After building one of the world's largest dams, the Three Gorges Dam on the Yangtze River, China is getting ready to outdo itself by building a massive structure on the Brahmaputra River that would be the largest hydroelectric project in the world.

If built, it will alter global sea level. But it won't be alone.

Most of the world's biggest dams have been built since 1950, flooding river valleys with giant man-made lakes. The cumulative effect of all this water storage has been to skim about 1.18 inches off the top of Earth's oceans.

But that's far from the whole story. The water behind dams is heavy — it pushes Earth's crust down and gravitationally attracts water from the ocean. The effect is small, averaging just .001 of an inch per year over the last sixty years according to a study in press in Geophysical Research Letters, but it works to counteract the sea level drop caused by storing water behind giant concrete walls.

The net effect of all this is that the oceans are actually rising about 20 percent faster than tide gauges would have us believe. Still, the vast majority of the roughly 3 millimeters (.12 inches) of annual rise is caused by the ocean expanding as it warms.

So if warming seas dwarf the dam effect, why do we care? In short, local effects. As study authors Julia Fiedler and Clinton Conrad of the University of

Hawaii write, sea level isn't uniform from one place to another — seas can rise faster in one region or slower in another, depending on a range of factors.

One of those factors is large dams. Coastal communities with a giant reservoir sitting inland may find the effects of sea level rise creeping up on them a little more quickly than they expect.

Image: AGU