What is Sea Level and How is it Measured? An Introduction

What is Sea Level and How is it Measured? An Introduction azs2 Wed, 05/21/2014 - 10:43

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)

What is Sea Level?

Click here for a transcript of the video What is Sea Level?

Sea level seems like a pretty easy concept, right? You just measure the average level of the oceans and that's that. But what about parts of the Earth where there aren't oceans? For example, when we say that Mt. Everest is 8850m above sea level, how do we know what sea level would be beneath Mt. Everest, since there's no sea for hundreds of kilometers? If the Earth were flat, then things would be easy - we'd just draw a straight line through the average height of the oceans and be done with it. But the Earth isn't flat.

If the Earth were spherical, it would be easy, too, because we could just measure the average distance from the center of the Earth to the surface of the ocean. But the Earth isn't spherical - it's spinning, so bits closer to the equator are "thrown out" by centrifugal effects, and the poles get squashed in a bit.

In fact, the Earth is so non-spherical that it's 42km farther across at the equator than from pole to pole. That means if you thought Earth were 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. This bulging is also why the Chimborazo volcano in Ecuador, and not Mount Everest, is the peak that's actually farthest from the center of the Earth.

So, how do we know what sea level is? Well, water is held on Earth by gravity, so we could model the Earth as a flattened & stretched spinning sphere and then calculate what height the oceans would settle to when pulled by gravity onto the surface of that ellipsoid. Except, the interior of the Earth doesn't have the same density everywhere, which means gravity is slightly stronger or weaker at different points around the globe, and the oceans tend to "puddle" nearer to the dense spots. These aren't small changes, either - the level of the sea can vary by up to 100m from a uniform ellipsoid depending on the density of the Earth beneath it.

And on top of that, literally, there are those pesky things called continents moving around on the Earth's surface. These dense lumps of rock bump out from the ellipsoid and their mass gravitationally attracts oceans, while valleys in the ocean floor have less mass and the oceans flow away, shallower.

And this is the real conundrum, because the very presence of a mountain (& continent on which it sits) changes the level of the sea: the gravitational attraction of land pulls more water nearby, raising the sea around it. So, to determine the height of a mountain above sea level, should we use the height the sea would be if the mountain weren't there at all? Or the height the sea would be if the mountain weren't there but its gravity were?

The people who worry about such things, called geodetic scientists or geodesists, decided that we should indeed define sea level using the strength of gravity, so they went about creating an incredibly detailed model of the Earth's gravitational field, called, creatively, the Earth Gravitational Model. It's incorporated into modern GPS receivers so they won't tell you you're 100m below sea level when you're in fact sitting on the beach in Sri Lanka which has weak gravity, and the model has allowed geodesists themselves to correctly predict the average level of the ocean to within a meter everywhere on Earth. Which is why we also use it to define what sea level would be underneath mountains... if they weren't there... but their gravity were.

Credit: minutephysics

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.

Sea Level Definitions

Sea Level Definitions azs2 Thu, 04/03/2014 - 15:55

In a perfect, non-moving, homogeneous sphere, the elevation of the Earth's liquid shell would be distributed equally about the center of gravity, and sea levels would be the same everywhere. However, the Earth is a heterogeneous, oblate spheroid that rotates on an axis and experiences gravitational influences from other planets and the sun. These factors, together with geographic variations of continents and submerged terrains, climate systems, water volume, tectonics, etc., the surface of the ocean, and hence sea level, change on various time scales, ranging from minutes to millennia. Therefore, it is a challenge to determine the exact sea level of the Earth, but it is done.

As a result of these complications when referring to sea level, geoscientists have to be a little bit more specific when they discuss "sea level." Hence, there are a number of different definitions for "sea level" that need to be understood.

Global Sea Level - the average height of the Earth's oceans combined (relative to the Earth's center). Influenced primarily by the factors that influence the volume of seawater, and size of ocean basins, etc. Often referred to as "Eustatic Sea Level"
Local (or regional) Sea Level - the height of seawater relative to a fixed point on land that is used as a continuous reference. Influenced by meteorological factors, tidal range, ocean currents, rates of subsidence/uplift. Also referred to a "Relative Sea Level"
Mean Sea Level (MSL) - the average height of seawater relative to a fixed datum established by a statistical average of water heights over a period of time. This is the most functional definition for sea level because it helps establish the elevation of all points on Earth (topographic elevation, and bathymetric elevation). In the U.S., MSL is often reported relative to the 1983-2001 NTDE (National Tidal Datum Epoch). Tidal datums need to be updated every couple of decades because sea level is not stable, and a new datum is likely to be announced.

