Imagine a fly landing on a spinning balloon—the balloon will tilt ever-so-slightly, shifting the fly to the outermost curve. Humans have caused a similar tilt, scientists have discovered, by building dams and reservoirs all over Earth.
Over the past two centuries, our dams have created almost 30,000 lakes and reservoirs across the planet, holding back 10,000 cubic kilometres of water. New Zealand alone has more than 400 large dams (as explored in our previous issue, these have had a profound effect on our eels and other native fish).
New research, led by Natasha Valencic of Harvard University, shows dams have also impacted the position of the north pole. After 1838, as dams went up all over Europe and North America, the pole retreated about 20 centimetres towards Russia. Then, after 1954, as dam building ramped up in Africa and Asia, it reversed direction.
In total, the pole has shifted about a metre over that time. The finding, says Valencic, shows the profound impact humans are having on the planet beyond the more widely understood shifts such as temperatures and sea levels.
And these impacts can play off each other. Earlier research found damming water up in lakes decreases global mean sea level. But these large bodies of water also compress the Earth’s crust and exert a gravitational pull on nearby seas, actually causing sea level to rise in some places, most notably the Bay of Bengal—surrounded by some of the most densely crowded areas on Earth.
With dam construction showing no sign of stopping, it’s crucial, says Valencic, “to keep track of this, and to make sure we understand the volume of these reservoirs, so we can understand what’s going on along these coastlines”.
