En-en adult card 22 sea level rise
Card #22: Rising Sea Levels
Since 1900, sea levels have risen by 20 cm. This is caused by the thermal expansion of ocean waters and the melting of glaciers and ice sheets.
It is important to note that the forecasts for the rise in water levels are very conservative. Some phenomena, understood qualitatively but not quantitatively, are simply not quantified in the IPCC report.
This is the case for moulins, for example. Moulins, or glacier mills, are shafts that carry melted water from the surface of a glacier or an ice sheet down to the bedrock. Once the water enters these passages, it lubricates the contact between the bedrock and the ice sheet, making it easier for the glaciers to drift towards the sea.
The figures for sea level rise will therefore most likely be revised upwards in future reports.
For the US, you can use the Sea Level Rise Viewer to show the extent of the phenomenon.
- Melting Sea Ice is not responsible for the rise of sea levels. More information on the link explanation page.
To go further
- Water hardly expands. How can ocean warming of a tenth of a degree result in a rise of the water level? The quick answer is that the ocean is 4,000 metres deep on average, so a very slight expansion is enough to make it a few centimetres deeper. A more complete answer is to talk about water's expansion coefficient. It happens to depend on the temperature. Between 0 and 4°C, it is negative, i.e. the water contracts when it warms up. This value of 4°C is precisely the value of the temperature at the bottom of many lakes. This is logical because water that is either colder or warmer is lighter than water at 4°C. It is at 4°C that the water is the densest, so it ends up at the bottom. If we look at the temperature of the ocean, over all latitudes and at all depths, it varies for the most part between 0°C and 10°C, with an average value probably around 4°C. So in theory, around this value, the expansion coefficient is zero. So how much can the ocean expand? In reality, it is in areas where the water temperature is higher that the ocean expands. When the water reaches 20°C, or even much higher temperatures, several tens of metres deep, then there is reason to see water dilation in these areas. We can then imagine how difficult it is to calculate the rise in the water level: to do so, we need to know with great precision the distribution of water temperatures, including at depth, and also to know the temperature rise in certain areas.
- Another important point: the rise in water levels is due both to an increase in its mass (melting of glaciers and ice caps) and to an increase in volume (expansion of water). This increase in water volume is not homogenous: it takes place in areas where water expands, i.e. in warm areas. The water level is not horizontal! Hot water floats in relation to cold water, it's just that we rarely have the opportunity to check it.
- The melting of the cryosphere (ice caps and glaciers) has become the dominant factor in sea level rise.
About 11% of the world's population lives below 10 metres, and these areas produce 14% of domestic products.
- Valérie Masson-Delmotte, Conference in Lyon, 58'51 https://www.youtube.com/watch?v=73qkhnhbyGI&feature=youtu.be (French)
- IPCC, Special report on cryosphere and oceans