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The third reference point used is the climatic conditions during prehistoric periods of warmer climate.

For climatic data from before the oldest ice in Antarctica was deposited, we have extensive geological data showing both climate and sea level.

Studies of ancient sediments and coastlines have shown that the current 400 ppm CO2 content in the atmosphere is not unique. The same CO2 content also occurred around 3-4 (ref 6) and 20 million years ago. (Ref 7)

During these periods, the average temperature was between 2 and 5˚C higher than today, with 3˚C as a moderate average used in this book.

In the same periods, the sea level is found to have been between 10 and 40 meters higher than today.

This great variation in sea level is mainly due to the movements of the earth's crust.

The surface of the earth consists of a number of tectonic plates moving relative to each other by a few millimeter a year. Some millimeters per year add up to several km. over some million years.

These movements are not purely horizontal. Both the Alps and the Himalayas are examples of areas where collision between two plates have resulted in the formation of large mountain ranges. Since the earth does not grow when mountain ranges grow, there must be other areas where the surface will sink accordingly.

Due to this up and down movement of the earth's surface, it is natural that coastlines that were deposited around the world several million years ago will be found at different heights relative to today's sea level.

When this up / down movement of the surface is taken into account, 10 to 40m can be rewritten to 25 ± 15m, where 25m is average sea level above current, while ± 15m is local variation due to the up / down movement of the earth's surface.

A 25m higher sea level than today in earlier periods of 400 ppm CO2 in the atmosphere means that if all the remaining ice had then melted, it would have given an additional sea level rise of 70m (caused by melting of all current ice) - 25m = 45m. This figure is used in the calculations in the next chapter.