The continental component of the hydrosphere—that is, lakes, rivers, wetlands, and groundwater—together with atmospheric water vapour, plays a fundamental role in shaping the climate.

Inland waters and the biodiversity of freshwater habitats are a valuable resource in economic, cultural, aesthetic, scientific, and educational terms. Their conservation and management are essential to the interests of all human communities. Yet this precious heritage is in crisis. The main threats to continental fresh waters include over-extraction; chemical, physical, and organic pollution of their natural reservoirs; alteration of surface and subsurface flow; the destruction or degradation of freshwater habitats; and the invasion of non-native species.

The cryosphere—water in solid form, comprising over 33 million km³ of ice, mainly in the polar ice sheets—plays an equally important role in determining climate. The cryosphere is far less mobile than liquid water and reflects most incoming solar radiation due to the whiteness of ice. Exchange between these two systems—solid and liquid water—occurs seasonally and appears as expansions or contractions of ice cover.

In many parts of the world, the availability of uncontaminated fresh water, in liquid or solid form, is declining sharply and is subject to intense competition among human populations. Protecting natural freshwater reservoirs, their ecosystems, and the biodiversity they support is therefore among the most urgent challenges we face.