It’s indisputable: Around the world, seas are rising at a faster rate than at any time in recorded history. On average, sea levels are up to 9 inches higher than they were in the late 1800s — and climate change is to blame.
But there’s more to this story than you might realize. Here are a few facts about sea-level rise that you might not know.
1. It’s not just the melting ice.
Most of us know that rising global temperatures are melting glaciers and ice sheets, causing seas to rise. But that’s only part of the picture. There’s another, less visible driver: a process called thermal expansion. As the ocean absorbs heat, its molecules move faster and spread out, making the ocean swell in size even though no extra water is added.
This phenomenon was behind nearly three-quarters of sea-level rise during the 20th century. And between 1993 and 2010, ocean warming caused about a third of the total rise we saw during that time.
So yes, melting ice matters. But the oceans themselves are also swelling with heat.
2. Seas aren’t rising at the same pace around the world.
It might seem like all the oceans would rise evenly — after all, they’re connected like one giant bathtub.
Of course, your bathtub probably doesn’t have shifting currents and stormy weather, which influence how ocean water moves — and where it ends up.
But the biggest factor by far is gravity, which has some unexpected pull:
Earth’s mass isn’t spread out evenly. Big features — like mountain ranges, dense rock and especially ice sheets — create stronger gravitational pulls. Ice sheets, in particular, are so heavy they actually draw ocean water toward them, raising sea levels in nearby regions.
But when those ice sheets melt, they lose mass — and their gravitational pull weakens. The water that was once drawn in starts to move away. Ironically, this means places close to melting ice, like Greenland or Antarctica, may see sea levels drop — while places farther away, like the U.S. East Coast, end up with more of that water and higher seas.
3. Some islands are in trouble — but not for the reason you might think.
In the western Pacific, sea levels are rising two to three times higher than the global average.
But experts say some Pacific islands won’t become unlivable because they’re underwater — they’ll run out of fresh water first.
Water is scarce on many low-lying Pacific islands to begin with. Most rely on a razor-thin “lens” of fresh water trapped underground beneath the island. This freshwater literally floats on top of salty seawater — sometimes it’s just a few inches thick.
But as seas rise, they push up the salty water beneath the island, squeezing and flooding this fragile lens. Saltwater can seep into wells or break through the surface, making the island’s water too salty to drink.
Scientists believe this will happen long before these islands are actually submerged. A U.S. military-funded study found that more than 1,000 low-lying islands could become uninhabitable by 2050 — not from flooding, but from thirst.
Pacific Island countries know what’s coming and are taking actions to prepare themselves and improve their water security. But it’s a race against time.
4. Seas could rise too fast for mangrove forests to keep up.
Mangrove forests are tough. They thrive where few trees can — right at the ocean’s edge, rooted in salty water and soft mud. They buffer coastlines from storms, shelter marine life and store massive amounts of carbon in their tangled roots.
But even mangroves can’t take everything the sea throws at them.
Recent research shows that in some parts of the world, seas may be rising faster than mangroves can adapt. These forests usually survive by trapping sediment in their roots and building themselves upward — or slowly creeping inland. But when the water rises too quickly, or when development blocks their path, mangroves can drown.
Without space to move or time to grow, even these salt-tolerant survivors may be overwhelmed.
Further reading: 11 facts you need to know about mangroves
5. We can’t stop sea-level rise entirely — but we can shape what happens next.
Some sea-level rise is now locked in. Even if we stopped burning fossil fuels tomorrow, the oceans will keep rising for centuries due to the heat already stored in the system.
But what we do today still matters. Cutting emissions, protecting coastal ecosystems like mangroves, and giving communities time and resources to adapt can mean the difference between manageable impacts — and devastating ones.
Want to support work that protects coastlines and the people who depend on them? Here’s one way.
Will McCarry is the content director at Conservation International. Want to read more stories like this? Sign up for email updates. Also, please consider supporting our critical work.
