Earth’s tides are shaped by more than the Moon. And they’re changing as the planet warms

Earth's tides are driven by more than just the Moon's gravitational pull, and they are changing alongside our warming planet. The Horizontal Falls in Western Australia's Kimberley region demonstrate the dramatic impact of tides, with some coastlines experiencing tidal ranges exceeding ten meters – roughly the height of a three-story building.

The Moon and Sun are the primary sources of Earth's tides. Although the Sun is significantly more massive than the Moon, its greater distance results in a weaker influence. The Moon's gravitational pull is strongest on the side of Earth closest to it, and weakest on the far side, creating a stretching effect on both the solid Earth and oceans.

This stretching generates two broad tidal bulges, one on the side facing the Moon and another on the far side due to the Moon's stronger pull on Earth's center compared to its pull on the distant water. As Earth rotates, different locations move into and out of these regions of higher sea level, leading to two high tides and two low tides each day in most places.

The Sun also contributes to tides, although in a less significant manner. When the Sun, Moon, and Earth align, their gravitational forces reinforce each other, resulting in larger tidal ranges called spring tides. Conversely, when the Sun and Moon form a right angle, their effects partially cancel each other out, leading to smaller tidal ranges, or neap tides.

However, the simplistic two-bulge model does not fully capture the complexity of tides. In reality, continents and underwater topography interrupt the oceans, causing seawater to move within ocean basins. In many cases, tides rotate around amphidromic points, where the tidal rise and fall nearly equals zero. Coastal geometry can further amplify tides through resonance.

When the natural rhythm of a bay matches the local tidal rhythm, the movements reinforce each other, contributing to exceptionally large tides in locations like the Kimberley coast and Canada's Bay of Fundy.

Additional factors, such as underwater mountains, ridges, and internal tides, also influence tide patterns. Internal tides are generated when tides flow over rough seafloor, converting some of their energy into waves within the ocean. These internal tides can travel thousands of kilometers and exert forces on surface tides, either removing or adding energy depending on their timing. Current tide models often overlook these springlike effects, but incorporating them could enhance model accuracy and efficiency.

As Earth's climate changes, so too do our oceans, with rising sea levels and altered coastlines. These changes can impact tide sizes. Evidence suggests that tides are indeed changing, and improved ocean models are increasingly capturing internal tides, as well as providing better measurements in complex coastal regions. Understanding how tides are changing is becoming crucial for coastal communities.

For instance, at Lakes Entrance in Victoria, Australia, changes in tidal range have been the primary cause of increased flooding, prompting a $4 million investment in drainage upgrades to protect homes and businesses. Better predictions of future tidal changes can help communities prepare for flooding, mitigate flood damage, and plan coastal development more safely.