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Geo-Joint: Sea Level Changes

Posted on February 09 2016

Sea level changes

When was the last time you sat down into a hot bath and the water was up to your chest, but barely covered your anklebones? That just couldn’t happen, given what we know about liquids flowing to conform to the shape of their container. But the rules change, or get more complicated, when you get to a container the size of an ocean basin. Because of these changed circumstances, some parts of the world are already suffering from sea level rise, and others are barely aware that it’s happening.

Of course, you’re always at a disadvantage if your home turf is barely above sea level, with no room to retreat. Some of the lovely islands of the South Pacific are in this predicament and even modest changes in sea level will force their occupants to find completely new locations on which to live. Many other coastlines will simply suffer great loss of habitat and infrastructure near the water’s edge and then pull back to higher ground. But whether the situation is an economic headache or a total disaster varies for some interesting reasons.

Continents, which seem like the solidest thing imaginable, are actually floating on a monstrously large sea of plastic rock called the mantle. Depending upon their mass and thickness, they depress the mantle structure to different extents. The continental masses don’t change significantly over the course of thousands of years, but an overburden of extra material, like miles-thick layers of ice and snow, can increase the weight of a landmass a great deal. Enough so, in fact, that the landmass not only rides lower on the mantle, but the added mass adds to the gravitational pull of the whole body. What’s the body pulling on? Well, amongst other things, the surrounding ocean, pulling the water closer to the land, causing an elevated local sea level. As ice masses melt away from such places as Greenland or Antarctica, the diminished mass means the once-covered land will both bounce back up (a process called isostatic rebound), and that the waters that lap its shores will be less strongly drawn toward it. Both of these actions will mean an effectively lower sea level along Greenland or Antarctica. However, for landmasses at some distance, the opposite is happening – the meltwater added to the volume of the sea, plus the migration of the water once piled up around the ice-crowned locales combine to contribute to higher water levels elsewhere.

The East Coast of the United States is and will continue feeling the effects described above, as the Greenland ice sheets melt. The sea level rise there is two or three times the world average of a few millimeters a year. That area has another thing going against it as well, elevation-wise. The massive ice sheets of the last ice age that once depressed Canada are long gone, but the rebound from the departed weight continues. As Canada rebounds, the U.S., once raised up, is now sinking. Think of two people sitting on a plank on a waterbed. If one is handed a bowling ball and sinks down some, the other will be lifted up. If the bowling ball “melted away”, the peoples’ elevations would even out – a relative loss for the one who never had the ball. Remember, the process is a whole lot slower between the two North American neighbors, so we’ve had centuries to get used to the shoreline we know, and we built a lot of stuff on it, but now it’s changing faster than ever before.

The loss of ice and the rise of Antarctica has its effects on distant lands as well. Add in the factors of tide, wind, and landform layout, and the interactions become extremely complicated. Thanks especially to satellite observations, scientists can actually measure the effect of all these different forces and calculate how much each will feed into the eventual sea level rise for different areas. This kind of information will be crucial for planning for future scenarios in a world that will inevitably experience changing coastlines.

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