Global Ocean Conveyor Belt

Transcription

1 Environmental Literacy Framework Global Ocean Conveyor Belt Focus Questions: How does the great ocean conveyor belt of currents control the Earth s climate patterns? The great ocean conveyor belt transports heat throughout the Earth s oceans. Follow the diagram below as you read about this important climate-controlling system of our Earth. Preview Time 50 min. Materials There is no real beginning or end in this conveyor system, but in the polar seas near Greenland, cold winter winds from northern Canada cool the surface waters starting a process of sea ice formation. Combined with evaporation, the process of sea ice formation squeezes out the salt from the forming ice. As this occurs, the surrounding waters become saltier and more dense. This dense water sinks to the bottom and flows south along North and South America. As it approaches Antarctica, it encircles the Antarctic continent, meets the Antarctic Bottom Water (AABW), It then flows northward into the three ocean basins where it gradually mixes with warmer waters, is warmed and rises to the surface in the Pacific. From there, it makes its way back to the Atlantic and becomes part of the wind driven surface currents eventually returning to the Greenland seas to begin the process again. The great ocean conveyor belt is responsible for northern Europe's moderate climate. Northward movement of heat in the Gulf Stream provides the British Isles and Scandinavia with milder temperatures than other landmasses at similar latitudes. The salinity of the North Atlantic is important because it drives the great ocean conveyor system. Decreasing salinity in the polar seas could affect ocean circulation. If the salinity of the North Atlantic surface water drops too low to allow the formation of deep-ocean water masses, the ocean conveyor belt would slow down or stop. This may have happened between 1400 and 1850 AD, contributing to what is known as the Little Ice Age when Northern Europe's climate became markedly colder. Red and blue food coloring Tap water Kosher salt (any salt works, but this type dissolves and leaves the water clear) Salty, blue ice cubes Turkey baster or plastic pipette Several insulated cups for hot and cold water Clear plastic shoebox Vocabulary (Terms) Deep ocean water masses Density Ocean conveyor belt Ocean current Surface current Thermohaline current Surface currents are driven by wind, but thermohaline currents are driven by density differences. Because this ocean conveyor belt is controlled by dense, cold water, it is frequently called thermohaline circulation (thermo = temperature, haline =salinity). 167

2 Environmental Literacy Framework Activity 3A-Global Ocean Conveyor Belt Prepare 1. A few days ahead, freeze salty, blue-colored water in an ice cube tray. 2. Fill a plastic shoebox 3/4 full of cold tap water. Let the water settle. 3. Predict what will happen when you add the salty ice cubes to the fresh water. Record your ideas in your journal and discuss your thoughts with your learning team. 4. Add three of the frozen blue, salty ice cubes to the fresh water in one end of the shoebox. Observe what you see. Draw a sketch of your shoebox and add notes about the layering. 5. Fill a cup with very hot tap water; add 3 drops of red food coloring to it and mix thoroughly. 6. Predict what will happen when you add the hot, red water to the shoebox. Use a turkey baster or pipette to slowly add the hot red water to the other end of the shoebox. Carefully observe the motion in the shoebox, from both the side and the top. Where does the red-colored warm water go? Was your prediction correct? 7. Optional: Expand your exploration by adding a third ice cube with fresh water, colored green, to your variables. 168

3 Ponder Trace the pattern of ocean circulation on the Thermohaline Circulation map as you read the text below. The great ocean conveyor belt is driven by cold, salty water sinking to the bottom in the polar seas around Greenland. Water must move in to replace the sinking water and thus the conveyor belt begins. The dense, deep current stays cold and dense until it reaches the Pacific Ocean where it mixes, warms and rises. It then works its way back to the North Atlantic where it cools again, continuing the cycle. Graphic: Robert Simmon, NASA. Minor modifications by Robert A. Rohde The conveyor belt is an important part of the global climate system as it is a major transporter of heat from the equatorial regions to the polar regions. 1. What would happen if the conveyor slowed down? Make a prediction about the change in climate of Northern Europe if the conveyor belt slowed or stopped. Make a prediction about the change in climate for northern Australia. (Use the diagram pictured above.) 2. As the cold, salty water sinks, it carries oxygen to the deep parts of the ocean allowing organisms to thrive. What do you think would happen to these organisms if the conveyor belt slowed down, or stopped? 3. How does this activity simulate the melting of the polar ice caps and the potential interference with the global ocean conveyor belt? 4. What is the great ocean conveyor belt, what are the processes that control it, and how does it influence Earth s climate? 169

