How Humanity's Use Of Water Is Nudging Earth's Tilt
In the last 200 years humanity has constructed over 6,800 dams for agricultural use, as drink-water storage and to generate hydroelectric power. Together, they hold so much water that Earth's tilt is changing.
Earth's geographic poles, which are the points around which the planet rotates, move with respect to the surface during a process called polar motion in response to the distribution of Earth's mass. The same physical principle can be seen on a much smaller scale during a hammer throw competition, as the swirling mass of the "hammer" — a metal ball attached by a steel wire to a grip — forces the athlete to wobble around the center of rotation.
Over geological time, the gravitational pull of Sun and Moon, growing or shrinking ice sheets and the slow drift of the continents will move mass around and cause Earth's poles to shift, but human activity can cause significant changes on a much smaller timescale.
'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,' says Natasha Valencic, a graduate student in Earth and planetary sciences at Harvard University and lead author of a new study.
Valencic and her colleagues used a global database of dams to map the locations of each dam and the amount of water each impounds, comparing their construction history with recorded shifts of Earth's poles.
From 1835 to 1954, many dams were built in North America and Europe, shifting these areas toward the equator. The North Pole moved 20.5 centimeters (8 inches) 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 57 centimeters (22 inches) toward the 117th meridian west, which passes through western North America and the South Pacific.
Over the entire period from 1835 to 2011, the poles moved about 113 centimeters (3.7 feet), with about 104 centimeters (3.4 feet) of movement happening in the 20th century.
Already in 2023, Seo et al. published a similar study in the journal Geophysical Research Letters focusing on groundwater use. By pumping water out of the ground and moving it elsewhere, humans have shifted such a large mass of water that the Earth tilted nearly 80 centimeters (31.5 inches) east between 1993 and 2010 alone.
The melting of glaciers and ice caps due to climate change, resulting in a mass shift from the poles to the equator, is expected to amplify this effect further, as a study published in the journal Science Advances suggested as early as 2016.
Observed polar motion (red arrow labelled 'OBS') due to water/groundwater mass redistribution
Earth's poles normally change by several meters within a year due to the planet's natural wobbling, so the observed changes don't run the risk of catastrophic consequences. However, such studies highlight how profoundly human activity can influence the entire planet.
The study,"True Polar Wander Driven by Artificial Water Impoundment: 1835–2011," was published in the journal Geophysical Research Letters.
Additional material provided by Samson Reiny for the American Geophysical Union.
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