Hydrosphere
The hydrosphere is all the water on, under, and above Earth's surface, including oceans, lakes, rivers, groundwater, ice, and atmospheric water vapor.
Definition
The hydrosphere is all the water on Earth, in every state: liquid oceans, lakes, rivers, and groundwater; solid ice sheets, glaciers, and sea ice; and atmospheric vapor and clouds, totaling about $1.39 \times 10^9\ \text{km}^3$ with oceans holding roughly $97\%$ of it and ice about $2\%$. Its reservoirs differ enormously in how long water stays in them, from about $9$ days for atmospheric vapor to centuries or millennia for groundwater and ice sheets and roughly $3{,}000$ years for the ocean, and the ocean's heat capacity, about a thousand times that of the atmosphere, makes the hydrosphere the climate system's flywheel, having absorbed over $90\%$ of recent human-caused heat. Water's high heat capacity, latent heats, and solvent properties make the hydrosphere the medium for heat transport, chemical weathering, and nearly every biogeochemical cycle on the planet.
Example
The Atlantic Ocean, Lake Norman, the Catawba River, the ice on Greenland, and the water in a well are all parts of the hydrosphere, and a hurricane moving Atlantic water into the atmosphere and dropping it as flooding rain that erodes the land and floods habitats shows all four Earth systems interacting at once. The Greenland and Antarctic ice sheets store enough water to raise sea level about $7$ and $58$ meters respectively, and during the last glacial maximum so much of the hydrosphere was locked in ice that sea level sat about $120$ meters lower, placing the North Carolina coastline tens of kilometers east of today's Outer Banks.
Key Insight
About $97\%$ of the hydrosphere is salty ocean water, leaving only a small fresh slice, most of it frozen, and it is what makes Earth look blue from space and stay habitable up close, since no other planet in the solar system has stable liquid water on its surface. The ocean also behaves as a chemical reactor rather than a passive reservoir, having absorbed roughly a quarter of human $CO_2$ emissions and buffered the atmosphere at the cost of acidification, with its slow mixing guaranteeing that this change will continue for centuries.