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Changes in Sea Level: Causes & the Effect on the Environment

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  • 0:01 Mean Sea Level
  • 0:56 Historic Sea Level Change
  • 2:37 Causes of Sea Level Change
  • 4:05 Effects of Sea Level Rise
  • 6:00 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

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.

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