Forget the ice-caps melting, the simple fact that water expands with higher temperatures could raise the sea levels considerably. Here I use back-of-the-envelope calculations to estimate this effect.
By 2050, the world is estimated to warm by 2C compared to the pre-industrial average, and some models predict that by 2100 the world could be 5C hotter than it was. [1] Currently the average temperature of the world is around 16.1C.[2] This would mean that for a 2C warmer planet, the temperature would change by 0.7% when compared to absolute zero (0 Kelvin).
In the range of 16C, water expands about linearly with temperature. A 2C increase will expand the ocean waters around 0.1%, but how much of the ocean will feel actually this temperature change?
We know that the top layer of the world’s oceans mix well, but the bottom is significantly calmer. These two layers are separated by the thermocline, where the temperature drops significantly. Below this layer, the temperature of the ocean is quite stable, and above it, the effects of the surface temperature are felt. This is especially evident from the seasonal changes in temperature of the ocean at various depths. While below the thermocline the temperature of the ocean does not change with season, above it, the oceans warm in the summer and cool in the winter. While this mixed layer varies by latitude and by season, it ranges from 10 to 200 m in depth, with a global average of around 50-100 meters. [3]
Modeling the world’s oceans as a thin spherical layer on top of the planet, the height increase would be equal to the expansion percentage times the mixing layer depth. This would amount to a 5-10cm increase for a 2C change, and a 13-25 cm increase for a 5C change in temperature. With 30% of the world living in low-lying coastal regions, this increase would be both environmentally and economically devastating. [4]
This estimate, of course, is quite rough as the mixing layer variability in different regions of the world and across seasons is quite high. Furthermore, global warming is predicted to change the mixing layer depths, which would affect this calculation. [5]
[1]https://www.nature.com/articles/d41586-020-01125-x
[2]https://www.ncdc.noaa.gov/sotc/global/201713
[3]https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2004JC002378
[4]https://blog.ucsusa.org/guest-commentary/what-is-the-cost-of-one-meter-of-sea-level-rise
[5]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5699439/
Melis Tekant 