Ice and rising seas
Seas have risen about 20 cm since 1901 and are rising faster. Learn which ice raises them and which doesn't, and why the rise keeps going for centuries.
THE MEASUREMENT
About 20 cm since 1901, and speeding up
Tide gauges, and since the 1990s satellites, show global average sea level rose about 20 cm between 1901 and 2018. The pace has picked up: roughly 1.3 mm a year in the early 1900s against about 3.7 mm a year in 2006–2018, nearly three times faster.
3.7 mm a year sounds like nothing. Over a 30-year mortgage on a seaside flat, it adds up to about 11 cm, and the rate itself is still rising.
Two reasons the sea rises
Warmer water expands
Water takes up slightly more space as it warms. The ocean has soaked up most of the extra heat, so it swells. Since the 1970s this has been roughly half of the rise.
Land ice melts
Mountain glaciers and the great ice sheets of Greenland and Antarctica send meltwater and icebergs into the sea. Most of the rest of the rise, and in recent years the bigger part.
Check yourself
Soroush reads that Arctic sea ice has shrunk a lot and worries this is what's flooding coasts. Arctic sea ice is frozen seawater, already floating. What should he know?
- He's right: Arctic sea ice is the main source of sea-level rise
- Floating ice already displaces its weight, so melting it barely moves sea level
- Melting sea ice lowers sea level, because ice takes up more space than water
- Sea ice only matters for sea level when it melts during the summer months each year
Show the answer
Floating ice already displaces its weight, so melting it barely moves sea level
Right. It's the ice cube in a glass: when it melts, the level hardly moves. Sea ice loss still matters for warming, as you'll see, but the sea rises from warming water and land ice.
WHY SEA ICE STILL MATTERS
A white lid on a dark ocean
Arctic sea ice doesn't raise the sea, but it matters for warming. White ice reflects most sunlight; open dark water absorbs most of it. Late-summer Arctic sea ice is now smaller than at any time since at least 1850. Less ice means more dark water, which absorbs more heat, which melts more ice. That's a feedback, and lesson 8 is all about them.
Wear a white shirt and a black shirt in the same sun and you feel the difference within minutes. The Arctic is swapping one for the other.
Check yourself
Does it raise sea level?
- Greenland's ice sheet melting
- Arctic sea ice melting in summer
- The ocean warming and expanding
- An iceberg that's already floating melts
- Himalayan glacier meltwater reaching the sea
- Ice on the sea around Antarctica shrinking in summer
Show the answer
Raises sea level: Greenland's ice sheet melting, The ocean warming and expanding, Himalayan glacier meltwater reaching the sea
Barely changes it: Arctic sea ice melting in summer, An iceberg that's already floating melts, Ice on the sea around Antarctica shrinking in summer
Projected rise by 2100, compared with around 2000 (IPCC, rounded)
| If emissions are… | Likely rise |
|---|---|
| Cut very fast | about 30–55 cm |
| Cut slowly (middle path) | about 45–75 cm |
| Very high | about 65 cm to 1 m |
Close to 2 m by 2100 can't be ruled out under very high emissions, because ice sheets could break up faster than models expect.
THE LONG TAIL
The sea keeps rising after warming stops
The deep ocean warms slowly, and ice sheets take centuries to respond. So sea level is committed to keep rising for centuries, even if warming stopped today. What our choices control is how fast and how far: every bit of warming avoided means a slower rise that coasts have more time to adapt to.
It's like a big pot you've just taken off the flame: the water is still heating through at the bottom, and a thick block of butter beside it keeps melting for a while.
Check yourself
Mahsa says: "If the world stopped all warming tomorrow, the sea would stop rising within a few years."
Show the answer
False
False. The deep ocean would keep warming and expanding, and the ice sheets would keep losing ice, for centuries. Stopping warming doesn't stop the rise; it limits how fast and how high it goes, which is still hugely worth doing.
WHY CENTIMETRES MATTER
Floods start from a higher floor
Nobody drowns in 20 cm of gradual rise. The damage comes from high tides and storm surges that now start from a higher base, so they reach further inland. Floods that used to be rare become routine. What matters locally is relative sea level: the sea rising plus the land sinking. In some coastal cities, pumping too much groundwater makes the land subside faster than the sea rises.
Jakarta is the well-known case: parts of the city have sunk metres, mainly from groundwater pumping, while the sea rises too.
Check yourself
Sahar lives in Rasht, near the Caspian Sea. She asks whether global sea-level rise will push the Caspian up too. What's the right answer?
- Yes, by the same amount as the oceans, since all seas are connected underground
- Yes, even faster, because the Caspian is small and fills up quickly
- No, because the Caspian is too far from the poles for meltwater to reach
- No: it's a closed lake, set by river inflow and evaporation
Show the answer
No: it's a closed lake, set by river inflow and evaporation
Right. The Caspian has no link to the ocean. Its level is set by what flows in, mainly the Volga, and what evaporates, so climate affects it through rain and heat, not through global sea level.
Mountain ice: the water towers
- Mountain glaciers are shrinking almost everywhere on Earth.
- Many rivers depend on snow and ice that melt slowly through spring and summer, just when farms need water.
- As glaciers shrink, meltwater can rise for a while and then fall once there's less ice left to melt.
- Warmer winters also mean more rain and less snow, so water arrives earlier and runs off faster (more on water in lesson 9).
Check yourself
Match each place to its main ice or sea-level concern
Show the answer
- A port on the Persian Gulf → Tides and storm surges reaching further inland
- A town on the Caspian coast → A lake level set by rivers and evaporation
- A valley fed by mountain snowmelt → Less snow, and water arriving earlier in the year
- A coastal city pumping lots of groundwater → Sinking land adding to the sea's rise
Lesson recap
- Global sea level rose about 20 cm from 1901 to 2018, and the rate has nearly tripled.
- Two causes: warming water expands, and land ice (glaciers, Greenland, Antarctica) melts. Floating sea ice barely changes it.
- Shrinking Arctic sea ice still matters: dark water absorbs more sunlight, a feedback.
- The rise continues for centuries; emissions decide how fast and how far.
- Damage comes from higher tides and surges, and sinking land can make local rise much worse.