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Sea Level: Past, Present and Future

Sea-Level-1

Earth from space today. Mouse over to see what Earth looked like in the depths of the last ice age, about 20,000 years ago. Millions of billions of tons more ice was frozen at the poles and sea level was 400 feet lower.

Images by Daein Ballard

The amount of water on Earth is essentially fixed. However, the amount of water in the oceans changes dramatically depending on whether the planet is hot or cold. When Earth is cold, such as during an ice age, billions of tons of water freezes in glaciers on mountaintops and at the poles, and sea level around the world drops. When Earth is hot, in contrast, this ice melts, ocean basins swell with the extra liquid, and the sea rises. If Earth were so hot as to be glacier-free, as it was about 40 million years ago, sea level would go up by about another 250 feet.

Most land-based ice is frozen at the poles in Greenland and in the East and West ice sheets of Antarctica. But, because the poles are so cold, most of the ice melting at present originates from mountain glaciers in lower latitudes. Due to their relatively small size, these glaciers have a relatively limited impact on sea level. All the world’s mountain glaciers—including ice on the summits of the Andes, the Himalayas, the Alps and the mountains of Alaska—contain only enough water to raise the level of the sea by about two feet.  The polar ice sheets, by comparison, are about 100 times bigger.

Today, Earth is warming and the seas are rising. One cause is the expansion of water as the ocean warms up. This effect, contributing to about half of current sea level rise, will play an increasingly smaller portion of the rise as further warming leads to faster melting of the vast polar ice sheets. The future of all coastal land on Earth depends on how much and how fast these polar ice sheets melt.