Light surfaces reflect more heat than dark surfaces. This is called the albedo effect. When the Earth's temperature dropped because of its position in orbit around the Sun, and the tilt of the axis, the ice sheets grew. This in turn caused more heat to be reflected and the ice sheets to expand further. Professor Iain Stewart uses coloured and plain ice cubes to demonstrate the albedo effect and glaciologist Croll's theory.
Students could annotate a series of simple sketch maps to illustrate the albedo effect. Using a feedback loop, they could show how expanding ice sheets in the northern hemisphere reflect more energy from the Sun, causing temperatures to fall and more sea ice to form.