Is the Earth getting heavier due to the growth of plants?

01 November 2009

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Question

Is the Earth getting heavier owing to plant photosynthesis converting energy into mass. And if so, does this affect the Earth’s spin?

Answer

We have actually looked at this in the past and the answer is, actually, yes the Earth is getting heavier...

Because E=mc2, Einstein's famous equation, (E) energy equals (m) mass, times (c), the speed of light squared. So, if you increase the energy in the system then the mass must also increase.

The Sun is adding energy to the Earth's system in the form of chemical energy; this arrives as light and is converted into chemical energy by photosynthesis.

Therefore, the Earth is gaining a little bit of weight in the form of the entrapment of that energy within plant chemistry.

But, compared with the 40,000 tons of dust and material that rains in on Earth from space every year, it's quite literally a drop in the ocean.

So, on the whole, the Earth is gaining a bit of weight as plants in the biosphere capture energy coming from the Sun, and the Sun warms the planet.

Comments

apparently earth also looses 90,000 tonnes each year in the form of gas (mainly hydrogen, helium) that escapes earths gravity. So if we put all things together it is estimated that earth is actually getting lighter...by about 50,000 tonnes each year

Surely, the mass of the earth must increase over time due to photosynthesis? A process that has continued for billions of years that must far exceed the loss of gaseous elements from the atmosphere into space.

This answer was prepared for the BBC Radio 4 programme "More or Less" in 2012...

There are factors that are causing Earth to both gain and lose mass over time, according to Dr Chris Smith, a medical microbiologist and broadcaster who tries to improve the public understanding of science.

Using some back-of-the-envelope-style calculations, Dr Smith, with help from physicist and Cambridge University colleague Dave Ansell, drew up a balance sheet of what's coming in, and what's going out. All figures are estimated.

By far the biggest contributor to the world's mass is the 40,000 tonnes of dust that is falling from space to Earth, says Dr Smith.

"[The dust] is basically the vestiges of the solar system that spawned us, either asteroids that broke up or things that never formed into a planet, and it's drifting around.

"The Earth is acting like a giant vacuum cleaner powered by gravity in space, pulling in particles of dust," says Dr Smith.

Another much less significant reason the planet is gaining mass is because of global warming.
The answer

    It's getting lighter, by about 50,000 tonnes in mass each year, but not due to space dust
    Some factors include:
    Gains: Mostly dust (like an asteroid, above) falling from space, plus increased energy from increases in the planet's temperature
    Losses: Mostly hydrogen, plus some helium and a tiny amount of lost energy

Nasa has calculated that the Earth is gaining energy due to rising temperatures. Dr Smith and his colleague Mr Ansell estimate this added energy increases the mass of Earth by a tiny amount - 160 tonnes.

This means that in total between 40,000 and 41,000 tonnes is being added to the mass of the planet each year.

Population growth and new buildings are not a factor, he says, because both of these are actually made up of existing matter on the planet.

But overall, Dr Smith has calculated that the Earth - including the sea and the atmosphere - is losing mass. He points to a handful of reasons.

For instance, the Earth's core is like a giant nuclear reactor that is gradually losing energy over time, and that loss in energy translates into a loss of mass.

But this is a tiny amount - he estimates no more than 16 tonnes a year.

And what about launching rockets and satellites into space, like Phobos-Grunt? Dr Smith discounts this as most of it will fall back down to Earth again.

But there is something else that is making the planet lose mass. Gases such as hydrogen are so light, they are escaping from the atmosphere.

"Physicists have shown that the Earth is losing about three kilograms of hydrogen gas every second. It's about 95,000 tonnes of hydrogen that the planet is losing every year.

"The other very light gas this is happening to is helium and there is much less of that around, so it's about 1,600 tonnes a year of helium that we lose."

So taking into account the gains and the losses, Dr Smith reckons the Earth is getting about 50,000 tonnes lighter a year, which is just less than half the gross weight of the Costa Concordia, the Italian cruise liner, that ran aground recently.

Clearly, compared to the immense size of the world, this is a tiny difference, a loss of just 0.000000000000001%.

Should we be worried?

It may seem a small amount, but is the world in danger of running out of hydrogen? Dr Smith is not worried.

"It would take trillions of years to empty the earth's oceans and since the planet is only about 5 billion years old, probably a hell of a lot longer than we have been here already."

Dr Chris Smith and David Ansell are part of Naked Scientists, a group of academics from Cambridge University who broadcast on BBC Radio about science issues

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