What is Sea Level and How is it Measured? An Introduction
The following pages look at what sea level change is, and what mechanisms drive sea level change on a planetary scale.
Before we investigate these mechanisms further, let’s ask a couple of fundamental questions: What is sea level anyway? How is it measured...and why has it fluctuated during the course of geologic time? And why is it not even across the globe? As you watch the following quick video, make a list of forces mentioned that influence sea level. The video clip (3:25) was published on Nov. 25, 2013, by MinutePhysics.
Video: What is Sea Level? (3:25)
The Minute Physics video introduces a few key concepts that make measuring sea level pretty complex:
- The Earth is not perfectly spherical, but an ellipsoid, due to its spin. This means that the Earth is “fatter” at the equator and slightly flattened at the poles, so that: “if you thought Earth was a sphere and defined sea level by standing on the sea ice at the North Pole, then the surface of the ocean at the equator would be 21km above sea level”.
- Differential density of the interior of the Earth so that “gravity is slightly stronger or weaker at different points around the globe, and the oceans tend to "puddle" nearer to the dense spots”.
- The mass of the continental plates creates a greater gravitational pull on ocean water than the ocean basin so that “mass gravitationally attracts oceans, while valleys in the ocean floor have less mass and the oceans flow away, shallower”.
These phenomena mean that there are peaks and valleys in the surface of the ocean – the ocean level is not uniform across the planet. These are important concepts to keep in mind as you read on.
We will also meet several other phenomena that drive sea level changes around the planet later in the module.