Sea levels are rising. They rose by 1.7 millimetres a year on average during the 20th century but the process is speeding up. Since the beginning of the 21st century, sea levels have risen by 3.1 mm a year.
There are two almost equally important reasons for this phenomenon. The first is that as they warm up, oceans dilate. The second is that terrestrial glaciers – the Andes, the Himalayas and the Alps for example – melt, and the water flows into the rivers and oceans. Contrary to popular opinion, the melting ice floe is not threatening our coasts. This paradox is called the ice cube effect: if a glass containing a few ice cubes is filled to the brim, it will not overflow when the ice melts.
If these terrestrial glaciers melt completely, they will make sea levels rise by about 50 centimetres. This is not much compared to what would happen if the ice in Antarctica and Greenland were to disappear. Sea levels would rise by 56 and 7 metres respectively!
Rising sea levels make coasts recede to differing extents depending on whether the phenomenon occurs near beaches or cliffs. On average, a one centimetre increase makes the coast recede by a metre but it can be much more. Thus, in Louisiana, the coast has receded by almost one metre a year over the past few years. Also, the sea is not still .Hot and cold currents move water masses and expand and contract oceans: certain areas are several metres higher than others. Similarly, the current increase of sea levels is very uneven.
The expansion of oceans is a phenomenon that has a lot of inertia: it will take quite a while to get started. Thus, all of Greenland’s ice is not expected to melt before at least a century, if all of it does indeed melt. But this also means that it will stop slowly: even if we stop producing greenhouse effect gases, sea levels will continue to rise for a long time… We must therefore anticipate the problem.