Melting Ice

Engage

10-30 minutes

Ask students their prediction about sea level rise. Provide 10-15 minutes of Internet research time or explore National Sea Level Rise together. Make a list of different predictions on the board based on different scenarios. (Note that in many places in Alaska sea level is actually in decline because of a combination of isostatic rebound after the weight of glaciers is gone or from plate tectonic induced land uplift)

1. During the period from 21,000 years ago to 2,000 years ago, the global sea level rose 120 meters.
2. Since the last ice age 18,000 years ago, sea level has risen about 130 meters. Most of the rise occurred before 6,000 years ago.
3. In the twentieth century, sea level began rising an average of 1.7 millimeters per year.
4. If this acceleration remains constant, then the 1990 to 2100 rise would range from 0.28 to 0.34 m
5. Climate models based on increasing greenhouse gases and a warmer atmosphere indicate that sea level may rise about 4 millimeters per year beginning in 1990, for a total of 0.44 meters by 2090 (International Panel on Climate Change).
6. Tide gauges in the United States show considerable variation because some land areas are rising and some are sinking. For example, over the past 100 years, the rate of sea level rise varies from an increase of about 0.36 inches (9.1 mm) per year along the Louisiana Coast (due to land sinking), to a drop of a few inches per decade in parts of Alaska. The rate of sea level rise increased during the 1993-2003 period compared with the longer-term average (1961-2003), although it is unclear whether the faster rate reflects a short-term variation or an increase in the long-term trend. (Intergovernmental Panel on Climate Change [IPCC]. 2007. IPCC Fourth Assessment Report—Climate Change 2007: The Physical Science Basis Summary for Policymakers.

Discuss the differences in predictions. Virtually all scientists agree that the sea level will rise in most locations in the world as the climate warms. However, there are many different predictions about how fast and how high the sea level rises might be. Some scientists have predicted that the sea level will rise about 4 inches by the year 2100, and others have predicted faster and higher rises in sea level. Scientists use complicated computer models to predict sea level rise, and must work with many variables that are not fully understood. Scientists do know that as water gets warmer, it expands, as demonstrated in Investigation 1C. Melting ice from glaciers and ice sheets will also add water to the ocean.

Ask students if they can think of other factors that scientists would need to know to make an accurate prediction. (Examples: the amount of ice that melts each summer and the amount of re-freezing that occurs each winter, and where ice melt occurs in relation to deep or shallow ocean basins areas or shallow nearshore areas). Does the type of ice matter? Does it make a difference if the melting ice comes from the sea or from the land? In this experiment, you will take a look at how melting land ice (glaciers and ice sheets) changes sea levels.

Explore

40-50 minutes

Ask students to remember what happened to the water level of the cup when the ice cube melted. Review with them that the activity was to simulate the melting of an iceberg or chunk of ice in the ocean. Based on their knowledge, ask them to predict what might happen to the sea level when glaciers or ice sheets melt. Ask students to write their prediction and their thinking in their science notebooks. Hopefully, students will predict correctly. But scientists can’t always assume their predictions are correct; so if possible, they conduct an investigation to test their prediction. Ask students how they might design an activity that simulates a glacier or ice sheet melting. (A couple of ideas are to mold a piece of clay into a mountain shape in the cup, or to place a piece of wood in the cup to represent land.) Discuss student ideas to make sure their ideas will work. Then pass out materials and let them carry out the investigation.
Each pair of students will need:

• Plastic cup or other container that will hold water
• Clay, rocks, or other material that does not float, to represent land in the container
• Ice cube
• Permanent marker or tape
• Water

Students will place the “land” in the container, and then pour water in until it is about half full.

Using a permanent marker or piece of tape, mark the water level on the container.

Place the ice cube on the “land.” When the ice cube has melted, check the water level in the container to see if it has changed. If it changed, mark the new level.

Explain

30 minutes

Allow students to share their predictions and findings. Ask for explanations for why the water level in the cup changed. Ask students to reflect on the following question in their science notebook: What does this experiment suggest might occur to the sea level if glaciers continue to melt?

Extend

5-35 minutes

Depending on time, and on students’ levels, you may also wish to discuss the idea of isostatic rebound (See the section on isostatic rebound in Teacher Background). To see the direction and magnitude of trends in sea level change in areas of the Gulf of Alaska, many of which are downward as rebound of the land more than compensates for sea level rise, go to NOAA’s Tides and Currents site and click on the Gulf of Alaska.
The Center for Remote Sensing of Ice Sheets has an interactive world map of sea level rise. 

Evaluate

10 minutes

Ask students to add to the ladder of learning in their science notebooks by adding a rung with information about land ice. Ask them to also reflect on the implications of rising sea levels for people who live in Alaska and other parts of the world.