The answer
Sea level would rise roughly 65 to 70 metres, flooding coastlines and many of the world's biggest cities, though it would take thousands of years.
Most of Earth's ice sits on land in two giant ice sheets: Antarctica and Greenland. Mountain glaciers hold a much smaller amount.
If all of Antarctica's ice melted, sea level would rise about 58 metres. Greenland would add about 7 metres, and all the mountain glaciers together less than half a metre. That adds up to about 66 metres; some estimates, including the USGS's, put it at about 70.
Ice that's already floating, like the sea ice at the North Pole, wouldn't raise sea level when it melts, just like melting ice cubes don't overflow a glass. Only ice on land adds new water to the ocean.
A rise like that would flood huge areas: much of Florida, the Netherlands and Bangladesh, and large parts of cities like London, Shanghai and New York. How fast this could happen is uncertain, but melting everything would take thousands of years. Today, sea level is rising about 4 millimetres a year and speeding up, partly from melting ice and partly because warmer water takes up more space.
What we know and what we don't
What physics tells us
- Melting all land ice would raise sea level roughly 65 to 70 metres (Antarctica about 58 m, Greenland about 7 m, glaciers well under 1 m).
- Melting floating sea ice does not raise sea level.
- Sea level is rising today, by about 4 mm a year.
Scientists aren't sure
- How quickly the ice sheets could melt. Scientists agree it would take thousands of years to lose them all, but the exact pace depends on future warming.
Go deeper
Why? Why doesn't melting sea ice raise sea level?
Floating ice already pushes aside its own weight of water. When it melts, it fills exactly the space it was taking up below the waterline. Land ice is different: it's sitting on rock, so melting it pours new water into the sea.
Why? Has Earth ever had no ice?
Yes. About 50 million years ago, Earth was much warmer and there were no big ice sheets at all. Crocodile relatives and palm trees lived near the Arctic.
Why? Is it all happening now?
Some of it is. Greenland and Antarctica are both losing ice, measured by satellites, and sea level has risen about 20 centimetres since 1900. Scientists are confident it will keep rising for centuries; how much depends on how much the planet warms.
Keep wondering
Sources
- How would sea level change if all glaciers melted?, USGS
- Ice sheet quick facts, National Snow and Ice Data Center
- Sea Level, NASA