Hydrosphere

The hydrosphere is all the water you can find on, under, and above the surface of Earth and other planets. This includes oceans, rivers, lakes, ice, snow, and even the water in the air. Earth's hydrosphere has been here for about 4 billion years and is always changing because of movements in the Earth's crust, like seafloor spreading and continental drift.

There is a huge amount of water on Earth—about 1.386 billion cubic kilometres of it! Most of this water is in oceans and is salty, making up about 97.5% of all the water. Only a small part, about 2.5%, is fresh water that we can drink. Most of this fresh water is frozen in ice and snow, especially in places like the Arctic and Antarctic. A very small amount is in lakes and rivers that we can easily use.

The weight of all this water is about 1.4 × 10¹⁸ tonnes, which is a tiny part of Earth's total weight. About 71% of Earth's surface is covered by oceans, which have an average salinity of about 35 grams of salt per kilogram of water. Water is very important for all living things, and it keeps changing all the time.

History

The word hydrosphere was first used in English in 1887. It came from a German word created by Eduard Suess.

Water cycle

Main article: Water cycle

The water cycle describes how water moves from one place to another on Earth. Water can be found in the air as clouds, rain, and snow, and on the ground in rivers, lakes, oceans, and underground. The Sun’s heat and gravity help move water between these places. Most water evaporates from the oceans and falls back to Earth as rain or snow.

Water is very important for life. Earth is often called the blue planet because two-thirds of it is covered by water. The water cycle helps create Earth’s atmosphere and supports all living things. Human actions, like building dams and causing pollution, can change how the water cycle works.

Recharging reservoirs

It takes a very long time for water to refill different places. For example, it can take 2,500 years to refill the oceans, and 10,000 years to refill ice and permafrost. Lakes can refill in about 17 years, and rivers can refill in just 16 days.

Specific fresh water availability

Fresh water is not spread evenly around the world, and some places can go from having too much water to not enough in just a few months. In 1998, most people—76% of the world—had less than 5,000 cubic meters of fresh water each year. About 35% of people did not have enough water at all. Only a small part, about 2.5%, of all Earth's water is fresh, and even less—just 0.25%—is easy for us to use.

Human impact

People have a big impact on the water all around us. Things like taking water from underground sources, building dams, and polluting water change how water naturally behaves. For example, an important underground water source in the United States, called the Ogallala Aquifer, is being used up much faster than it can refill. This could cause big problems for food and farming.

Many rivers are also blocked by dams and other structures, which stops them from flowing freely. This can hurt animals and plants that need clean water. Pollution from things like fertilizers and acid rain also harms our water, making it less safe for both people and nature. We depend on healthy water for drinking, travel, fishing, growing food, energy, and fun activities.