A Primer On Sea Level Rise

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Sea level rise refers to the average increase in the water level of the Earth’s oceans. As ice sheets and glaciers melt, they add more water. A huge toolkit of increasingly sophisticated instruments, deployed across the oceans, on polar ice, and in orbit, reveals significant changes among globally interlocking factors that are driving sea levels higher. In the last decades, the reality of a rising sea due to the impact of human activities on the geological scale has gained wide scientific acceptance: Earth’s seas are rising as a direct result of a changing climate.

Ocean temperatures are increasing, leading to ocean expansion. You’d think that sea level rise would be constant across all the world’s major waterways, right? I mean, it’s like when you fill a bathtub with more water, the entire tub gains volume — isn’t that so?

Actually, no. Sea level rise is uneven due to ocean dynamics and Earth’s uneven gravity field. This has led to regions around the world addressing sea level rise in different ways that makes local life there more predictable and easier, and, so, human interpretations of sea level rise have varied widely.

The globally averaged trend toward rising sea levels masks deeper complexities. Regional effects cause sea levels to increase on some parts of the planet, to decrease on others, and even to remain relatively flat in a few places.

Why is Sea Level Rise Primarily a Human Construct?

Mean sea level is routinely used as a baseline for a variety of measures, including altitude. “We forget that sea level is far from a natural index, but a product of technically and culturally determined assumptions,” Wilko Graf von Hardenberg writes in Issues in Science and Technology. “We forget that sea level is far from a natural index but a product of technically and culturally determined assumptions.”

To understand humanity’s future challenges in an environment that is less and less comparable to the one that nurtured our species over millennia, it’s important to read appreciate the history of both oceanic sciences and surveying in a way that clarifies their relationship with the environments. New ways of understanding these relationships may be the necessary outcome of unprecedented environmental changes in the Anthropocene.

The use of sea level connotes lots of different meanings:

• a baseline for measuring change yet more than a mere boundary marker
• a reference point for altitude — in the eighteenth century it became increasingly common to relate elevations further inland to sea level
• a gauge of the current geological epoch, the Holocene, which began approximately 11,700 years ago
• a reminder of the instability created by climate change  — a material gauge of variations in the world’s climate

Mean sea level is one of many height reference points that exist concurrently with cultural appreciate for methods used to assess rates of change over time.  Sea level measurement was created within a specific social and cultural setting as a tool to make the world more legible, von Hardenberg continues.

“Sea level becomes quintessential in shaping the environment as we know it. Human cultural conceptions of what sea level is, which individual points should be singled out from the continuous curve of tidal movements, and how absolute and relative changes can be assessed are historical constructs that have a substantial impact on how humans imagine and frame the environment.”

Over the last two centuries, human ideas about sea level and how it varies have changed drastically. Algal growth on the foundations of Venice buildings, for example, has been a benchmark for the depth soundings undertaken to assess the impact of siltation and sedimentation for centuries. These markers are of an extremely local nature and only record the level of the tide in a single canal on a specific palazzo.

“Human cultural conceptions of what sea level is, which individual points should be singled out from the continuous curve of tidal movements, and how absolute and relative changes can be assessed are historical constructs that have a substantial impact on how humans imagine and frame the environment,” according to von Hardenberg.

The preeminence of the North Atlantic in historic sea level data is a peculiar product of people’s attempts to live and thrive in the littoral spaces of Western Europe. The subsequent rise of capitalism, the first wave of industrialization, and the growing infrastructural needs of nation-states “gradually divided land and sea into reciprocally alien worlds.” New notions of property, management, and control meant, von Hardenberg continues, that space became the object of “clear epistemic and legal categories.”

Phenomena characteristics of the North Atlantic, where most early measurements were taken, were superimposed on all the seas under the assumption that processes such as sea level rise would be uniform across the globe. We’ve learned since that other factors come into play when considering sea level rise:

• the gravitational pull exerted by land and ice masses
• the loss of attractive force caused by the melting of major ice masses
• global glacial melt, which will affect the regions around the equator more significantly

As humans degrade and alter the planet, sea level rise is already impacting widely different locations like Miami, the Netherlands, Bangladesh, and the island states of the South Pacific. Each location requires a separate and regionally-appropriate adaptive response.

Today’s Sea Level Rise Indicators

NOAA’s Climate.gov offers some important statistics about the nature and effects of sea level rise over the last 150 years.

Global mean sea level has risen about 8–9 inches (21–24 centimeters) since 1880. The rising water level is mostly due to a combination of melt water from glaciers and ice sheets and thermal expansion of seawater as it warms. In 2023, global mean sea level was 101.4 millimeters (3.99 inches) above 1993 levels, making it the highest annual average in the satellite record (1993-present).

In some ocean basins, sea level has risen as much as 6-8 inches (15-20 centimeters) since the start of the satellite record. Regional differences exist because of natural variability in the strength of winds and ocean currents, which influence how much and where the deeper layers of the ocean store heat.

The global mean water level in the ocean rose by 0.14 inches (3.6 millimeters) per year from 2006–2015, which was 2.5 times the average rate of 0.06 inches (1.4 millimeters) per year throughout most of the twentieth century. By the end of the century, global mean sea level is likely to rise at least one foot (0.3 meters) above 2000 levels, even if greenhouse gas emissions follow a relatively low pathway in coming decades.

Past and future sea level rise at specific locations on land may be more or less than the global average due to local factors: ground settling, upstream flood control, erosion, regional ocean currents, and whether the land is still rebounding or resettling from the compressive weight of vanished Ice Age glaciers. In the United States, the fastest rates of sea level rise are occurring in the Gulf of Mexico from the mouth of the Mississippi westward, followed by the mid-Atlantic. Only in Alaska and a few places in the Pacific Northwest are sea levels falling today.

Trends can reverse in the future if the world follows a pathway with high greenhouse gas emissions.