why is the balance of gases in the atmosphere necessary?

Answer 1

The largest component of the atmosphere is nitrogen, it accounts for nearly four fifths of the atmosphere but serves no real purpose (sort of).

The second most prevalent gas is oxygen, this accounts for just over one fifth of the atmosphere, this is essential to all lifeforms that breath.

The third greatest gas by volume is argon and together with oxygen and nitrogen accounts for over 99.9% of the atmosphere.

There are a great many other gases existing in very small amounts, a lot of them don't have any direct influence. Of these 'trace' gases it's the ones we call the 'greenhouse gases' that have a very important role in that they regulate the temperature of our planet, without any of them Earth would be 33 degrees C colder, a frozen wilderness on which life would never have evolved.

Changes in the concentrations of greenhouse gases can cause the planet to warm up or cool down and although the amounts involved are very small in comparison to the atmosphere as a whole, any changes in concentrations affect our climate. The main greenhouse gases are carbon dioxide, methane and nitrous oxide and levels of all three are increasing sharply.

One other important gas is water vapour, this is part of the hydrological (or water) cycle and if it weren't for water vapour there'd be no precipitation. Changes to the amount of water vapour will affect the amount of precipitation but it's not something humans have much influence over as there's a maximum amount of water vapour the atmosphere can contain (4% by volume but dependent on temp and pressure), once this limit is reached (saturation vapour point) the excess is either deposited on surfaces as dew or it forms water droplets which fall as precipitation of some kind. Water vapour is also a greenhouse gas and is by far the greatest by volume but in comparison to the other GHG's it's pretty weak and ineffective.

Answer 2

because we live in a very fragile environment, the slightest changes to the atmospheric gasses could have profound affects on all plant and animal life.Certain plants and animals are more "picky" on how they will live, as well as were they will live. For example, if you took away the carbon from the air, and soil away, people would not be able to sustain life on earth, because as we know, carbon is necessary in the production of DNA, or deoxy-ribon-nucleic-acid. people could not even recreate themselves without it.