4 Practice Got the Big Idea? As the Earth's temperature warms, the polar ice caps may melt, allowing the fresh water that has been locked for hundreds of thousands of years in the glaciers and ice sheets to drain into the ocean. The addition of this fresh meltwater will reduce the salinity of the oceans. As demonstrated in this activity and also in the activity "Cold, Salty Deep," fresh, cold water does not sink as deeply as salty and cold water. A decrease in the salinity of the water in the ocean where the deep-water formation sites are located has the potential to alter the global ocean conveyor belt. Since this global ocean current plays an important role in the Earth s climate system as well as providing deep-water oxygen and other gases to the ocean, many living organisms that depend on this circulation system would be affected. Get ready to present Come up with a question to engage your audience about the great ocean conveyor belt. Ask them how it affects their lives. Prepare a visual to show them how the sinking of cold, salty, dense water creates the movement of the current. Practice your explanation of how the deep ocean conveyor is a critical part of the global climate system and the source of deep-water nutrients. Present Set up your demo and ask your audience to predict what will happen to the cold, salty ice cube as you add it to the plastic shoebox of fresh water. Then add the hot, red water to the bottom of the other end of your plastic shoebox. If they do not understand why the warm red water rises and the cold, salty water sinks, explain that salty water is denser than fresh water, and that cold, salty water is even denser than cold, fresh water causing currents and layering in the ocean. Engage them with your visual and explain that this is called a thermohaline current, driven by the waters from the Arctic and Antarctic. The sea ice in the polar regions "squeezes" the salt out as it freezes, leaving very cold, very salty water under the ice. This cold, salty water is very dense, and it begins sinking deep and moving toward the Equator. As the Earth warms and there is less sea ice formation, thermohaline currents may change. Make sure your visitors understand how important this current is to many systems including living organisms in deep water habitats, oceanic and atmospheric circulation, and humans. 170

5 Background Information for the Teacher Activity NSES CLEP CLEP In this hands-on activity, learners create a model which demonstrates how colder, saltier water sinks under warmer, fresher water. This temperature and density phenomenon drives the ocean currents, which distribute heat around our planet. NSES Physical Sci Standard B: Energy is transferred in many ways. Heat moves in predictable ways, flowing from warmer objects to cooler ones, until both reach the same temperature. The sun is a major source of energy for changes on the earth's surface Earth Science Std D: Water, which covers the majority of the earth's surface, circulates through the crust, oceans, and atmosphere in what is known as the "water cycle." Water evaporates from the earth's surface, rises and cools as it moves to higher elevations, condenses as rain or snow, and falls to the surface where it collects in lakes, oceans, soil, and in rocks underground. Global patterns of atmospheric movement influence local weather. Oceans have a major effect on climate, because water in the oceans holds a large amount of heat. The sun is the major source of energy for phenomena on the earth's surface, such as growth of plants, winds, ocean currents, and the water cycle. History and Nature of Science Std G: Scientists formulate and test their explanations of nature using observation, experiments, and theoretical and mathematical models. 2A: Earth s climate is influenced by interactions involving the Sun, ocean, atmosphere, clouds, ice, land, and life. Climate varies by region as a result of local differences in these interactions. 2B: Covering 70% of Earth s surface, the ocean exerts a major control on climate by dominating Earth s energy and water cycles. It has the capacity to absorb large amounts of solar energy. Changes in ocean circulation caused by tectonic movements or large influxes of fresh water from melting polar ice can lead to significant and even abrupt changes in climate, both locally and on global scales. 2F: The interconnectedness of Earth s systems means that a significant change in any one component of the climate system can influence the equilibrium ofthe entire Earth system. Positive feedback loops can amplify these effects and trigger abrupt changes in the climate system. 171

6 Background Information for the Teacher CLEP 7A: Melting of ice sheets and glaciers, combined with the thermal expansion of seawater as the oceans warm, is causing sea level to rise. Interconnectedness of Earth s systems means that a significant change in any one component of the climate system can influence the equilibrium of the entire Earth system. Positive feedback loops can amplify these effects and trigger abrupt changes in the climate system. ELF Hydrosphere 1 b: Water transports energy, solutes, and sediments as it moves through the water cycle s different reservoirs. Oceanic energy transport has a major impact on regional and global climate. Hydrosphere 2: The ocean circulates water around the Earth on time scales varying from seasonal to hundreds of years. ELF Hydrosphere 2 b: Thermohaline circulation is driven by differences in the density of water masses due to changes in salinity and temperature. This circulation incorporates intermediate and deepwater currents in a threedimensional pattern. ELF Hydrosphere 2 d: Plate tectonic motions change the size and shape of ocean basins, and alter coastlines and features on the seafloor. These changes influence ocean circulation patterns over long timescales. NSES: National Science Education Standards ( CLEP: Climate Literacy Essential Principles ( ELF: Environmental Literacy Framework ( Additional Resources: Animations The Great Ocean Conveyor Belt NOAA animation ocean_conveyor_belt.html kits/currents/08affect.html animations.htm 172

