What is Sea Level?
Sea level seems straightforward - it's where the ocean surface meets the sky. But scientists measure something more specific: the average height of the ocean surface relative to a fixed reference point on land.
The ocean surface is never completely still. Waves move up and down constantly, tides raise and lower the water several times each day, and storms create temporary surges. When scientists measure sea level, they're looking at the average of all these variations - the mean water level that would result if all the waves and ripples were smoothed out.
Sea level varies across the globe. The Pacific Ocean's average sea level is actually several centimeters higher than the Atlantic Ocean's, due to differences in ocean currents, water temperature, and gravitational effects.
How Do We Measure It?
Tide Gauges
For more than a century, scientists measured sea level using tide gauges - instruments installed at harbors and coastal locations that continuously record the height of the water surface. These mechanical and electronic devices provide localized, highly accurate measurements.
Satellite Altimetry
Starting in the 1990s, satellites revolutionized sea level measurement through radar altimetry. Satellites orbit Earth approximately 1,300 kilometers above the surface, emitting microwave pulses that travel down to the ocean, bounce off the water surface, and return to the satellite. By precisely measuring this journey time, scientists calculate the ocean surface height.
Try It: Identify the Trend
Look at a satellite sea level time series from 1993 to 2024. At first, the chart looks noisy with measurements jumping up and down. But step back and notice: the peaks are getting higher, and the troughs are getting higher too. The entire oscillating pattern is shifting upward.
Reading a Sea Level Data Chart
Sea level data is typically displayed as a time series graph: a line chart with time on the horizontal axis and sea level height on the vertical axis. This format clearly shows how sea level changes over months, years, or decades.
You'll usually see a seasonal oscillation - a regular up-and-down pattern repeating every year caused by temperature changes. In summer, water expands as it warms; in winter, it contracts as it cools.
Beyond the seasonal wiggle, you'll see a longer-term trend: the overall direction the line is moving over years or decades. This is the signal scientists are most interested in.
Sea Level
The average height of the ocean surface at a specific location, calculated by smoothing out short-term variations like waves and tides.
Mean Sea Level
The average sea level calculated over a long time period (typically one year), used as the standard reference for measuring ocean heights.
Tide Gauge
An instrument positioned at coastal locations that continuously records the height of the water surface, providing long-term sea level measurements.
Radar Altimetry
A satellite-based measurement technique that uses microwave pulses bounced off the ocean surface to calculate the satellite's distance to the water.
Trend
The overall direction or tendency in data over time; in sea level data, the trend shows whether the ocean is rising or falling on climate timescales.
Thermal Expansion
The increase in volume of water as temperature increases; this is a major contributor to sea level rise as oceans warm.
Check Your Understanding
1. What is mean sea level?
2. Why did satellites revolutionize sea level measurement?
Try It: Identify the Trend
Here's how to practice identifying trends in sea level data. Imagine you're looking at a satellite sea level time series from 1993 to 2024, based on real data from TOPEX/Poseidon and Jason satellites.
The data shows measurements taken approximately every ten days at a location in the central Pacific Ocean. At first glance, the chart looks noisy—measurements jump up and down all over the place due to tides, storms, and other short-term variations. But when you step back and look at the overall pattern, you notice something important: the peaks of the oscillations are getting higher, and the troughs are getting higher too. The entire oscillating pattern is shifting upward over the thirty-one year period.
To identify the trend more clearly, imagine drawing a straight line through the middle of all the wiggles, representing the average direction over time. This is called a trend line, and it shows that sea level at this location has risen approximately 95 millimeters (about 3.7 inches) over the thirty-one year period. That's roughly 3 millimeters per year—a rate that matches the global average.
Now imagine you're looking at data from a location near a major ice sheet in Greenland. Here, the sea level change is much more dramatic. Over the same thirty-one year period, sea level might have risen 150 millimeters or more. This accelerated rise reflects the massive amounts of water entering the ocean as glaciers and ice sheets melt.
In contrast, at a location in the Pacific island of Kiribati, the sea level rise might appear less dramatic—only about 90 millimeters—when measured by satellite. But when you combine this with the fact that the land itself is sinking due to subsidence, the relative sea level rise (what actually matters for the island) is much more severe.
Key Vocabulary
Sea Level: The average height of the ocean surface at a specific location, calculated by smoothing out short-term variations like waves and tides.
Mean Sea Level: The average sea level calculated over a long time period (typically one year), used as the standard reference for measuring ocean heights.
Tide Gauge: An instrument positioned at coastal locations that continuously records the height of the water surface, providing long-term sea level measurements.
Radar Altimetry: A satellite-based measurement technique that uses microwave pulses bounced off the ocean surface to calculate the satellite's distance to the water, from which sea level height is derived.
Satellite Altimeter: An instrument on orbiting satellites that emits and receives radar signals used to measure sea level from space.
Trend: The overall direction or tendency in data over time; in sea level data, the trend shows whether the ocean is rising or falling on climate timescales.
Thermal Expansion: The increase in volume of water as temperature increases; this is a major contributor to sea level rise as oceans warm.
Subsidence: The sinking of land due to geological processes like groundwater extraction or post-glacial isostatic adjustment; subsidence can make relative sea level rise appear faster than the actual rise in ocean height.
Next: Level 2
You now understand what sea level is, how scientists measure it, and how to read basic sea level charts. In Level 2, you'll learn to identify patterns in sea level data more systematically. You'll practice distinguishing between short-term variations and long-term trends, and you'll begin to understand what causes these patterns. You'll also learn how artificial intelligence can detect patterns in vast datasets that would be impossible for humans to spot manually.
Ready to advance? Continue to Level 2: Identifying Patterns in Sea Level Data.