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Right now ocean tides are big and the next few million years will see them becoming bigger.

That鈥檚 because our scattered continents are surrounded by ocean basins that are just the right size to make really powerful tides. In contrast, whenever all Earth鈥檚 continents clump together into a single supercontinent, the tides go limp.

For the last 3 billion years, Earth鈥檚 continents have been conducting a stately dance. Roughly every 400 to 500 million years they cluster together into one giant supercontinent, before fragmenting again. The last supercontinent was Pangaea, which existed from 335 to 175 million years ago.

at Bangor University in the UK and his colleagues wondered if the position of Earth鈥檚 continents, and hence the sizes of the ocean basins, might affect the strength of tides. To find out, they studied predictions of how the continents will move over the next 250 million years, and modelled how those movements might affect the tides.

A high tide is a bulge in the water, caused by the gravitational pull of the Moon. The biggest 鈥渕ega-tides鈥� occur when the width of an ocean basin is a multiple of the width of the tidal bulge. This creates 鈥渞esonance鈥�: the Moon鈥檚 pull reinforces the way the water naturally slops about in that particular ocean, leading to a higher tide.

Future oceans

The team found that the current Atlantic basin has just entered a resonant phase.

鈥淭oday鈥檚 tides are unusually strong,鈥� says at Royal Holloway University of London in the UK. They may be three times as powerful as the average seen in Earth鈥檚 history.

What鈥檚 more, Atlantic tides may as much as double in strength over the next 5 to 10 million years.

In around 18 million years, the widening Atlantic will outgrow its resonant width. But in 50 million years, the narrowing Pacific will start to produce mega-tides as it too enters a resonant phase.

A quiet patch will follow, when the Pacific becomes too narrow to resonate. But then tides will rev up again when a newly formed Pan-Asian Ocean reaches a resonant width around 100 million years from now.

Finally, the closing Atlantic Basin will have one last resonant tidal hurrah, in around 150 million years. Then, from 200 million years onwards, a new supercontinent called Aurica will dominate. Ocean tides will enter 100 million years of slumber, with less than 40 per cent of the push-pull energy of today鈥檚 tides.

The power of tides

This could help to explain 鈥渁noxic鈥� episodes in the distant past, when the oceans became starved of oxygen. One anoxic event around 93 million years ago wiped out around one-third of marine invertebrates and finished off some of the swimming reptiles of the dinosaur era, like the ichthyosaurs.

鈥淲eaker tides mean the ocean is less mixed, which could lead to more oxygen-starved shallow water regions,鈥� says Green.

Ups and downs in tidal energy could also resolve a long-standing conundrum about the Moon.

Today鈥檚 strong tides are slamming the brakes on Earth鈥檚 spin, so the Earth and Moon are getting further apart. But if the two have always been separating at the current rate, it would imply the Moon is only 1.5 billion years old. That is odd, because moon rocks collected by the Apollo astronauts show that the Moon is around 4.5 billion years old. But if the tides were weaker in the past, the Earth and Moon would have moved apart slowly, which fits with the Moon鈥檚 true age.

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