in the Deep Future
The most compelling reason to worry about sea level rise in the future comes from sea level estimates from the past. Ancient coral reefs and relic beach deposits attest to large changes in the sea level associated with past climate changes. Periods of low sea level, such as the last glacial maximum, are more difficult to document in this way because the relic seashore is now submerged under more than a hundred meters of water. In some locations such as Barbados, the land surface itself is rising out of the ocean faster than the flooding of the ocean, leaving the traces of the ancient shorelines exposed for scientific inspection. As described in Chapter 4, oxygen isotopes in CaCO3 shells deposited in the deep ocean also carry information about ancient sea levels.
Sea level during the last glacial time, 20 millennia ago, was about 120 meters lower than today (Chapter 5). Most of that missing water was tied up in the North American and European ice sheets. The temperature at this time was about 5–6°C colder than today, on a global average. Figure 17 shows the difference in temperature on the horizontal axis, and the change in sea level on the vertical axis, for this and other climate changes past and future.