Water is heavy. When we move enough of it, it can change the tilt of Earth’s axis.

It’s that axial tilt that creates our seasons. And the tilt is always changing, due to changes in Earth’s gravity.

Natural phenomena like earthquakes and volcanoes can redistribute the weight of continents, affecting Earth’s gravity. But so can the movement of water.

Researchers recognized that water melting out of glaciers and ice sheets into the ocean had redistributed the weight of that water from the colder latitudes to the equator, where seawater accumulates in a bulge around Earth.

When they calculated the resulting gravitational changes, their numbers didn’t add up. Until they also factored in the weight of the groundwater humans had extracted.

We use groundwater mainly for irrigation and in city water systems, which discharge into rivers that eventually lead to the sea. This moves groundwater from where it’s concentrated underground to soils and oceans.

Across the globe, humans have pulled more than two trillion tons of water from aquifers over two decades. And, the scientists realized, weight redistribution from groundwater alone had changed the tilt of Earth’s axis, causing the North Pole to move three more feet!

They plan to use this new understanding to look for other connections between Earth’s water and gravity, and how droughts might also have altered Earth’s axial tilt.