When we look at a picture of Earth from space, there are areas of green, blue, and white. Most people will recognize(辨认) that the green island, the blue is water, and the white is clouds. Taken as a whole, Earth is mainly blue. That is because of the amount(数量) of water it has that covers the surface. So, how much of Earth is water? About 71% of Earth's surface is covered in water. Of that 71%, the oceans hold about 96.5% of the water. The water found in oceans is all salt water. The remaining 3.5%of the water is fresh water, which can be found in lakes, rivers, glaciers(冰川), and in the ground. Freshwater is what people use for farming, raising animals, drinking, and daily activities such as taking showers and washing clothes or dishes. Does the amount of water stay the same? For the most part, yes. Water on Earth is part of a closed system. That is, water is always moving and changing forms between ice, liquid, and gas. The form of the water changes all the time, but the total amount of water on Earth remains the same. If Earth is not necessarily running out of water, why are people worried? Why do we need to think about saving water? First, humans can only use fresh water, which is only about 3.5% of all of Earth's water. Second, of this 3.5%, only about 1% is available for human use. Most fresh water is found in glaciers far from where anyone lives. Third, the amount of water that is useful for humans is being used faster than it can be cleaned and put back into the environment. Now, knowing the facts about water, it is our job to make sure we use Earth's water in a good way. |