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 activitiesÃ¢â‚¬â€mainly withdrawals from ancient aquifers, some chemical syntheses, and combustion of fossil fuelsÃ¢â‚¬â€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.
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