The hydrosphere is the total amount of water on, within, and above Earth’s surface. This system includes water in liquid, solid, and gaseous forms, encompassing oceans, lakes, rivers, glaciers, groundwater, and atmospheric water vapor. Covering about 71% of Earth, it shapes the environment and influences nearly all of Earth’s systems.
Where Water Resides
The majority of Earth’s water resides in its oceans, which account for about 97% of the total global water volume. This immense saltwater reservoir dominates the planet’s surface.
The remaining 3% is freshwater, much of which is not easily accessible. A significant portion of this freshwater is locked away in ice caps and glaciers, making up approximately 2.1% of all Earth’s water and about three-quarters of its freshwater. Groundwater, stored beneath the Earth’s surface in aquifers, constitutes another substantial freshwater reservoir. Smaller amounts are found in lakes and rivers, and a tiny fraction exists as water vapor in the atmosphere.
The Global Water Cycle
Water on Earth is in constant motion, driven by solar energy and gravity, through the global water cycle. This continuous circulation moves water between the atmosphere, land, and oceans. The cycle begins with evaporation, where solar energy transforms liquid water from oceans, lakes, and rivers into water vapor, which then rises into the atmosphere. Plants also contribute through transpiration, releasing water vapor from their leaves.
As water vapor ascends into cooler atmospheric layers, it undergoes condensation, changing back into liquid droplets or ice crystals to form clouds. These clouds eventually release water back to the Earth’s surface as precipitation, which can take the form of rain, snow, or hail. Precipitation falling on land can then follow various paths.
Some water flows over the surface as runoff, eventually reaching rivers, lakes, and ultimately the oceans. Other precipitation soaks into the ground through infiltration, replenishing soil moisture and percolating deeper to recharge groundwater reserves. This groundwater can remain underground for varying periods, from weeks to thousands of years, before flowing into streams, lakes, or oceans.
Water’s Dynamic Movement
Water also moves dynamically within its reservoirs, beyond the water cycle’s phase changes. Ocean currents are continuous, directed movements of seawater driven by wind patterns, water density differences, and Earth’s rotation. These currents distribute heat from warmer equatorial regions towards the poles and carry colder water back towards the tropics, influencing global and regional temperatures.
Deep ocean currents, often called the “global conveyor belt,” are driven by variations in water temperature and salinity. This slow, deep circulation moves water, nutrients, and heat across ocean basins over long timescales. Waves, generated primarily by wind, represent the movement of energy through water, causing water molecules to oscillate in circular patterns. Tides are the rhythmic rise and fall of ocean water levels, primarily caused by the gravitational pull of the Moon and Sun. Rivers and streams also exhibit dynamic movement, with water flowing downhill under gravity, carving pathways across landscapes.
Connecting Earth’s Systems
The hydrosphere interacts extensively with Earth’s other major systems: the atmosphere, lithosphere (land), and biosphere (life). Water vapor in the atmosphere influences weather patterns, cloud formation, and precipitation, acting as a medium for heat transfer and contributing to atmospheric humidity. This interaction helps regulate global climate and can lead to significant weather events.
Water also shapes the lithosphere through erosion and deposition. Rivers carve valleys and canyons, transport sediment, and build floodplains and deltas. Glaciers similarly reshape landscapes through their slow, powerful movement, eroding rock and depositing material. Coastal areas are continually molded by the erosive power of waves and currents.
For the biosphere, water sustains all known life forms. It serves as a universal solvent, dissolving and transporting nutrients within organisms and ecosystems. Water provides habitats for countless aquatic species and is important for biological processes, from cellular functions to temperature regulation in living beings. Without water, diverse ecosystems and life on Earth would not exist.