Measuring Sea Level

Measuring Sea Level azs2 Thu, 07/30/2015 - 15:19

Measuring Sea Level Using Tide Gauges

Measuring sea level using gauges has a 200-year history. Today, the technology has changed, but the principles are the same as before, and some gauges provide very long and reliable records of water levels that can be used to observe sea level change trends. For example, the Fort Point tide gauge in San Francisco has more than 100 years of record that we will access later.

Sea level is often measured locally by tide gauges (and averaged over tidal cycles) that detect high and low points in a given period of time. Local tide gauges are especially useful for people who work or recreate in coastal areas and need to know what the water level ranges will be. These data points are also important for detecting water levels during storms and other events, as well as in the long-term investigation of relative water level change (rise or fall). Tide levels are also measured by floating buoys, which may also be used to detect tsunami waves. We will use tide gauge data to investigate sea level changes in different locations in the Module 4 Lab.

Measuring Sea Level Using Satellite Altimetry

With the advent of satellite altimetry in the 1960s, measurements of the sea surface took on a whole new level of accuracy. Between 1996 and 2006, altimetry took off with multiple satellites orbiting the Earth, providing much better coverage and data resolution. These measurements utilize multibeam methods that are very precise and can measure changes in elevation on the Earth's surface to great precision in the range of centimeters. These methods have shown that water bodies are not flat, but are incredibly dynamic and have high and low spots due to factors such as gravitational variability described above. Data such as ocean circulation, sea level rise, and wave heights can be measured. These measurements have provided insight into the links between the ocean and the atmosphere and how the connections drive climate. Satellite altimetry data collection began in earnest with the launch in 1992 of the TOPEX/Poseidon joint satellite mission between NASA and CNES, the French space agency. TOPEX/Poseidon proved data previously impossible to obtain. The next generation of satellites to collect these data was the NASA Jason satellites. They have been collecting data since Jason 1 was launched in 2001. Jason 2 was launched in 2008, while Jason 3 is presently collecting altimetry data. Each mission lasts about 5 years. Meanwhile, the European Space Agency’s Sentinel 3 satellite is collecting similar data, as shown below.

Satellite with altimeter interacting with a GPS satellite and ground stations while collecting data and measuring water vapor.

Jason 2 Satellite – How it works.

Credit: The source of this material is the COMET® Website of the University Corporation for Atmospheric Research (UCAR), sponsored in part through a cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). ©1997-2017 University Corporation for Atmospheric Research. All Rights Reserved.

How Satellite Altimetry Works

As the figure illustrates, satellite altimetry measurements are obtained by a system of instruments carried on a satellite orbiting the Earth. The instruments include an altimeter and antenna, which measure sea surface height; a radiometer, which measures atmospheric disturbances, and a GPS system for precisely determining the satellite’s location. The altimeter transmits rapid (1700/second) pulses of microwave energy towards the Earth, which reflect back to the satellite. The average round-trip time of these pulses is accurately measured to determine the exact distance between the satellite and the sea surface (range). Water vapor measurements are also made as the level of water vapor affects the rate of transmission of the pulses, and a correction must be made to obtain the final range, which is accurate to 2 cm. This range must be referenced to the reference ellipsoid, which is an approximation of the Earth’s surface (the sphere flattened at the poles discussed above). The GPS receiver onboard and ground-based radio receivers track the satellite’s exact location. Using these data, sea surface height can be accurately measured. In addition, the ocean surface topography (the highs and lows depicted on the images) are obtained through calculations. This information is key to understanding the ocean’s surface as a dynamic and complex terrain and to determining changes over time.

The Jason satellites have revealed critically important information that was not available prior to the mid-1990s. As technology develops and more data are added to the database, our understanding of the changing ocean increases. Among the many scientific goals of the Jason and other altimetry satellite systems currently in use, are to extend the time series of ocean topography measurements begun in 1992 and to monitor the changes in global mean sea level and its relationship to global climate change. Since the mid-1990s, there has been explosive growth in ocean and climate studies, and multiple altimetry satellites have provided longer and more accurate measurements and have led to better spatial and temporal coverage and resolution. These accurate and detailed measurements, in turn, inform predictive science on sea level change.

In addition, important information on ocean circulation and the relationships between heat transport and other variables such as nutrients and salt content are obtained, as well as measurements of wave height. These data can be used in modeling that informs our understanding of tides, weather, and other dynamic phenomena at work on our planet. This technology continues to add knowledge and understanding of our ocean.