What Causes Sea Level Changes? Short-Term and Long-Term Factors

Picture this: you’re standing at the beach, watching the tide roll in and out. The water rises, the water falls-it’s all very predictable, right? But what if I told you that sea levels are constantly shifting, sometimes over hours and sometimes over millennia? Understanding what drives these changes, from the daily ebb and flow to the long-term trends that reshape our coastlines, is crucial in a world where global mean sea level has risen about 8-9 inches since 1880.

Sea level change isn’t just one story-it’s many stories happening at once. Some causes play out in days or weeks, while others unfold over centuries. Let’s dive into what makes the ocean rise and fall, from the temporary to the transformative.

When the ocean breathes: Short-term sea level changes 🔗

If you’ve ever lived near the coast, you know the ocean has moods. Some days it’s calm and predictable, other days it seems to reach higher than usual, flooding parking lots and streets. These short-term fluctuations happen for several fascinating reasons.

The rhythm of tides 🔗

The most obvious short-term cause is, of course, the tides themselves. Twice a day, the gravitational pull of the moon and sun causes the ocean to bulge, creating high and low tides. But not all high tides are created equal. During certain times of the year, when the moon is closest to Earth or when the sun and moon align, we get what coastal residents call “king tides”-exceptionally high tides that can be several inches higher than normal. While tides are predictable, they create the baseline on which other short-term factors build.

Storm surges: When weather pushes water 🔗

Now imagine a powerful storm approaching the coast. As it moves across the ocean, something dramatic happens: the water level can rise by several feet or even meters. This is a storm surge, and it’s one of the most dangerous short-term sea level changes.

Storm surges occur primarily because strong winds push surface water toward the coast. As this water piles up against the shore, especially in shallow coastal areas, it has nowhere to go but up and inland. Think of it like pushing water in a bathtub toward one end-the water level rises dramatically at that end.

But wind isn’t the only player. The atmospheric pressure at the center of a storm also matters. In the eye of a hurricane or cyclone, the atmospheric pressure drops significantly. This low pressure allows the sea level to rise by approximately 1 centimeter for every millibar drop in pressure-scientists call this the “inverse barometer effect.” Lower pressure on the ocean surface means less force pushing down on the water, so it rises slightly. While this effect is smaller than wind-driven surge, it adds to the overall water level increase.

How the atmosphere squeezes and releases 🔗

Even without storms, atmospheric pressure changes can affect sea level over days or weeks. High-pressure systems push down on the ocean surface, lowering sea level slightly, while low-pressure systems allow it to rise. These changes are usually measured in centimeters rather than meters, but they’re part of the complex dance of forces that determine where the ocean surface sits at any given moment.

Ocean currents: Rivers within the sea 🔗

The ocean isn’t a static bathtub-it’s constantly moving. Ocean currents, driven by wind, temperature differences, and salinity variations, can cause water to build up next to the land, raising local sea level. For example, when certain currents slow or change direction due to seasonal patterns or weather variations, coastal sea levels can rise temporarily. These changes typically last from days to months and can vary by several centimeters.

The big picture: Long-term sea level drivers 🔗

While short-term changes grab headlines during hurricanes and floods, long-term changes are reshaping our world. These are the slow-motion transformations that occur over decades, centuries, and millennia.

Thermal expansion: The ocean gets bigger as it warms 🔗

Here’s a physics fact that has enormous consequences: water expands when it heats up. As global temperatures continue to warm, the ocean absorbs much of this heat, causing the water to expand and sea levels to rise. This process, called thermal expansion, has been responsible for roughly half of observed sea level rise over the past few decades.

The effect is more subtle than you might think. We’re not talking about water boiling-even a small temperature increase across the vast volume of the ocean translates to significant sea level rise. From the 1970s through the early 2000s, thermal expansion and melting ice contributed roughly equally to sea level rise. Today, however, ice melt has accelerated and contributes even more.

When ice returns to the sea 🔗

Imagine the weight of ice sheets up to three kilometers thick pressing down on Greenland and Antarctica. Now imagine that ice melting and flowing into the ocean. This transfer of water from land to sea is the most straightforward long-term contributor to rising sea levels-and it’s accelerating dramatically.

