Is sea level rising by the same amount everywhere?

That last one is strange and important. Ice sheets kilometers thick pressed the crust down, and thousands of years after they melted the land is still rising back up. Where it is rising, relative sea level can fall even while the ocean gains water. Where the land is sinking instead, the local rise outruns the global average.

Sea level is rising, and at an increasing rate, and with continued ocean and atmospheric warming it will likely keep rising for many centuries at rates higher than this century’s. That is the global statement. The more useful question is what it means at a particular stretch of coast, and the answer is that it varies.

Global sea level trends and relative sea level trends are different measurements. Just as the surface of the Earth is not flat, the surface of the ocean is not flat either, and the sea surface is not changing at the same rate everywhere. Rise at a specific location may be more or less than the global average.

NOAA’s list of reasons runs as follows. Subsidence, upstream flood control, erosion, regional ocean currents, variations in land height, and whether the land is still rebounding from the compressive weight of Ice Age glaciers. Several of those are about the land rather than the water, which is why relative sea level can fall somewhere while the ocean gains volume.

The measurement follows the distinction. NOAA uses tide stations and satellite laser altimeters, and tide stations around the globe tell us what is happening locally, as height measured along the coast. Why it matters is a matter of where people live. In the United States almost forty percent of the population lives in relatively high-population-density coastal areas, and globally eight of the world’s ten largest cities are near a coast. One caution: this page names the local factors without sizing them.