During these periods high tides are a little higher than usual and low tides are a little lower than usual. Twice a month, when the sun, the moon and Earth are nearly in alignment, Earth experiences “spring” tides. Each tidal cycle causes coastal waters to rise and fall in a predictable pattern, dictated by the orbits of these bodies. Tides come in several flavors, all ultimately tracing back to interactions between the moon, the sun and Earth. “It wasn’t really until about 10 years ago that we started to appreciate that it’s occurring on a much wider level,” says Haigh. (After the weir was built, the tides did increase about as much as they were expecting.) But only recently have scientists collected modern, precise tide gauge data from around the world, showing just how widespread tidal changes have become. In 1899, builders predicted that tides would increase in the Ems River upstream from a weir they planned to construct. Those tides rhythmically submerged and revealed the Rising Tide sculpture, underwater artist Jason deCaires Taylor’s 2015 installation at Vauxhall in London.ĬREDIT: THAMES FESTIVAL TRUST Tides of historyĮngineers have known for at least a century that tides can change locally. The tidal range along the Thames River has quadrupled since the time of the Romans, as engineers narrowed and deepened the river for navigation. “What people don’t realize is that if tidal range is increasing, it will exacerbate that even more,” says Ivan Haigh, an oceanographer at the University of Southampton, UK. Changing tides could add to that problem and leave some coasts at even greater risk of flooding. In many coastal cities, seawater is now lapping higher than it ever has. As people burn more fossil fuels and put more heat-trapping greenhouse gases into the atmosphere, global warming is melting ice caps and causing the volume of the oceans to expand. Perhaps the biggest challenge is how changing tides might add to the risks of sea level rise. Either way, the shifting tides have big implications for hundreds of millions of coastal residents. The nature of those shifts is complicated: In some locations the tidal range grows more dramatic, whereas in others it shrinks. Dredging river channels like the Ems or filling in coastal wetlands can trigger shifts. Rather, people are causing these changing tides. But oceanographers have recently started to realize that tides in many places around the world are undergoing notable changes, in ways that can’t be explained by interactions among celestial bodies. Most people think of tides as regular and predictable - the rise and fall of coastal waters, caused by the gravitational tug of the moon and the sun, forecast down to the minute with a mariner’s tide table. “That’s why we have tide tables.” He was amazed to discover, he says, that not only could tides undergo long-term changes, but that they could change by so much. “I had always assumed tides were constant,” says Talke, now an oceanographer at California Polytechnic State University in San Luis Obispo. Over the last 120 years the tidal range - the distance between high and low tide - has quintupled in the Ems estuary. Those shifting tides stirred up sediment from the river bottom and muddied its waters. Physical World A deeper understanding of the Grand Canyonīut those changes also changed the rhythm in which tides ebbed and flowed into the river from the sea.
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