Photo of sunrise over the ocean by NASA astronaut Reid Wiseman.


The seas of the Earth are rising, a direct result of a changing climate. Ocean temperatures are increasing, leading to ocean expansion. And as ice sheets and glaciers melt, they add more water. An armada of increasingly sophisticated instruments, deployed across the oceans, on polar ice and in orbit, reveals significant changes among globally interlocking factors that are driving sea levels higher.

Yet the globally averaged trend toward rising sea levels masks deeper complexities. Regional effects cause sea levels to increase on some parts of the planet, decrease on others, and even to remain relatively flat in a few places, including, in recent decades, on the California coast. Thermal expansion of seawater can be the product of regional phenomena, such as El Niño, the periodic warming of the eastern tropical Pacific. But some of these regional cycles so far show no direct link to long-term global climate change—despite, at times, independently exerting a powerful short-term influence on global climate.

And while Greenland, Antarctica and most of the world’s glaciers are melting, a distinction must be made between glacial discharge into the oceans, a more permanent type of ice loss, and changes in the precipitation and evaporation that is feeding those glaciers and ice sheets, which fluctuate on the scale of decades.

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