Sea Level Rise

You've probably seen a pot filled to the brim with soup rise up before it even boils. When a liquid warms, its molecules move around more, and its volume grows. This is called thermal expansion (volume increasing as temperature rises).

The ocean works on the same principle. About 90% of the extra heat trapped in the climate system by global warming is absorbed by the vast ocean. Even a tiny rise in seawater temperature adds up to an enormous amount of swelling, because there is so much water.

In fact, roughly a third to a half of the sea level rise over the past few decades came not from melting ice but from warmed seawater swelling on its own. The expansion of invisible water molecules is lifting the whole ocean.

Of course, ice melting and flowing into the sea is also a huge cause. But here you need an important distinction. When ice floating in a cup of soda melts, the soda doesn't spill out of the cup. Ice that is already floating pushes aside water equal to its own weight.

So sea ice floating on the ocean, like that in the Arctic Ocean, barely raises sea level when it melts. The real problem is when ice resting on land melts, such as in Antarctica, Greenland, and high mountain ranges.

When a huge amount of fresh water locked up on land melts and pours into the sea, it's like pouring fresh water into the cup from outside. The great ice sheets of Greenland and Antarctica and mountain glaciers are melting faster, and in the 21st century, melting land ice has become a bigger factor than thermal expansion.

When we say sea level rise, it sounds like all the world's oceans fill to the same height, like a bathtub. In reality, they don't. Because of Earth's rotation, winds, ocean currents, and differences in gravity, the rate of rise differs from place to place.

For example, when the giant Antarctic ice sheet melts, gravity weakens around it, so the sea near Antarctica can actually drop, while the ocean far away near the equator swells even more. On top of that, in coastal cities where the ground itself is sinking because of excessive groundwater pumping, the rise people actually experience is much larger.

Even a rise of a few dozen centimeters can send seawater flooding into a city at high tide and greatly worsen storm surges driven by typhoons. Salt water also seeps into groundwater and ruins drinking water and farmland, so it turns into a compound disaster.