December 1, 2010
|Ocean meets land, Western Cape, South Africa, March 2009.|
A global change in sea level. The key word in the definition is global— eustasy is not used to refer to local variations in sea level. Rather, a eustatic change in sea level occurs when there is a global change in (a.) the total amount of water in the oceans and/or (b.) the total volume of the ocean basins.
Changes in the total amount of water in the oceans are most often related to glacial-interglacial cycles. Or, to put it more simply, to hot-cold cycles. When the planet is hotter, there is more liquid water in the oceans. When the planet is colder, there is more solid water stored on land as glaciers and ice sheets. There is also more ice stored as ice sheets covering parts of the ocean. When the planet is hotter and there is more liquid water, global sea level rises. When the planet is colder and there is less liquid water, sea level drops.
Motions of Earth’s tectonic plates can also affect sea level. Movement of the plates over millions of years changes the shape of the ocean basins. Although plate tectonic changes (millions of years) occur more slowly than glacial-interglacial cycles (thousands of years), tectonic motion nonetheless can have a big influence on global sea level. When the plates are arranged in such a way that the oceans are wider, sea level will be lower. When the plates are arranged in such a way that the oceans are narrower, sea level will be higher.
In addition to eustatic changes in global sea level, there are also local changes in sea level. Local sea level changes are caused by regional factors, such as local tectonic uplift/depression, gravity, ocean temperature, and ocean currents. For example, sea level in Iceland (one of my favorite geologic locales) dropped significantly at the end of the last ice age ~13,000-10,000 years ago. During the last ice age (or glacial period), sea level in Iceland was higher by ~50-60 meters. This is because during the last glacial period, Iceland truly was Iceland… the entire island was covered by a thick ice sheet, at least 1km thick and possibly as thick as 2km. The ice sheet even extended beyond the island to the edge of the continental shelf. The large mass of the ice sheet depressed Iceland downwards, raising local sea level. When the ice sheet rapidly melted ~13,000 years ago, Iceland rebounded. That is, the land started rising upwards again. This caused local sea level to drop. Note that this drop is sea level has to be attributed to local rebound. This is because eustatic sea level was rising as ice melted in Iceland and all over the world.
A simple way to think about eustatic sea level is to think of an ocean like a giant bathtub. When there’s more water, the water level in the bathtub is higher everywhere. When there’s less water, the water level in the bathtub is lower everywhere. Now, extent this metaphor a little. I’ve already explained that plate tectonic movements can change the shape of the ocean basins. So, maybe think of the bathtub as a crazy Alice in Wonderland bathtub that changes shape and size with time. You should also think about the bathtub walls, which are bumpy. These higher-mass bumps have a gravitational influence on the water. The bathwater is attracted to these higher masses. This is true in your bathtub at home, but the effect is so small it’s negligible. On a larger scale in the ocean, the effect is measurable and significant. The surface of the oceans is not level– and not just because of waves and tides and such. The surface of the ocean is bumpy, and these bumps match topography. This is one way we can tell the topography of the ocean floor… from outer space! Satellites actually measure the height of the ocean and infer topography from this. Where the water is higher, the underlying topography is higher. Finally, I encourage you to think outside the bathtub. Just as the bumps on the surface of the bathtub have an influence on the water, so does everything surrounding the bathtub. The continents and the ice sheets also influence the water. Again, the water is attracted to higher masses.
Pretty cool, huh? But why do we care about this, unless we’re interested in seafloor topography? We care because gravity may actually play a role in sea level rise as a result of anthropogenic global warming. Jerry Mitrovica, a geophysicist at the University of Toronto, warns that if you take gravity into account, sea level does not rise evenly as a result of global warming. Rather, sea level may rise more than expected some places on the planet and may actually fall elsewhere. This means that sea level rise may be more catastrophic than previously realized for certain regions.
I worry over global warming and eustasy. Unfortunately, I am also lazy. Even after drinking a cup of tea out of my global warming mug, I usually drive the 5 minutes to work rather than walk or bike. I am lazy other ways, too– I waste paper, I don’t recycle everything, I drink far too many beverages in wasteful cans, I forget to turn lights off sometimes, and I sometimes buy bottled water. But I am proud to be working as an Earth Scientist, striving to better understand how the planet works. There are many scientists working hard (myself included) trying to figure out how we might be able to geo-engineer– or at least better understand– the planet so that we can mitigate global warming and sea level rise.
Many of us are lazy. Fortunately, we are also smart and we also have science. I’m going to go fill my global warming mug with another cup of tea and then I’m going to go back to picking carbonate crystals for several hours. Because maybe, just maybe, my research will help us understand a little more about the planet and maybe, just maybe, these little carbon dioxide-storing crystals can help us geo-engineer a way out of global warming and eustatic sea level rise. Not from my research alone. But maybe my little piece of research can help, in some little way. And maybe, just maybe, working hard at science will make up (just a little) for my laziness in other aspects of my life. Or maybe not. But at least my little piece of research will help us understand our planet just a little bit better.