The level of the ocean is almost constantly changing, depending on natural variations in the Earth’s climate. As the climate warms and cools, the melting and freezing of water on super-enormous scales leads to rises and falls in sea-level. However, since the beginning of the 19^th century the rate at which sea levels have been rising is faster than it has been for thousands of years. Since 1901 the average sea level has increased by about 20 centimetres, and continues to rise at a rate of about 2 mm per year, largely due to man-made climate change which has raised global temperatures by, on average, half a degree. This has melted ice from the land and also led to “thermal expansion” of seawater (as seawater gets warmer it expands). Scientists think it is very likely that this has increased the impact of coastal storm surges and that it has led to more severe flooding. But how far could sea levels possibly rise in the future?The three major sources of frozen water on the planet are in the Arctic, the Antarctic, and Greenland. As the Arctic is just floating ice and snow, sea levels would not rise if it melted (in the same way that your drink does not overflow when the ice cubes in it melt). If all the ice on Greenland melted, sea level would rise by 7 meters. If all the ice Antarctica melted, then sea levels would rise by a terrifying 61 meters. Thankfully, the whole of the Antarctic is unlikely to melt any time soon as the average temperature there is less than -30⁰C. Greenland is a lot more vulnerable as average winter temperatures are between about -5 and -20⁰C, and temperatures in summer are above freezing.

It is impossible to say how far sea levels will rise in the future, because there are many things we do not know, such as how much CO₂ humans will continue to release into the atmosphere and for how long. So what scientists do is make predictions based on different scenarios (some optimistic and some more pessimistic). Under all of these scenarios scientists predict that, in the next 100 years, the rate of sea level rise will continue to increase, possibly up to 16 mm per year. The best case scenario is that sea levels rise by about 20 cm by 2100, but under the worst case scenario almost 1 meter of sea level rise would be possible. This may not sound like a lot, but this would put many coastal regions underwater. In the USA, about 10% of coastal land is less than 1 meter above sea level. Whichever scenario turns out to correct, one thing is certain: coastal areas will increasingly experience submergence, flooding and erosion throughout the 21st century and beyond.

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This post is an excerpt from “Do Fish Sleep?: and 38 other ocean mysteries”, by David Aldridge. It is available to buy on kindle here for the reduced  price of $1.46 (99p) until the end of January. To read it, you do not need to own a Kindle device, just the Kindle App which is a free download for smartphones or tablets. It can also be read with“Kindle Cloud Reader” on a PC or Laptop.