If the amount of water on Earth is constant, why do oceans rise?

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Dear Tom,
You have said the Earth is a closed system in which the amount of water is constant. So where do we get the extra water to make the oceans rise?
— Roger Anderson, Winthrop Harbor
Dear Roger,
The amount of water on the Earth is constant, or nearly so. Actually, the amount is increasing ever so slightly due to volcanic eruptions expelling water vapor into the atmosphere, but, for all practical purposes, the amount of water (as a gas, liquid and as snow and ice) can be considered to be constant. The world’s oceans are rising due to the distribution of water: snow and ice on land, on average, are melting, and that water ultimately ends up in the oceans. The total amount of water is essentially unchanged, but more of it is finding its way from land into the oceans. Secondarily, the world’s oceans are warming, and water expands slightly as it is heated, adding to the volume of the oceans.
The amount of water on the Earth is constant, or nearly so.
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