Does population growth result in an increase in the mass of the earth?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

05 November 2006

Eric Ayienga says: “On Earth, most living things reproduce: Human beings (there is population growth), animals, plants etc. This then implies that the earth's weight/mass should be increasing. Because of this then we should expect that the period of revolution around the sun should change (since this is dependent on the masses involved) causing weather etc to also change. Is the change in mass/weight negligible or does the principle of matter cannot be created or destroyed still apply. I hope it’s not a silly question.”

No, Eric, it is not a silly question. This year, the population of the world will increase by 77 million people. With the average mass of a person being about 30kg (from 1.5kg babies to 70kg adults), the total additional human mass will be 2.31 billion kilograms. Therefore, you are correct in suspecting that the growth could have some effect on the planet.

But is your suspicion cause for worry? First, the total mass of the Earth is about 6 trillion-trillion kilograms. That is, 6,000,000,000,000,000,000,000,000kg! Now one may ask where the guy who weighed it was standing, but that is a story for another day. Suffice it to say that an increase of 2.31 billion kilos represents only 0.000,000,000,000,0385 percent of the planet’s total mass – quite insignificant.

Indeed, the total biomass on the planet (i.e., all animals and plants) accounts for only 0.000,000,03 percent of the Earth’s mass – still inconsequential. And, having said that, I should add that I suspect that humans are probably the only living things whose number is increasing: we are depleting the forests and killing the animals to extinction!

Secondly, and probably more important, an increase in the mass of living things does not affect the mass of the planet at all. Human babies, for example grow in their mothers’ wombs before they are born. This growth results from the food that the mother eats. After birth, the young ones continue to feed from the mothers’ milk. This also comes from her food. All the food comes from the Earth. So, no matter how large our population becomes, it shall not affect the total mass of the planet. The same goes for other living things.

The third point is a little more complicated. Even if the mass of the Earth increased, the orbital distance from the sun would not change (I say “orbital distance” because it varies as the planet goes round the sun). The reason is that the planet stays in its path due to the balancing of two forces – the sun’s gravitational pull and the “centrifugal force” arising from the earth’s motion.

Now, both these forces are directly proportional to the plant’s mass. So, if the mass is raised, the two would increase by equal amounts thereby remaining balanced. Thus there would be no change in the Earth’s orbital distance from the sun. Nevertheless, we may still want to know whether the mass of our planet increases. Well, that is also a story for another day.