Sea Level Rise
Just like a heated kettle boiling over, warming temperatures cause ocean water to expand and land-based ice to pour into the sea.
Definition Sea level rise is the gradual increase in the average height of the world's oceans, caused by the thermal expansion of warming seawater and the melting of glaciers and ice sheets on land. This rise puts coastal cities and low-lying islands at serious risk of flooding and submersion.
Warming Water Swells and Expands
If you fill a pot with soup to the brim and turn on the heat, you'll notice the liquid rising even before it starts boiling. When liquids warm up, their molecules move more vigorously and take up more space. This is called thermal expansion (the increase in volume caused by rising temperature).
The oceans work the exact same way. Due to global warming, Earth's vast oceans absorb more than 90% of the excess heat trapped in the atmosphere. Even if the water temperature increases by only a fraction of a degree, the sheer volume of the global ocean means that swelling adds up to a colossal amount of water.
In fact, about one-third to half of the sea level rise observed over recent decades isn't from melting ice, but from warmer seawater expanding on its own. The invisible push of expanding water molecules is actively lifting the surface of our global seas.
Land Ice Poses a Bigger Threat Than Floating Ice
Of course, melting ice pouring into the oceans is another major culprit. However, there is a crucial distinction to understand. When ice cubes float in a glass of water and melt, the glass doesn't overflow. That's because floating ice already displaces a volume of water equal to its own weight.
This means that when sea ice floating in the Arctic Ocean melts, it barely raises sea levels at all. The real problem comes from ice sitting on land, such as the massive ice sheets in Antarctica, Greenland, and mountain glaciers.
When immense amounts of freshwater locked on land melt and cascade into the ocean, it is just like pouring fresh water into an already full glass. As melting accelerates across Greenland and Antarctica, land ice is becoming the single most powerful driver pushing sea levels higher.
A Closer Look: Sea Levels Don't Rise Evenly Everywhere
When we talk about sea level rise, you might picture the global ocean filling up uniformly like water in a bathtub. In reality, it doesn't work that way. Due to Earth's rotation, prevailing winds, ocean currents, and variations in gravity, the rate of sea level rise varies significantly by region.
For instance, as massive Antarctic ice sheets melt, the local gravitational pull weakens, causing sea levels near Antarctica to drop while water piles up more around the distant equator. In coastal cities where ground subsidence occurs due to excessive groundwater pumping, the local sea level rise feels even more severe.
Even a rise of a few inches can cause seawater to spill onto streets during high tides and drastically worsen storm surges from hurricanes. Furthermore, saltwater seeping into coastal aquifers contaminates drinking water and ruins farmland, triggering complex cascading disasters.
๐ค Common misconceptions
Melting Arctic sea ice is the primary cause of sea level rise.
Sea ice floating in the Arctic Ocean already displaces water, so melting it does not noticeably raise sea levels. The true primary drivers are the thermal expansion of warming seawater and the melting of land-based ice sheets in Antarctica and Greenland.
๐งบ Where you meet it
Sea level rise is driven by two main forces: 'thermal expansion,' where warming seawater swells in volume, and the runoff of melting land-based glaciers and ice sheets.