Is the Earth getting heavier?

12 October 2008

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Question

Is the Earth getting heavier? As the human population grows, are we putting on weight as a planet?

Answer

Chris - Yes but perhaps not for the reason that he was suggesting.

People often think that, as we increase our population, the weight of the people comes from nowhere. People just weigh more and the Earth therefore weighs more. That's not true. The Earth is an isolated system where all the of the mass that was on it to start with doesn't go anywhere. You don't make mass from nowhere, so people have got to gain weight by taking mass from elsewhere on the Earth: in other words, eating food that's come out of the Earth and into their bodies.

But, considering the planet as a whole, does that get heavier over time? The answer is yes, it can.

Every year, Earth gains about the weight of two aircraft carriers landing on it: two "HMS Ark Royals", or about 40,000 tonnes-worth of debris, which lands on Earth from space.

You can demonstrate this for yourself. If you put a big plastic sheet or a white sheet on your grass in the garden on a nice day, leave it for a few hours and then run a magnet over it. You can often find specks have just fallen down from outer space and landed on your magnet. Debris, dust and other stuff raining in from space contributes a huge amount of weight to the Earth every single year.

Comments

Just like the movement of the continents, earth gaining weight from space debris, while small in our small time wise perspective, over millions of years it can add up. i saw estimates from 30-100K tons per annum. Now multiply that by, say 64 million since the dinosaurs 100,000 x 64,000,000 = 64x10^11 tons.

I understand that the mass of water doesn't change when going from solid state to liquid state, however if water is more dense than ice, would that affect the overall weight of the planet if several glaciers were melting at the same time?

The mass of water is constant - you haven't made more water molecules; it's just the volume that has changed, because the particles are more "spread out" in ice than in the liquid state.

Actually, the Earth loses about 50,000 tonnes each year, despite the 40,000 tonnes of space dust that we gain. As the earths core looses energy, we loose mass. In addition, we loose helium and hydrogen, which can escape (although minimally) the Earths gravitational pull. It would take trillions of years to loose all of the hydrogen, even at the rate of 95,000 tonnes lost each year. Although we only loose about 1,600 tonnes of helium each year, it is of more concern as it is far more scarce.

Sources?

You are right the Eath is becoming lighter

If the earth gets heavier, does that affect the gravitational pull of the sun? Making earth closer to the sun as the earth gets heavier?

Yes, if the planet gains mass then it will exert an increased gravitational attraction upon other bodies. However, as mentioned in the answer above, the amount of mass added to the Earth every year may sound like a lot - of the order of a some tens of thousands of tonnes. Relative to the mass of the Earth though - which is about 6 million million million million kilograms (6x1024) the added mass is absolutely trivial makes a near-negligible different to Earth's gravitational field. Thus its effects may be disregarded in the grand scheme of things.

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