History of Sea Level
Scientists who study ancient climate have shown a close relationship between temperature and sea level reaching back millions of years. During the most recent Ice Age, for example, when the world was about eight degrees F colder than it is now and glaciers covered much of the Northern Hemisphere, sea level was as much as 400 feet lower than it is today. During the warm interglacial period that came just before the last Ice Age, global average temperature appears to have been just a few degrees warmer than today, and the seas were very likely at least 20 feet higher. The likely temperature difference is well within the range of temperature increases we could see this century due to climate change.
If you look more closely at the past 2,000 years, you can see that sea level also goes through some less dramatic changes. For the first 900 years A.D., it was very steady. Then it rose gradually for the next 600 years or so, then fell even more gradually until about 1900. But at that point — just as greenhouse gases and global temperatures began to increase significantly — sea level began to rise faster than at any time over the previous 2,000 years, and has climbed about eight inches over the past century. That rise, climate scientists agree, is likely to accelerate.
Next section: Sea Level Projections