7 Background Information for the Teacher In this example of the journey of the great ocean conveyor belt, we begin with the process that forms the deep ocean mass known as the Mid-Atlantic Deep Water that forms in the North Atlantic Ocean. Off the coast of Greenland, especially during the fall and winter months, cold winds from northern Canada and Greenland cool the surface waters, causing them to freeze and form new sea-ice. Sea ice formation, combined with surface evaporation, creates cold, salty, and very dense ocean water. The cold, dense, and salty water sinks to the bottom of the ocean and begins to flow south along the ocean floor near the coasts of North and South America. As it approaches Antarctica, it encircles the Antarctic continent. Eventually the cold, deep water flows northward and splits into the three ocean basins. There, it moves upwards (due to upwelling) and warms as it flows onward. The cool dense water then becomes part of the wind-driven surface currents, eventually returning to the seas off the shore of Greenland to begin the process again. This journey can take up to one thousand years to complete. The conveyor belt is an important part of the global climate system as it is a major transporter of heat from the equatorial regions to the polar regions. For example, the oceanic conveyor belt and the wind-driven surface currents are responsible for northern Europe's moderate climate. Northward movement of heat in the Gulf Stream (a wind-driven surface current) provides the British Isles and Scandinavia with milder temperatures than landmasses at similar latitudes on other continents. As Earth s temperature warms, the polar ice caps may melt, allowing the fresh water that has been locked for hundreds of thousands of years in the glaciers and ice sheets to enter the ocean, thus reducing the salinity of the oceans. If the salinity of the North Atlantic surface water drops too low to permit the processes that contribute to the formation of deep-ocean water masses, the oceanic conveyor belt could slow down or even stop. The conveyor system has shut down in the past; for example, it shut down between 1400 and 1850 A.D., contributing to what is known as the Little Ice Age. During this period, Northern Europe's climate became markedly colder. Answers to Student Ponder Questions: 1. What would happen if the conveyor slowed down? Answer: Some of the Earth's areas would be warmer while others would be colder. An example is England. It is predicted that if the Gulf Stream slowed, the British Isles, as well as Scandinavia, would become much colder. 2. As the cold, salty water sinks, it carries oxygen to the deep parts of the ocean allowing organisms to thrive. What do you think would happen to these organisms if the conveyor belt slowed down, or stopped? Answer: Ocean habitats would change and organisms would have to move or adapt. As the current moves northward it carries heat from the low latitudes (near the equator) to the polar regions. If this heat is not transported north, but is trapped in the low latitudes, the ocean temperatures may increase. 173

8 3. How does this activity simulate the melting of the polar ice caps and the potential interference with the global ocean conveyor belt? Cold water is denser than warm water, but in this case the cold water is also salty, making it very dense. The red, warmer water is less dense and floats on top of the cold, dense bottom water. 4. What is the global ocean conveyor belt, what are the processes that control it, and how does it influence Earth s climate? Answer: The global ocean conveyor belt transports heat, as well as dissolved nutrients and gases, throughout the Earth s oceans. There are four major deep-water formation sites in the global ocean. These sites are found in the following four places: In the North Atlantic there are two sites-one in the Greenland-Norwegian Sea, and the other in the Labrador Sea. In the Southern Ocean near Antarctica, they are in the Weddell Sea and the Ross Sea. On a short-term time scale (decades or centuries), these deep-water formation sites and subsequent currents are the forces that give the "push" that drives the great ocean conveyor belt. These formation sites are in polar regions, near areas where seasonal sea ice forms. The coldest and densest of these deep-water masses forms in the Weddell Sea off the coast of Antarctica. It is known as Antarctic Bottom Water. Unlike surface currents, which are driven by wind, thermohaline currents are driven by density differences in ocean water. Because the ocean conveyor belt is controlled by dense, cold water, it is frequently called thermohaline circulation (thermo= temperature; haline =salinity). There is no real beginning or end in this conveyor system, since it is a continuous loop. Glossary Unit Activity Vocabulary Word Definition Hydrosphere Global Ocean Conveyor Belt Density The calculated mass per unit volume of a substance (Less dense fluids and gases float on more dense fluids and gases unless they mix. Hot air is less dense than cold air, which is why hot air balloon rise.) Hydrosphere Global Ocean Conveyor Belt Ocean current A continuous and directed movement of the oceans water due to winds, waves, temperature, density, or the movement of the Earth Hydrosphere Global Ocean Conveyor Belt Thermohaline current The thermohaline (thermo = heat; haline = salinity) circulation of the oceans refers t o the deep- w a t e r current that is driven by cold dense salty water and warm surface waters. 174