I’m sure you’ve heard this many times before… “glaciers are melting and sea level is rising”. At first glance, this is a simple concept that many people have already become tired of hearing about. But allow me to add a bit more intrigue; the intricacies of how sea-level rise is experienced differently around the world is so much more complex and so much more fascinating!
On the most basic level, the global ocean is like a swimming pool. If you add water to a swimming pool, the water level will rise at the same speed in the entire pool regardless of which side you add the water to. So if ice is melting near the Earth’s poles, it will cause the sea level to rise everywhere, even at the equator. But the global ocean is so much larger and so much more complex than a simple swimming pool. A myriad of other processes are working to affect sea level on a smaller local scale, including differences in the amount of ocean warming (which causes water to expand and sea level to rise locally) and slight differences in the local gravitational pull (click here to read a captivating post about how icebergs can alter sea level with their own gravitational pull!). There are so many processes that interact to influence local sea level that I can’t possibly discuss all of them in this one post, so I will give you this map as a teaser and then move on to explain just one of the processes affecting local sea-level rise: land literally shifting beneath our feet.
Sea-level changes around the world measured by satellite radar altimeters from 1993 to 2015. Notice that while sea level is rising (warm colors) in most places, the rate of that rise varies, and there are even some places where sea level is falling (cool colors).
Imagine standing very still on a beach and watching everything move around you. You can feel the wind brush your skin. Sand grains are swept up in the breeze and tickle your legs. The waves crash in front of you with a calming, periodic roar. Sea spray floats in the air and rests on your cheeks. If you were to stand there for several years, you may notice that the ocean is slowly creeping closer and closer towards you. From your perspective, sea level appears to be rising. But think again… perhaps it’s actually you that is falling.
This is the idea behind local sea-level rise due to vertical land motion. As I discussed in my last post, The Levee Dilemma, many coasts around the world are sinking beneath the ocean. The ground naturally compacts over time from gravity. This compaction causes the ground to lose volume and sink, called land subsidence. As long as enough sediment is deposited on top to rebuild the land surface, the land level will appear to remain the same. But, we have built structures like levees and dams that cut off the flow of sediment to the land. We are also causing the land to lose more volume and sink faster by pumping out things like ground water and natural gas. So in many places around the world, particularly low-lying deltas, sea level appears to be rising much faster than it actually is because the land is also sinking. For example, in Vietnam’s Mekong Delta, the sea level is rising about 0.3 cm/year but the land is subsiding about 3 cm/year. That means that the land is sinking ten times faster than sea level is rising!
On the other hand, if the beach you are standing on is rising faster than sea level, it would appear to you as if sea level were actually falling. There are several processes that can cause the land surface to rise, such as glacial rebound. Think of a memory foam mattress. When you lay down on the mattress, you compress the foam, and the impression of your body on the mattress remains for a few seconds even after you stand back up. In the case of glacial rebound, also referred to as isostatic rebound, the earth’s surface is the memory foam mattress, the glacier is your body, and it takes MUCH longer for land surface to rebound. Land undergoing glacial rebound rises back up at about the same rate that your finger nails grow, so it can take thousands and thousands of years for it to bounce all the way back up. If the beach you are standing on is currently rebounding from a glacier that melted up to several thousand years ago, you may see the sea level falling when it is actually just rising more slowly than you are. For example, Iceland is bouncing back up at an astonishing rate of 3.5 cm/year! If you want all the nitty gritty details from the source, click here to read the original scientific article. But if you’re in the mood for more light reading, you can read an accessible summary of that article here. For more information on glacial rebound in general, check out this blog post.
So the next time you hear that “glaciers are melting and sea level is rising”, don’t just brush it off as as old news; remember that this fairly simple phenomenon is experienced differently all around the world. People living on large muddy deltas are sinking and slowly being engulfed by the ocean, while those living at higher latitudes where glaciers are melting are springing upwards and beating the sea level in its own race.