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Can we actually measure rising sea levels with Plymouth Rock?

For many reasons, the iconic landmark cannot be used to disprove climate change.

Plymouth Rock has been moved multiple times, and was once broken in half. Steven Senne/Associated Press

July 11, 2022

Earlier this month a tweet claiming that, if sea levels are indeed rising, Plymouth Rock should be underwater gained traction online to the tune of almost 27,000 likes.

A post like this, inaccurate for multiple reasons, getting a lot of online attention is nothing new. It happens every day. But it does open the door for a deeper exploration of the famous landmark itself, how sea levels are measured, and why claims like this can be so misleading.

Plymouth Rock’s multiple homes

First, the basics. The idea that Plymouth Rock’s relation to the ocean can be used to determine the validity of rising sea level warnings is based on an untrue statement: that the rock is in the exact same spot today as it was hundreds of years ago.

In fact, there is no written historical record of Plymouth Rock from the days when pilgrims first set foot on the Massachusetts shore. The two primary sources written by those pilgrims, William Bradford’s Journal Of Plymouth Plantation and Mourt’s Relation, do not mention the rock at all. It wasn’t until over a century after the pilgrims landed that the first references to Plymouth Rock appear, according to Pilgrim Hall Museum.

In 1775, a newspaper report regarding the capture of British supplies by a George Washington-allied ship captain based in Plymouth mentions “the same rock our ancestors first trod when they landed in America.”

According to a history of Plymouth written in 1835, the location of Plymouth Rock and its importance was an oral tradition passed down from father to son. In 1741, according to this history, a wharf was to be built over the rock. A 95-year old man named Thomas Faunce, who was descended from early Massachusetts pilgrims, was moved to tears when he heard about the wharf, according to the Plymouth Rock Foundation. He was reportedly carried three miles down to see the landmark, where he “bedewed it with his tears and bid to it an everlasting adieu.”

In 1774, animated by a fresh patriotic spirit, many townspeople gathered to remove and dedicate the rock. As they attempted to mount it on a carriage, the rock split in two. The top half was moved near the town square as a symbol of American liberty, while the bottom half remained in place. Over the next hundred years, as the rock became more famous, tourists came to chip away pieces of the bottom half, according to the Smithsonian.

In 1880, the two halves were reunited back at the shoreline. After being stuck together with mortar, the famous “1620” inscription was made.

In 1920, to celebrate the 300th anniversary of the landing of the Pilgrims, Plymouth’s entire waterfront was redesigned. The rock was temporarily relocated while construction took place. The shoreline was rebuilt so that the rock, when placed in its original site, would be at water level, according to the Pilgrim Hall Museum.

A swing in sea levels

Over the years, Plymouth Rock has periodically been covered with water. This mostly happens when supermoons create unusually high tides known as king tides. Over time, the height of tidal systems is gradually increasing due to climate change. In general, high tides are getting higher and extending further inland than in previous years, according to the EPA.

Movable objects like Plymouth Rock are not accurate ways to measure rising sea levels. In some cases, immovable bedrock and clear identifying marks for past sea levels can be used, Reuters reported, but those are not applicable to this scenario.

Measuring how the sea level rises is tricky because the ocean does not have one steady level, according to the National Oceanic and Atmospheric Administration. Tectonic plates move land masses relative to water, while other factors like tides and currents come into play. Tide stations around the globe and satellite lasers are used by experts to measure the sea level. The sea level rises at different rates around the world. This data is used to determine the average height of the entire ocean worldwide. With that information in hand, scientists can compare the current sea level to previous recordings.

The sea level along the United States coastline is projected to rise an average of 10 to 12 inches between 2020 and 2050, according to a federal report released this year. That will be the same as the rise measured over the past 100 years, from 1920 to 2020.

With that in mind, experts predict that tide and storm surge heights will increase and reach even further inland. What we consider “damaging” floods today will likely occur an average of 10 times more frequently by 2050.