Discuss global warming by talking about what global warming means and how it differs from climate change. Climate change has occurred throughout the earth’s history, and it can refer to any change that affects climate, such as precipitation, temperature and winds. Natural causes of climate change include fluctuations in the heat of the sun, changes in the earth’s orbit and an increase in volcanic activity. Global warming refers specifically to a rise in the temperature of the earth’s atmosphere, and is widely believed by scientists to be caused by human activity.
If the earth’s temperature rises, then surface water temperatures will also rise, and ice floating in the Antarctic and Arctic will melt. This may cause students to think that ocean levels will rise, as a result. To illustrate the fact that ocean levels will not rise, place several ice cubes in a bowl and fill it to the top with water. Observe what happens as the ice melts. Water expands when it freezes, and since the icebergs are already in the water, they will not significantly affect its total volume by melting.
To contrast with the previous experiment, fill a dish with water, then place a piece of wood in the water, and on top of the wood, two or three ice cubes. As the cubes melt, the water in the dish will rise, and at the same time the wood will rise higher in the water, too. From this we see that when land ice such as glaciers and icecaps melt, the water runs into the ocean and causes it to rise. However, without the weight of the ice pressing down on them, landmasses will also rise.
Ask your students: “If all the fresh water that’s frozen in ice caps melts and runs into the ocean, what will happen to it?” (It will become salt water.) Many countries of the world depend on water that runs down from ice packs and glaciers for drinking and farming. Ask, “What will happen to those people if their drinking water runs into the ocean?” Some of the other impacts of global warming include deforestation -- when trees disappear because it is too hot for them -- desertification -- when good land turns into desert -- and floods -- when low-lying lands are flooded by rising sea levels as a result of melting glaciers and ice caps.