Earth’s magnetic poles have been shifted away from the planet’s axis of rotation because of the huge amount of water stored behind man-made dams, a study has found.

The Earth’s crust sits atop a molten rock core, which means it can shift in different directions relative to the magma below it. Any time that mass is redistributed around the planet’s surface, as when ice sheets grow or shrink, causes the outermost rock layer to move around. This means that different areas of the surface end up sitting directly over the axis of rotation. The geographic poles then pass through different spots on the surface than before, a process called true polar wander.

A study published in Geophysical Research Letters finds that the construction of nearly 7,000 dams from 1835 to 2011 shifted the poles about a metre in total and caused a 21mm drop in global sea levels. Together, these dams hold enough water to fill the Grand Canyon twice over.

While the polar shift is relatively small, scientists say it could help them understand how the poles will move if major glaciers and ice sheets melt because of climate change.

“As we trap water behind dams, not only does it remove water from the oceans, thus leading to a global sea level fall, it also distributes mass in a different way around the world,” said Natasha Valencic, a graduate student at Harvard University and lead author of the study. 

“We’re not going to drop into a new ice age because the pole moved by about a metre in total, but it does have implications for sea level.”

The researchers used a global database of dams to map the locations and the amount of water 6,862 dams impound. 

Their results showed global dam building caused Earth’s poles to shift in two distinct phases. From 1835 to 1954, many dams were built in North America and Europe, shifting these areas toward the equator. The North Pole moved 20.5cm toward the 103rd meridian east, which passes through Russia, Mongolia, China and the Indochina Peninsula.

Then, from 1954 to 2011, dams were built in East Africa and Asia, and the pole shifted 57cm toward the 117th meridian west, which passes through western North America and the South Pacific.

Over the entire period of 1835 to 2011, the poles moved about 113cm, with around 104cm of movement happening in the 20th century.

The results show that researchers need to take water impoundment into consideration when calculating future sea level rise. In the 20th century, global sea levels rose by 1.2mm per year on average, but humans trapped a quarter of that amount behind dams.

“Depending on where you place dams and reservoirs, the geometry of sea level rise will change,” she said. “That’s another thing we need to consider, because these changes can be pretty large, pretty significant.”