39.4 Changing levels of carbon dioxide . . .422 39.5 The global carbon budget . . . . . . .422
Why it matters
Earth contains all the elements needed for life. These elements are constantly being removed from and released back into the environment.
Understanding these cycles helps us to manage natural resources more carefully.
Earth provides living things with all the matter they need to survive. Matter such as water, carbon and nitrogen is constantly cycled through the living and non-living parts of Earth. Various processes and chemical reactions move this matter from living organisms to the non-living part of Earth and back again. In this chapter we will look at how water and carbon are cycled on Earth.
39.1 The water cycle
Water is an essential component of life on Earth. It exists in three states: solid (ice), liquid (water) and gas (water vapour). It covers more than three-quarters of Earth’s surface. It never runs out because it is constantly moved through the living and non-living parts of Earth in a process known as the water cycle. Why is it that when wet clothes are hung out on the line on a warm dry day they dry out? This is a process called evaporation and is one part of the water cycle. Water vapour condenses in clouds and falls back to Earth in many forms such as rain, snow and sleet. This is called precipitation. This water then collects in rivers, lakes and streams and eventually flows into the oceans.
Activity Build a model of the water cycle 416
Apparatus Large metal or plastic bowl Small plastic container Water
