Most of the living biomass is water, and this unique triatomic compound is also indispensable for all metabolic processes. The water molecule is too heavy to escape the earth's gravity, and juvenile water, originating in deeper layers of the crust, adds only a negligible amount to the compound's biospheric cycle. Additions due to human activities—mainly withdrawals from ancient aquifers, some chemical syntheses, and combustion of fossil fuels—are also negligible. Both the total volume of Earth's water and its division among the major reservoirs can thus be considered constant on a timescale of 103 years. On longer timescales, glacial–interglacial oscillations shift enormous volumes of water among oceans and glaciers and permanent snow. The global water cycle is energized primarily by evaporation from the ocean (Fig. 2). The ocean covers 70% of the earth, it stores 96.5% of the earth's water, and it is the source of approximately 86% of all evaporation. Water has many extraordinary properties. A very high specific heat and heat capacity make it particularly suitable to be the medium of global temperature regulation. High heat of vaporization (approximately 2.45 kilojoules per gram [kJ/g]) makes it an ideal transporter of latent heat and helps to retain plant and soil moisture in hot environments. Vaporization of 1 millimeter per day (mm/day) requires 28 or 29 watts per square meter (W/m2), and with average daily evaporation of approximately 3 mm, or an annual total of 1.1 m, the global latent heat flux averages approximately 90 W/m2. This means that approximately 45 quadrillion watts (PW), or slightly more than one-third of the solar energy absorbed by the earth's surface and the planet's atmosphere (but 4000 times the amount of annual commercial energy uses), is needed to drive the global water cycle. The global pattern of evaporation is determined not only by the intensity of insolation but also by the ocean's overturning circulation as cold, dense water sinks near the poles and is replaced by the flow from low latitudes. There are two nearly independent overturning cells—one connects the Atlantic Ocean to other basins through the Southern Ocean, and the other links the Indian and Pacific basins through the Indonesian archipelago. Measurements taken as a part of the World Ocean Circulation Experiment show that 1.3 PW of heat flows between the subtropical (latitude of southern Florida) and the North Atlantic, virtually the same flux as that observed between the Pacific and Indian Oceans through the Indonesian straits. Poleward heat transfer of latent heat also includes some of the most violent low-pressure cells (cyclones), American hurricanes, and Asian typhoons. Oceanic evaporation (heating of equatorial waters) is also responsible for the Asian monsoon, the planet's most spectacular way of shifting the heat absorbed by warm tropical oceans to continents. The Asian monsoon (influencing marginally also parts of Africa) affects approximately half of the world's population, and it precipitates approximately 90% of water evaporated from the ocean back onto the sea. Evaporation exceeds precipitation in the Atlantic and Indian Oceans, the reverse is true in the Arctic Ocean, and the Pacific Ocean flows are nearly balanced. Because of irregular patterns of oceanic evaporation (maxima up to 3 m/year) and precipitation (maxima up to 5 m/year), large compensating flows are needed to maintain sea level. The North Pacific is the largest surplus region (and hence its water is less salty), whereas evaporation dominates in the Atlantic. In the long term, oceanic evaporation is determined by the mean sea level. During the last glacial maximum 18,000 years ago, the level was 85–130 m lower than it is now, and satellite measurements indicate a recent global mean sea level rise of approximately 4 mm per year. The difference between precipitations of evapotranspiration has a clear equatorial peak that is associated with convective cloud systems, whereas the secondary maxima (near 50°N and °S) are the result of extratropical cyclones and midlatitude convection. Regarding the rain on land, approximately one-third of it comes from ocean-derived water vapor, with the rest originating from evaporation from non-vegetated surfaces and from evapotranspiration (evaporation through stomata of leaves). Approximately 60% of all land precipitation is evaporated, 10% goes back to the ocean as surface runoff, and approximately 30% is carried by rivers (Fig. 2). Because the mean continental elevation is approximately 850 m, this river-borne flux implies an annual conversion of approximately 370×1018 J of potential energy to kinetic energy of flowing water, the principal agent of geomorphic denudation shaping the earth's surfaces. No more than approximately 15% of the flowing water's aggregate potential energy is convertible to hydroelectricity. The closing arm of the global water cycle works rather rapidly because average residence times of freshwater range from just 2 weeks in river channels to weeks to months in soils. However, widespread construction of dam means that in some river basins water that reaches the sea is more than 1 year old. In contrast to the normally rapid surface runoff, water may spend many thousands of years in deep aquifers. Perhaps as much as two-thirds of all freshwater on Earth is contained in underground reservoirs, and they annually cycle the equivalent of approximately 30% of the total runoff to maintain stable river flows. Submarine groundwater discharge, the direct flow of water into the sea through porous rocks and sediments, appears to be a much larger flux of the global water cycle than previously estimated.