When we think of sea level rise, we probably have a fairly simple explanation for what happens. The glaciers and polar icecaps melt, the resulting water flows into the world’s oceans and they rise accordingly. But, in actuality, the process is far more complicated than that. Consider Antarctica as an example—usually neglected because of its remoteness and the incorrect assumption until recently about its relative stability.
Antarctica’s ice sheets are about 1.5 times the area of the United States, some of them nearly 5 km thick, and they contain enough water to raise the world’s oceans about 60 metres if they were all to melt. The mass is so large that it actually creates a significant gravitational attraction. That is to say, Antarctica pulls water toward itself like the moon creates tides. Consequently, as its ice sheet melts, the gravitational attraction is reduced and the water goes elsewhere, thereby increasing the rise of sea levels in other locations.
Indeed, our whole planet is a patchwork of individual gravitational sites of greater or lesser intensity—measuring these by satellites when they change is one of the ways we can actually measure ice loss and the effects of added water to sea levels.
A simplistic understanding of Earth’s geology considers it to be a floating crust on a core of molten magma. While this is true, the gravitational intensity differs from place to place, depending on the thickness and the mass of the crust’s material, and now on the distribution of water.
Moving the mass of ice to water by melting the planet’s glaciers and ice caps creates very interesting effects. The weight of water, where the crust is thin and flexible, causes the land to subside, thereby increasing the effect of sea level rise. Forcing down the crust in some places causes other parts to rise, thereby reducing in those localities the effect of more water in the oceans. “Ice loss from Greenland’s ice sheet, for example, decreases sea levels on the nearby shores of north-western Europe and eastern Canada, and raises them around South America. Losses of ice from Western Antarctica push water up around the coastlines of North America, Australia and Oceania.” (“Out of Sight Out of Mind”, The Economist, March 30, 2024).
Melting ice has other noteworthy effects. Since it is fresh water, it is inclined to sit on the surface of the denser salt water. This slows the so-called thermal halide currents that move colder polar water to the tropics and warmer tropical water to the poles. These currents not only have a dramatic effect on climate by distributing heat, but also determine the amount of sea level rise in specific locations—if the current is moving toward a land mass, it is inclined to raise sea levels; if moving away it is inclined to lower them. And the changing wind patterns and the increasing strength of weather systems on a warmer planet cause storm surges which can raise sea levels by multiple metres over short periods. This can cause catastrophic flooding and shoreline erosion.
Another effect of melting ice is to change the actual rotational axis of Earth. It tends to wobble anyway, up to 10 metres over time. But the added water to the world’s oceans has had a measurable effect on this axis. Since 2005, according to studies by NASA and Scientific American, the polar north is actually shifting eastward toward Greenland at about 6 cm per year. About 90% of this shift can be attributed to the redistribution of mass—from ice to water—on the planet.
For us, sea levels are critical to how we live and move around our planet. London and Tokyo already have flood gates protecting them from exceptional surges in sea levels. So does Venice. Most other coastal cities do not, including Miami, a city that seems to be both in the state of Florida and in a state of denial. But other cities are going to become vulnerable to the dance that occurs between the water that is being added to the oceans and the land’s decision to rise or subside. The east coast of the US is particularly vulnerable to sea level rise. So, too, are the coastal refinery centres of the southern United States. The Economist reports that a $57 billion barrier is being planned to protect Galveston and the Houston area from anticipated flooding. Many other coastal cities, including Vancouver, are also considering what to do—the present estimate for only “improving” its existing dyke system is $9.5 billion.
As the global ice melts, as it inevitably will, this is a mere sample of the costs, the calculating and the measuring that is going to take place as we confront the lottery of sea level change. The big winners living on the rising land will breathe a sigh of relief, but the big losers on the subsiding land will struggle with eventual and certain inundation. But everyone else living in coastal locations will also have to confront the reality of what is happening. In retrospect, the cost of reducing greenhouse gas emissions compared to the havoc caused by a hotter planet will seem so cheap that we will wonder why we ever missed the opportunity. But we did. And because it’s already too late, we will have to confront the inevitable consequences of our own folly.
Ray Grigg for Sierra Quadra
Top image credit: Sea Level Rise is real – Photo by atramos via Flickr (CC BY 2.0)