Ice loss from the Greenland Ice Sheet increased seven-fold from 34 billion tons per year between 1992-2001 to 247 billion tons per year between 2012 and 2016. Mountain glaciers worldwide are also retreating at unprecedented rates. Every time a glacier calves an iceberg or meltwater flows from an ice sheet into the ocean, global sea level rises just a tiny bit-but those tiny bits add up across the thousands of glaciers and ice sheets around the world.

The Earth still remembers the ice age 🔗

Here’s something remarkable: the land beneath our feet is still moving in response to ice sheets that disappeared 20,000 years ago. During the last ice age, massive ice sheets up to three kilometers thick covered much of North America and Northern Europe. The sheer weight of this ice pressed the land downward by up to half a kilometer.

When the ice melted, something interesting happened. The land that had been compressed started to slowly rebound upward-and it’s still rising today. This process, called post-glacial rebound or glacial isostatic adjustment, means that land around Hudson Bay and central Scandinavia is still rising by over a centimeter per year.

But there’s a flip side. Areas around the edges of former ice sheets, which had bulged upward during the ice age, are now sinking. This “forebulge collapse” is particularly problematic for places like Chesapeake Bay and much of the eastern U.S. seaboard, where sinking land compounds the effects of rising seas.

Tectonic movements and sediment shifts 🔗

The Earth’s crust isn’t just rebounding from ice-it’s also moving due to tectonic forces. Regions near active plate boundaries can experience uplift or subsidence that affects local sea levels. Additionally, sediment compaction, especially in river deltas and areas where groundwater or oil has been extracted, causes the land to sink gradually. In some parts of the Gulf Coast, this subsidence adds several millimeters per year to the effective rate of sea level rise experienced by coastal communities.

A century of change: Recent sea level trends 🔗

So what does all this add up to? The story of the last hundred years tells us a lot about where we’re headed.

The acceleration we can measure 🔗

The rate of global sea level rise has more than doubled from 0.06 inches per year throughout most of the twentieth century to 0.14 inches per year from 2006-2015. That acceleration is the fingerprint of climate change. As greenhouse gas concentrations increase, the ocean absorbs more heat (thermal expansion) and more land ice melts-both processes feeding into faster sea level rise.

In 2023, global mean sea level reached a new record high, standing 101.4 millimeters (nearly 4 inches) above 1993 levels. That might not sound like much, but consider this: high-tide flooding is now 300% to more than 900% more frequent than it was 50 years ago in many U.S. coastal communities. Small changes in baseline sea level have enormous impacts on flood frequency.

Not all coasts are created equal 🔗

Here’s where it gets complicated: the ocean doesn’t rise uniformly everywhere. Some regions experience sea level rise faster than the global average, while others see slower rise or even falling sea levels. In Alaska and parts of the Pacific Northwest, ongoing post-glacial rebound causes land to rise faster than sea level, resulting in a net decrease in local relative sea level-for now.

Meanwhile, the Gulf Coast experiences some of the fastest sea level rise rates in the United States, driven by a combination of global sea level rise, land subsidence, and changes in ocean circulation. The same forces that push water around the globe-winds, currents, and the Earth’s rotation-create regional differences that can be substantial.

The human fingerprint 🔗

While natural processes like post-glacial rebound still play a role, the recent acceleration in sea level rise bears the unmistakable signature of human influence. Burning fossil fuels has warmed the planet, causing both thermal expansion and accelerated ice melt. Extracting groundwater and oil causes land to subside in some regions, effectively raising local sea level even faster.

Looking ahead, projections suggest that even with significant reductions in greenhouse gas emissions, we can expect at least another foot of global sea level rise by 2100. On a pathway with high emissions, sea level could rise by 7 feet or more by century’s end-enough to reshape coastlines and displace millions of people.

What do you think? How might your community be affected by ongoing sea level changes? And what steps do you think we should take to prepare for a future with higher seas?

References

1. https://www.climate.gov/news-features/understanding-climate/climate-change-global-sea-level
2. https://www.noaa.gov/explainers/tracking-sea-level-rise-and-fall
3. https://education.nationalgeographic.org/resource/storm-surge/
4. https://en.wikipedia.org/wiki/Storm_surge
5. https://en.wikipedia.org/wiki/Post-glacial_rebound
6. https://www.antarcticglaciers.org/glaciers-and-climate/sea-level-rise-2/recovering-from-an-ice-age/