The Horizontal Falls are an unusual natural phenomenon on the coast of Western Australia's Kimberley region, where tidal flows cause waterfalls on the ebb and flow of each tide. James D. Morgan/Getty ImagesIn some parts of the world, the sea barely rises and falls with the tides. In others, the difference between high and low tide is dramatic. Along Western Australia’s Kimberley coast, for example, it can exceed ten metres – roughly the height of a three-storey building.Ocean tides shape coastlines and marine ecosystems, affect coastal communities, and even influence the melting of Antarctic ice shelves. But why are tides so large in some places and so small in others? And as our oceans change, will the tides change too?The pull of the Moon and SunThe Moon and the Sun both produce tides on Earth through their gravitational pull. Although the Sun is far more massive, the Moon is much closer, so it has a stronger influence on our ocean tides.The Moon’s gravitational pull decreases with distance. It is strongest on the side of Earth that’s closest to the Moon, and weakest on the far side. This difference stretches both the solid Earth and oceans. But because water moves more freely than rock, the effect is most visible in the ocean. Ocean tides are the regular rise and fall of sea level driven by the gravitational pull of the Moon and the Sun. This video shows tidal elevation amplitude simulated by a global tide model around Australia (green equals higher than mean sea level while blue equals lower than mean sea level) In a simplified picture, this creates two broad tidal bulges. One forms on the side facing the Moon. A second forms on the far side because the Moon pulls Earth’s centre more strongly than it pulls the more distant water.As Earth rotates, different locations move into and out of these regions of higher sea level. This helps explain why many places experience two high tides and two low tides each day.The Sun produces a similar but weaker effect. When the Sun, Moon and Earth line up, their effects reinforce one another, producing larger tidal ranges than usual – known as a spring tide. When the Sun and Moon form a right angle as viewed from Earth, their effects partly offset one another, producing smaller tidal ranges or a neap tide. The size of Earth’s tides change depending on the position of the Moon and the Sun. ttsz/Getty Images More than gravityTides measured by coastal gauges and satellites look very different from this two-bulge picture. Continents interrupt the oceans, so seawater instead moves back and forth within ocean basins.In many ocean basins, tides rotate around points where the tidal rise and fall is almost zero, known as amphidromic points. Tidal ranges generally become larger farther away from these points.Coastal geometry can amplify tides further through a process called resonance. Each bay has a natural rhythm at which its water sloshes back and forth, determined by its shape and depth. When this rhythm matches that of the local tides, each movement reinforces the next.Resonance contributes to the exceptionally large tides of the Kimberley coast and Canada’s Bay of Fundy, where the tidal range can reach 16 metres.Underwater mountains and ridges also affect tides. When tides flow over rough seafloor, some of their energy is converted into waves within the ocean, known as internal tides. The complex seafloor of the Indonesian archipelago, for example, generates powerful internal tides. When internal tides break, they can drive ocean mixing, helping distribute heat and nutrients.Some internal tides break near where they form, while others travel thousands of kilometres. As they travel, they exert forces on surface tides. Depending on their timing, these forces can behave like a brake or a spring – either removing energy from the tide like a brake, or taking energy in one part of the cycle and giving it back later like a spring. Most tide models overlook this springlike effect. Our recent study showed that including this effect allows models to reproduce observed tides accurately without directly simulating internal tides, which can take a lot of costly computer power.This improvement could make models more efficient and reliable for reconstructing past tides – offering clues to how Earth’s rotation and the Moon’s orbit have evolved – and predicting future tides.Tides in a changing oceanIsaac Newton explained tides using gravity in 1687. More than three centuries later, scientists are still uncovering how tides evolve with our changing oceans.Climate change is raising sea levels and altering coastlines. It is also warming the upper ocean and making the ocean more stratified. These changes can affect the size of tides.Measurements already suggest that tides are changing. Changes in ocean stratification may help explain some of these changes. Meanwhile, improved ocean models are capturing internal tides in greater detail, and new satellites can measure tides in complex coastal regions that were previously difficult to observe from space.For coastal communities, understanding how tides are changing is increasingly important. At Lakes Entrance, one of Victoria’s most climate-exposed coastal communities, changes in tidal range have been the main cause of increased flooding. The local council is now seeking $4 million for drainage upgrades to protect homes and businesses and support future development. Better predictions of future tidal changes could help communities prepare for flooding, reduce flood damage, and plan future coastal development more safely.Luwei Yang receives funding from the Australian Research Council. Callum Shakespeare receives funding from the Australian Research Council.