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| Earth on Aug 23, 2011

The tides are a changing

Scientists have discovered that ocean tides along the eastern coast of North America were substantially different 10,000 years ago.

Ocean tides are often considered one of the more stable and predictable forces of nature, but a new scientific study has discovered that tides have changed substantially since prehistoric times and may change again in the future.

The term “tides” refers to the alternating rise and fall of the surface level of large bodies of water caused by the effects of gravity exerted by the sun and the moon on the Earth. Most coastal areas experience a semidiurnal tidal pattern consisting of two high tides and two low tides every day. However, a few coastal areas have diurnal tides consisting of one high tide and one low tide every day. High tides become especially pronounced during the full moon and the new moon and are called spring tides.

Illustration of tides. Image Credit: Wikimedia Commons.

In 2011, scientists from Oregon State University, the University of Pennsylvania, the University of Toronto, Tulane University and the University of Leeds completed a research project that details how tides have changed along the east coast of North America over the past 10,000 years. To conduct their research, the scientists used a high-resolution ocean model to reconstruct ancient tides at 1,000 year intervals from the end of the last glacial maximum up until the present day.

Their findings offer compelling evidence that tides can and do change.

Specifically, the scientists calculated that around 8,000 to 9,000 years ago tides were much higher along many coastal areas in the United States – a difference between low and high tide of 10 to 20 feet (3 to 6 meters) versus the current tidal range of 3 to 6 feet (1 to 2 meters). The scientists suspect that the large amplification in tides during the end of the last ice age was due to the lack of the extensive continental shelf system that exists today. Continental shelf systems have large areas of shallow, overlying water that can act to dissipate incoming tidal energy before it reaches the shoreline.

Interestingly, the scientists calculated that tidal conditions around the Bay of Fundy in Canada were much smaller 6,000 to 7,000 years ago than they are today. Presently, tidal ranges in the Bay of Fundy are the highest in the world and approach 40 feet (12 meters).

Hopewell Rocks in the Bay of Fundy, Canada were formed by tidal erosion. Image Credit: Martin Cathrae.

While the featured scientific study on tides isn’t the first to propose and analyze prehistoric changes in tidal patterns, the study is the first to do so at such high levels of resolution. The scientists are hoping that their results will be more widely incorporated into other scientific disciplines in the future.

In a July 29, 2011 press release, lead author David Hill, associate professor in the school of civil and construction engineering at Oregon State University, commented that:

Scientists study past sea levels for a range of things, to learn about climate changes, geology, marine biology. In most of this research it was assumed that prehistoric tidal patterns were about the same as they are today. But they weren’t, and we need to do a better job of accounting for this.

Further, Dr. Hill stressed the importance of conducting paleoceanographic research when he stated:

Understanding the past will help us better predict tidal changes in the future. And there will be changes, even with modest sea level changes like one meter. In shallow waters like the Chesapeake Bay, that could cause significant shifts in tides, currents, salinity and even temperature.

The paper describing ancient tides along a portion of the Atlantic Ocean is currently in press and will soon be published in the Journal of Geophysical Research.

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