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The oceans form an integral part of the global water cycle: precipitation, evaporation, and rivers move large amounts of water between land and the oceans. Since the amount of water on Earth doesn’t change, a change of the amount of water in one component must be offset against the other. Because the water cycle varies over time, the amount of water stored in the ocean varies over time as well. For example, heavy rains over Australia, Southeast Asia, and the Amazon Basin -- and the subsequent storage of large amounts of water on land -- caused a drop in global sea level of 1 centimeter during the 2010-2011 La Niña cycle.
Humans also directly change the amount of water on land, and thus the amount of water in the oceans. The global construction of large dams in rivers has led to the impoundment of an amount of water equal to 3 centimeters of sea level. On the other hand, in many places, groundwater from non-recharging aquifers is consumed by human civilization, and this water eventually ends up in the oceans, adding to global sea-level rise.