pacific Coasts 1. Shoreline erosion problems are different along the opposite coasts 2. Atlantic and Gulf coasts a. Broad, gently sloping coastal plains b. Tectonically quiet regions c. Development occurs mainly on the barrier islands (also called barrier beaches or coastal barriers) 1. Barrier islands face the open ocean and 2. Receive the full force of storms 3. Pacific Coast a. Relatively narrow beaches backed by steep cliffs and mountain ranges b. A major problem is a significant narrowing of many beaches 1. Sediment for beaches has been interrupted by dams and reservoirs 2. Narrow beaches allow for the erosion of the sea cliffs c. Shoreline erosion varies considerably from one year to the next largely because of the sporadic occurrence of storms VIII. Coastal classification A. Emergent coasts 1. Develop because of a. Uplift of an area, or b. A drop in sea level 2. Features of an emergent coast a. Wave-cut cliffs b. Wave-cut platforms B. Submergent coasts 1. Caused by a. Land adjacent to the sea subsides, or b. Sea level rises 2. Features of a submergent coast a. Highly irregular shoreline b. Estuaries – drowned river mouths IX. Tides A. Daily changes in the elevation of the ocean surface B. Causes of tides 1. Tidal bulges are caused by the gravitational forces of the a. Moon, and to a lesser extent the b. Sun C. Spring and neap tides 1. Spring tides a. Occur during new and full moons b. Gravitational forces of the Moon and Sun are added together c. Especially high and low tides d. Large daily tidal range 2. Neap tides a. Occur during the first and third quarters of the moon b. Gravitational forces of the Moon and Sun are offset c. Daily tidal range is least D. Other factors that influence tides 1. Shape of the coastline 2. Configuration of the ocean basin E. Tidal currents 1. Horizontal flow of water accompanying the rise and fall of the tide 2. Types of tidal currents a. Flood current – advances into the coastal zone as the tide rises b. Ebb current – seaward-moving water as the tide falls 3. Areas affected by the alternating tidal currents are called tidal flats 4. Occasionally form tidal deltas F. Tides and Earth’s rotation 1. Tidal friction against the ocean floor acts as a weak brake that is steadily slowing Earth’s rotation Global Climate Change a. b. The day is increasing by 0.002 second per century Over millions of years this small effect becomes very large 183 2. The length of each day must have been shorter and the number of days per year must have been longer in the geologic past Answers to the Review Questions 1. The climate system includes the atmosphere, hydrosphere, geosphere, biosphere, and cryosphere. 2. Proxy data refers to indirect evidence of past climate changes. Examples of proxy data include seafloor sediments, glacial ice, fossil pollen, and tree-growth rings. Proxy data are necessary in the study of climate change because recorded historical data are lacking beyond recent history. 3. Seafloor sediments are useful recorders of worldwide climate change because the numbers and types of organisms living near the sea surface change with the climate. 4. The characteristics of each tree ring, such as size and density, reflect the environmental conditions (especially climate) that existed during the year when the ring formed. 5. Clean, dry air is composed mainly of nitrogen and oxygen. A small percentage is made up of the inert gas argon. Two significant variable components of air include water vapor and liquid particles. 6. Most of the Sun’s energy is emitted as shorter wavelength electromagnetic radiation. 7. Approximately 50 percent is absorbed by Earth’s surface, 30 percent is reflected back into space, and 20 percent is directly absorbed by clouds and the atmosphere’s gases. These percentages can vary greatly because the percentage of light reflected and scattered back to space will vary greatly depending on atmospheric conditions. 8. The atmosphere is heated mainly by terrestrial radiation from Earth because the gases in the atmosphere readily absorb the longer wavelength terrestrial energy. The shorter wavelength rays emitted by the sun pass through the atmosphere without much of the energy being absorbed. 9. Carbon dioxide and water vapor are the primary heat absorbers in the lower atmosphere. 10. The greenhouse effect refers to the heating of the lower atmosphere by the absorption of terrestrial radiation by carbon dioxide and water vapor. These gases act much like the glass in a greenhouse in that they allow shorter wavelengths to pass through them while absorbing longer wavelength. 11. Large volcanic eruptions, such at Mount Pinatubo in the Philippines, appear to have measurable shortterm effects on global temperatures. It is thought that fine-grained debris and huge quantities of gases erupted from such volcanoes will filter out a portion of the incoming solar radiation, which in turn will lower temperatures in the troposphere. 184 CHAPTER 21 12. Two examples of possible climate change linked to solar variability include sunspot frequency related to temperature and drought conditions on Earth. Despite much research, no connection between solar variations and weather has yet been well established. 13. Carbon dioxide levels in the atmosphere have been rising for more than 150 years due to the use of coal and other fossil fuels. 14. Based on the increase in global average surface temperature during the twentieth century when carbon dioxide levels were increasing, temperatures in the lower atmosphere are likely to increase in the future as carbon dioxide levels continue to increase. 15. Other trace gases that are contributing to future global temperature change include methane, nitrous oxide, and chlorofluorocarbons. 16. Climate feedback mechanisms are those possible outcomes when the climate system is altered. Some examples include warmer surface temperatures and increased cloud cover. 17. The main sources of human generated aerosols are the sulfur dioxide emitted during combustion of fossil fuels and also as a consequence of burning vegetation to clear agricultural land. Aerosols have a net cooling effect on the atmosphere because they reduce the amount of solar radiation by influencing cloud formation. Aerosols, once released into the troposphere, will remain there for only a few days. 18. Four potential consequences of global warming are 1) increases warming due to increased greenhouse gases; 2) a wide difference in the impact of climate change on various regions; 3) the disappearance of vulnerable ecosystems, and 4) widespread water shortages and associated food supply issues. Lecture outline, art-only, and animation PowerPoint presentations for each chapter of Earth, 9e are available on the Instructor’s Resource Center CD (0131566911).