The source of life is water. Life began in the
oceans, which represent the largest stock of water
on Earth. Much less water is stored below the land
surface in the form of fresh groundwater, amounting
to not quite 0.8% of the earth’s total water
reserves, while lakes and rivers combined only
contribute a further 0.007%. Therefore terrestrial
life depends primarily on the global water cycle.
This cycle makes the land productive by the infusion
of fresh water precipitation, originating from
the salt water of the oceans. However, in many
regions the precipitation does not provide a sufficient
or reliable source for the sustained presence
of plants and animals. In fact, in all regions, precipitation
proves to be highly variable in time and
space, and human activities that have led to global
warming have also increased the variability and
intensity of rainfall.