© Jiuguang Wang
Which type of ice causes a rise in sea level when it melts: formations on land, like glaciers, or formations in the water, like icebergs?
In this lesson, students will
- learn that ice formations on land will cause a rise in sea level when they melt, whereas ice formations on water will not cause a rise in sea level when they melt.
- learn that ice is less dense than water.
- learn that ice displaces water equal to the mass of the ice.
Teacher tip: This activity can be performed as a demonstration or in student groups.
This activity was selected for inclusion in the NSF/NOAA/DOE-funded Climate Literacy and Energy Awareness Network’s (CLEAN’s) collection of educational resources! CLEAN is a nationally recognized project in which climate scientists and educators rigorously review and select high-quality online resources to be part of a collection. It was also used by the National Parks Service to create a demonstration video.
For Student Groups
- two identical clear food storage boxes (approximately 6 inches square) per group
- 8 sticks of classroom modeling clay per group
- 1 ruler per group
- 1 tray of ice cubes per group (may need to start storing ice cubes ahead of time)
- 1 liter of water per group
- Sea Level Rise Worksheets (1 per student)
- one clear food storage box (approximately 6 in square) per group
- 4 sticks of classroom modeling clay per group
- blue food coloring
- ice cube trays
- 4-5 intensely colored ice cubes per group
- 1/2 liter of refrigerated water per group
Have a discussion about global climate change and its impact on sea level rise. Ask students where there is a lot of ice in the world. Is the ice on land or on water? Does it matter whether the ice is on land or water? Will one or both cause sea level to rise when they melt?
Give each student the Sea Level Rise worksheet.
- Guide students through the development of a question about the melting of ice and sea level rise. Which type of melting will cause a greater increase in sea level?
- Have each student make a prediction.
- Explain the steps in the activity and in the methods section of the worksheet have each student write down, in their own words, the steps involved in this investigation. Go over the steps slowly and in stages. Tell students they have to write clearly and with enough detail so another student could follow the same steps. Or give students written instructions.
- Tell students that they will need to record their measurements and write down their results, so to pay attention as they perform the investigation.
- Place half of the clay into one side of each box. Form the clay to represent land rising out of the ocean. In one box, form a level place at the highest part as shown below. Make rivers on the land if you like.
- Place as many ice cubes as possible on the level place formed with the clay in the first box.
- Place the same number of ice cubes next to the clay in the second box, so that they are resting on the bottom of the container.
- Pour water into the container where the ice is resting on the bottom until the ice floats. Be sure the ice is floating, not resting on the bottom. If this occurs, add more water.
- Pour water into the second container with the ice resting on the clay (be careful not to disturb the ice cubes) until the water levels in the two containers are approximately equal.
- On their Sea Level Rise Worksheets, have students record initial measurements of water height (in mm) using a ruler. For visual impression you may wish to draw a line in the clay where the water height begins for each container.
- Leave the setup. If possible, have students take measurements every hour and record the results on their worksheets. You can also leave the setup for several hours or overnight and just record the final measurement after the ice has melted.
- Have students measure new water heights and make observations about what occurred once the ice melted. Make sure students enter their measurements on their worksheets.
- Have students include the answers to the following questions in their conclusions on the worksheet.
- In which “situation” did the water level rise more?
- How do the results compare with your predictions?
- Why do you think this happened?
- Use the information in the teacher background section to help students understand their results.
- Have another discussion about global climate change. Why might we be concerned about sea level rise? Coastal areas will be flooded. People will lose their homes. Some fresh water resources will become too salty to use. Habitat loss will occur. What can we do to help slow this process by using less fossil fuel? Take public transit instead of driving, eat local foods, turn off lights and electrical equipment when not in use, plant a tree, reduce, reuse and recycle.
Conduct the extension activity as described for the original activity with the following modifications:
- When freezing the ice cubes, put lots of blue food coloring into the water before filling the trays. This will make colored ice (the darker, the better).
- Add salt to the water that will be poured into the boxes in a ratio of 3 tablespoons of salt to 1 liter of refrigerated water. This makes your water approximately the same salinity as the ocean. Teacher tip: It is important to use cold water for this activity because, as discussed earlier, liquids become denser as they cool. If the water from the melting ice is significantly colder than the saltwater in the pan, the difference in density (caused by variation in temperature) may negate the effect of the difference in salinity. This could cause the colored fresh water to sink instead of float.
- Use the colored ice in place of regular ice, and build your “landlocked ice” box as previously outlined, using the cold, salty water to pour into the container.
- Allow the ice to melt and watch where the fresh water (blue color) accumulates.
- Have a discussion about density, and what causes the observed result. Ask students to write in their own words why the blue colored water floats on the clear water.
- Discuss potential problems associated with a layer of freshwater floating on the ocean in the North Atlantic (see teacher background).
global climate change: the alteration of average global temperature, rainfall, and wind patterns as a result of increased atmospheric greenhouse gases
greenhouse gases: gases in Earth’s atmosphere that absorb and reradiate heat near the surface of the planet
density: a measurement of compactness. For solids, this is usually measured as mass per unit volume. For substances dissolved in water, this is usually measured as parts per thousand or million.
displacement: the forced relocation of water due to a submerged or partially submerged object occupying fluid space
Student Group Version
Global climate change is becoming a threat to our current way of life on Earth. One consequence of climate change is the melting of ice caps, glaciers, and sea ice, including polar ice in Greenland and Antarctica. Substantial melt of these massive glaciers will cause a rise in sea level along coastlines throughout the globe. This activity will explore how melting ice impacts sea level.
