Sea level rise is a natural process, but recent trends are concerning because it is increasing at an alarming rate.
In this lesson, you’ll learn what causes changes in sea level and what effect rising sea level has on humans and the environment.
Mean Sea Level
Last weekend, I decided to take a trip to the beach. It’s just so relaxing to watch the waves roll in and out along the shore. I stayed all day, long enough to see the tide go out and then come back in again. Watching the water level change like this got me thinking about how the sea level is changing in a more permanent way than just with these daily tides.When we talk about sea level, we’re referring to the surface level of the oceans. But as you can see, this changes all the time due to tides, wind, and other factors.
To account for this variation, scientists instead calculate a mean sea level, which is the average surface level of the oceans. A mid-point of the high and low levels is taken over time, and this value of sea level is used to determine elevations elsewhere on Earth, like the hill in your driveway and the top of Mount Everest.
Historic Sea Level Change
Sea level changes by small amounts all the time, but it may also change drastically over longer periods of time. In fact, global sea level was about 100 meters lower during the last ice age, 36,000 years ago, when much of the oceans were frozen in glaciers. Since then, sea level has been slowly and continually rising as those glaciers melt and return their frozen water back to the oceans.We know that significant changes in sea level occur over a geological time scale because of evidence in the fossil record. Remains of sea creatures have been found in places like the Sahara Dessert and the mountains of South America – neither of which is currently covered by water! It’s unlikely that these creatures were displaced by wind or migrated there on their own.
Instead, it tells us that these now-dry places were at one time covered by ocean waters.Rocks also tell us about sea level change over long periods of time. Glaciers are humongous (sometimes covering entire continents!) and move very, very slowly. As they move, they pick up all kinds of debris, rocks, and sediments. Some of these are boulders as big as houses, while others are very fine silt, sand, and clay.
As the glaciers move and melt, they leave this debris behind and geologists can tell by the type of debris where it was moved from. For example, when the debris left behind is sandy or muddy, scientists know that this came from the oceans that froze into a glacier.
Causes of Sea Level Change
The freezing and melting of glaciers is not the only cause of sea level change, but it is an important one because it is directly related to changing global temperatures. As Earth’s temperature continues to increase, the remaining glaciers continue to melt and return water to the ocean.When sea level changes because the amount of water in the oceans changes, we call this eustasy. While this is a natural process, people are greatly concerned because of the accelerating rate of sea level rise due to glacial melting – that is, the sea level isn’t just rising; it’s rising faster and faster all the time along with increasing global temperatures.Changes in Earth’s geology also affect sea level.
When sea level changes because the elevation of the land itself changes, we call this isostasy. Earth’s crust is dynamic, and the tectonic plates that comprise it are slowly but constantly moving. This can change the vertical structure of Earth, increasing or decreasing the height of land both above and below ocean surfaces.
Increasing global temperatures may also cause sea level to rise through thermal expansion. This is a process in which water molecules warm and expand. As water molecules expand, they take up more space, increasing the volume of the oceans and pushing the sea level higher.
Effects of Sea Level Rise
So if sea level rise occurs naturally, why is everyone so concerned about it? Well, when ocean waters encroach onto land, this can cause numerous problems for both humans and other organisms that inhabit coastal areas.Ocean water is very salty, and not all organisms can survive in such a saline environment.
Many plants and animals will either die or be forced to look elsewhere for habitat as salty ocean water moves farther inland from current shorelines. This salt water may also find its way into the freshwater underground and in aquifers, which is where we get water for things like drinking and crop irrigation.Erosion is another major issue of sea level rise. Water is very good at eroding land, and as ocean levels rise, our coastlines are at a greater risk for being washed away. This erosion also leaves us more vulnerable to coastal storms, like hurricanes.
Coasts, marshes, and wetlands provide protection from storm damage, and as these lands disappear into the seas, our cities and communities are at greater risk for harm.Don’t think that the oceans themselves aren’t affected by these changes. Glaciers are made of freshwater, and when they melt back into the oceans, it decreases the water’s salinity. Just like coastal organisms, oceanic organisms are also adapted to a certain salinity, and when this changes, both the plants and animals that depend on these conditions may not be able to survive.
Considering that 80% of the world’s organisms live in the oceans, this is a BIG deal!Humans are also going to be forced to adapt. More than half of the world’s population currently lives near or along a coastline, but more and more are flocking there all the time. Clearly, this conflicts with rising sea level, and as coastlines continue to disappear, these people are going to have to find new places to call home.
Earth is a dynamic place, and our oceans are no exception. Sea level, or the surface level of the oceans, can and does change on an hourly, daily, or even geological time scale.
Because it is constantly changing with the tides, wind, and other factors, the exact sea level is difficult to determine. Therefore, scientists usually calculate mean sea level, which is an average surface level of the oceans.Sea level change, which can be either an increase or a decrease, is most often related to changes in global temperatures as ocean waters freeze into glaciers or frozen glaciers melt back into the oceans. When sea level change occurs because the amount of water in the oceans changes, this is eustasy. However, as is sometimes the case, if sea level changes because the elevation of the land itself changes, this is isostasy.
Regardless of the cause, the effects of sea level rise are both numerous and problematic. Changes in ocean salinity, increased shoreline erosion, increased risk of storm damage, and habitat loss for humans and other coastal organisms are just a few issues that come with rising sea level. Currently, much of the world’s population lives near or along a coastline. But if the current rate of sea level rise continues, there may be less coastline available for living.
By the end of this lesson, you should be able to:
- Explain the difference between sea level and mean sea level
- Describe some of the causes of sea level rise
- Recall the role of glaciers in sea level rise
- Discuss how sea level rise can negatively affect terrestrial and oceanic organisms