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High tides could be up to 4 centimetres higher in many places by 2100 as a result of the reduced mixing of ocean layers as seas get hotter, according to one of the first computer simulations of this effect.

These increases will be in addition to the overall rise in sea level聽and mean there will be a higher risk of flood defences being overtopped when storm surges coincide with high tides.

鈥淕enerally with coastal flood risk analysis and storm surge projections, the tide is considered static,鈥� says at the University of Rostock in Germany. 鈥淚t鈥檚 fairly new that people are looking at changes in tides.鈥�

The average global sea level has already risen by around 0.2 metres since 1880 due to the melting of ice on land and the expansion of the oceans as they warm, and it could rise by more than 1 metre by 2100.

This rise in sea levels is already resulting in more frequent flooding during high tides in some places around the world. But the tides are changing too, says MacPherson.

鈥淲e can鈥檛 just put a future storm surge on a present-day tide and expect it to be 100 per cent accurate,鈥� he says.

Changes in the shape of coasts due to sea level rise, erosion and human engineering can change tides at a local level. Changes in the density of the oceans also have an effect.

This is because there are layers of different densities within the ocean. Tidal forces produce waves within these layers, and these internal waves affect the surface tides.

As surface waters get warmer, however, they are less likely to mix with colder water below, leading to the density layers becoming more pronounced.

Observations and computer modelling suggest this stratification effect is already resulting in , according to preliminary work by another team. These changes occurring now vary from place to place but are mostly decreases.

So, MacPherson and his colleagues decided to model what would happen by 2100 under a high-emissions scenario. They found the effect varies from place to place but mostly resulted in increases, which could be up to 40 millimetres.

For instance, according to their results, tidal levels will increase on the east coasts of Britain and the US. The researchers presented their findings at a meeting of the European Geosciences Union in Vienna, Austria, last week.

The team members had expected to find overall decreases, because they thought more tidal energy would go into the waves within ocean layers and less into pushing surface waters around. They now plan further studies to try to understand why most tides increased in the computer model.

Because this area of study is so new, the results should be seen as preliminary, but they show this effect is significant, says MacPherson. 鈥淚t鈥檚 something that we should consider.鈥