Pranav Dilip Sheth @pranavSheth94 - Twitter Profile

7 days ago

There is a wall of concrete in China holding back so much water that 2 scientists sat down and calculated what it does to the length of a day on Earth. It does something. They measured it. The Three Gorges Dam sits on the Yangtze. Its reservoir holds about 40 cubic kilometres of water, roughly 10 trillion gallons, and it holds it 175 metres above sea level. That is 39 trillion kilograms of water, lifted up and parked in one place. The physics is the same as a figure skater. Arms pulled in, they spin faster. Arms out, they slow down. Move mass away from the axis of a spinning object and it slows. Earth is a spinning object. In 2005, Benjamin Fong Chao at NASA Goddard and Richard Gross at the Jet Propulsion Laboratory worked out the numbers. A full Three Gorges reservoir would lengthen every day on Earth by 0.06 microseconds, and shift the position of the pole by about 2 centimetres. The reservoir first reached that level in October 2010. Nobody has ever missed a train because of it. That is not the point. The point is the category of thing that normally does this. The 2004 Sumatra earthquake, the one that caused the Indian Ocean tsunami, shortened the day by 2.68 microseconds. The 2011 Japan earthquake sped the planet up by about 1.8 microseconds. Those are earthquakes. Tectonic plates. Forces that build over millions of years. The dam did it with standing water, because somebody decided to put it there. And it is not alone. A study out of Harvard found that the water held behind roughly 7,000 dams built since the 19th century has shifted the position of the North Pole by about 1 metre, and very slightly lowered global sea level, because all that water is sitting on land instead of in the ocean. Groundwater does it too. Between 1993 and 2010, humans pumped out around 2,150 gigatonnes of it. That raised sea level by more than 6 millimetres and moved the Earth's rotational axis about 80 centimetres to the east. We are already at the scale where the planet responds. Now the part that matters for anyone living in India or Bangladesh. In July 2025, machinery arrived at the edge of a canyon in Medog county, in southeastern Tibet. Premier Li Qiang announced the start of construction on what will be the largest hydropower project on Earth. It is on the Yarlung Tsangpo. The river drops about 2,000 metres in the space of 50 kilometres there, through one of the deepest canyons in the world. That drop is the reason engineers want it. The projected output is around 3 times that of Three Gorges. The cost is put at about 170 billion dollars. Completion is expected around 2035. The Yarlung Tsangpo does not stay in Tibet. It crosses into India, where it is called the Brahmaputra, and then flows into Bangladesh. Millions of people plant and harvest according to when that river rises and falls. Fish migrate on it. The silt it carries builds the land they farm on. All of that is decided upstream, in another country, by whoever operates the gates. Nobody has publicly calculated what a structure 3 times the size of Three Gorges does to the spin of the planet. And it is going into the Himalaya, which is the most seismically active mountain range on Earth, and which spent this week demonstrating exactly how fast water can come down it. The old assumption was that geology moves slowly and humans move fast, on separate timescales that never really touch. That assumption is finished. We are now building at the scale where the Earth notices.

san_x_m's tweet photo. There is a wall of concrete in China holding back so much water that 2 scientists sat down and calculated what it does to the length of a day on Earth.

It does something. They measured it.

The Three Gorges Dam sits on the Yangtze. Its reservoir holds about 40 cubic kilometres of water, roughly 10 trillion gallons, and it holds it 175 metres above sea level.

That is 39 trillion kilograms of water, lifted up and parked in one place.

The physics is the same as a figure skater. Arms pulled in, they spin faster. Arms out, they slow down. Move mass away from the axis of a spinning object and it slows.

Earth is a spinning object.

In 2005, Benjamin Fong Chao at NASA Goddard and Richard Gross at the Jet Propulsion Laboratory worked out the numbers. A full Three Gorges reservoir would lengthen every day on Earth by 0.06 microseconds, and shift the position of the pole by about 2 centimetres.

The reservoir first reached that level in October 2010.

Nobody has ever missed a train because of it. That is not the point.

The point is the category of thing that normally does this.

The 2004 Sumatra earthquake, the one that caused the Indian Ocean tsunami, shortened the day by 2.68 microseconds. The 2011 Japan earthquake sped the planet up by about 1.8 microseconds.

Those are earthquakes. Tectonic plates. Forces that build over millions of years.

The dam did it with standing water, because somebody decided to put it there.

And it is not alone. A study out of Harvard found that the water held behind roughly 7,000 dams built since the 19th century has shifted the position of the North Pole by about 1 metre, and very slightly lowered global sea level, because all that water is sitting on land instead of in the ocean.

Groundwater does it too. Between 1993 and 2010, humans pumped out around 2,150 gigatonnes of it. That raised sea level by more than 6 millimetres and moved the Earth's rotational axis about 80 centimetres to the east.

We are already at the scale where the planet responds.

Now the part that matters for anyone living in India or Bangladesh.

In July 2025, machinery arrived at the edge of a canyon in Medog county, in southeastern Tibet. Premier Li Qiang announced the start of construction on what will be the largest hydropower project on Earth.

It is on the Yarlung Tsangpo. The river drops about 2,000 metres in the space of 50 kilometres there, through one of the deepest canyons in the world. That drop is the reason engineers want it.

The projected output is around 3 times that of Three Gorges. The cost is put at about 170 billion dollars. Completion is expected around 2035.

The Yarlung Tsangpo does not stay in Tibet. It crosses into India, where it is called the Brahmaputra, and then flows into Bangladesh.

Millions of people plant and harvest according to when that river rises and falls. Fish migrate on it. The silt it carries builds the land they farm on.

All of that is decided upstream, in another country, by whoever operates the gates.

Nobody has publicly calculated what a structure 3 times the size of Three Gorges does to the spin of the planet.

And it is going into the Himalaya, which is the most seismically active mountain range on Earth, and which spent this week demonstrating exactly how fast water can come down it.

The old assumption was that geology moves slowly and humans move fast, on separate timescales that never really touch.

That assumption is finished. We are now building at the scale where the Earth notices.

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