Water is an unusual liquid because it expands when it freezes. In general, liquids do not expand upon freezing, but rather contract and become denser as temperature drops. Like other liquids, as water begins to cool, it becomes more and more dense. But, because of the physical structure of the water molecule, it continues to become denser until just before freezing, when it expands. This expansion occurs at the point that freezing begins (around 4°C). At this temperature water molecules arrange themselves into a crystal lattice structure that is significantly less dense than the liquid form. Because of this decrease in density at the point of freezing, ice always floats on water.
When objects are totally submerged in water, they displace an amount of water equal to their volume. However, because ice floats on water and is not completely submerged, ice does not displace an amount of water equal to its volume. Instead, it displaces less than its total volume of water. The water that floating ice displaces is equal to the volume that the ice would take up if it melted and became water again. In other words, floating ice displaces water equal to the mass of the ice. When ice melts, the mass of the ice is conserved, but the crystal lattice structure of ice disappears and the volume decreases and becomes equal to the volume of water it displaced in its ice form.
Therefore, when floating ice melts, the melted water is equal only to the volume of the ice that was submerged. This means that when floating ice melts, it contributes no additional volume to the body of water. We see this phenomenon when we let ice melt in a glass of water. The water does not overflow because the ice has already displaced water equal to the volume it will take up upon melting.
Ice already in the oceans does not contribute to sea level rise, but ice covering land will contribute to sea level rise upon melting. Greenland, for example, is covered by vast quantities of continental ice. The melting of this ice will contribute to sea level rise. The sea ice in the area of the North Pole is floating in water and thus the melting of this ice will not contribute to sea level rise.
Local relevance: Although many of the icy areas that could melt with climate change are far away from the San Francisco Bay area, this issue is extremely relevant locally. According to maps created by the Bay Conservation and Development Commission for the San Francisco Chronicle, a one-meter rise in sea level would submerge parts of Corte Madera, San Rafael, Hayward, Newark and much of the Silicon Valley shoreline. In San Francisco, Mission Bay housing and office developments, Caltrain tracks, Candlestick Point redevelopment, Heron's Head Park, parts of Treasure Island, and the San Francisco and Oakland airports would all be under water.
In this activity, students will learn which masses of ice pose the biggest threat for rising global sea level and why.
Water is a wonderful solvent. This means many substances dissolve in water, adding molecules to the liquid. As molecules are added to a particular volume of water, the density increases. Thus, seawater is noticeably denser than fresh water and fresh water will float on the surface of seawater.
In the North Atlantic, a phenomenon based on this concept drives a process known as thermohaline circulation or the “great ocean conveyor belt.” In this area, surface water moving north from lower latitudes becomes saltier (due to evaporation) and colder as it moves northward. This causes the density of the water to increase, and the water eventually sinks as it enters the North Atlantic. When the water sinks, it drives a current that plays a significant role in global ocean circulation. The sunken water (it’s colder and more dense) slowly flows along the bottom of the ocean back toward the lower latitudes where it eventually rises, like a conveyor belt, to the surface and starts the journey north again. Thermohaline circulation is extremely important in maintaining hospitable climates around the globe because it contributes to the overall circulation of warm water from near the equator towards the poles.
Glacial melting in Greenland has caused some concern because of the potential for significant increase of fresh water in the North Atlantic. If melting rates continue to increase with global warming, a layer of freshwater could theoretically form in the North Atlantic. This fresh water could mix with the salty, dense water of the North Atlantic and stop the sinking of North Atlantic water, thereby altering the driving force behind the great ocean conveyor belt current. Although projections are speculative, scientists suggest that a disruption in this circulation could lead to a cold climate shift in Europe as well as unpredictable changes in other parts of the globe.
In this exercise, students will be able to visualize differences in water density and relate this to potential consequences of increased glacial melting.
- 1e. Students know matter has three forms: solid, liquid, and gas.
- 1f. Students know evaporation and melting are changes that occur when objects are heated.
- 1g. Students know that when two or more substances are combined, a new substance may be formed with properties that are different from those of the original materials.
Investigation and Experimentation
- 5c. Use numerical data in describing and comparing objects, events, and measurements.
- 5d. Predict the outcome of a simple investigation and compare the result with the prediction.
Investigation and Experimentation
- 6b. Students will develop a testable question.
- 6f. Students will select appropriate tools and make quantitative observations.
- 6g. Students will record data by using appropriate graphic representations, and make inferences based on those data.
- 6h. Students will draw conclusions from scientific evidence and indicate whether further information is needed to support a specific conclusion.
- 6i. Write a report of an investigation that includes conducting tests, collecting data or examining evidence, and drawing conclusions.
Physical Sciences: Density and Buoyancy
- 8a. Students know density is mass per unit volume.
- 8c. Students know the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid the object has displaced.
- Greenpeace International. Sea level rise. Retrieved July 9, 2008 from http://www.greenpeace.org/international/campaigns/climate-change/impacts/sea_level_rise/
- United Nations Environment Program. Great ocean conveyor belt. Retrieved July 9, 2008 fromhttp://www.grida.no/climate/vital/32.htm