The Fitness of the Atmosphere

Earth's physical features-–mass, structure, temperature and so on–are "just right for life". Such features alone are not enough to allow life to exist on Earth however. Another vital factor is the composition of the atmosphere.

In science-fiction movies sometimes space travelers and explorers come across planets with breathable atmospheres: they seem to be lying all over the place. If we could explore the real universe, we'd discover that this isn't true at all: the possibility of another planet's having an atmosphere that we could breathe is most unlikely. That's because the atmosphere of Earth is specially designed to support life in a number of crucial ways.

The atmosphere of Earth is composed of 77% nitrogen, 21% oxygen, and 1% carbon dioxide. Let's start with the most important gas: oxygen. Oxygen is vitally important to life because it enters into most of the chemical reactions that release the energy that all complex life-forms require.

Carbon compounds react with oxygen. As a result of these reactions, water, carbon dioxide, and energy are produced. Small "bundles" of energy that are called ATP (adenosine triphosphate) are used in living cells are generated by these reactions. This is why we constantly need oxygen to live and why we breathe to satisfy that need.

The interesting aspect of this business is that the percentage of oxygen in the air we breathe is very precisely determined. Michael Denton writes on this point:

Could your atmosphere contain more oxygen and still support life? No! Oxygen is a very reactive element. Even the current percentage of oxygen in the atmosphere, 21 percent, is close to the upper limit of safety for life at ambient temperatures. The probability of a forest fire being ignited increases by as much as 70 percent for every 1 percent increase in the percentage of oxygen in the atmosphere.

The present oxygen level is at a point where risk and benefit nicely balance.

That the proportion of oxygen in the atmosphere remains at this precise value is the result of a marvelous "recycling" system: Animals constantly consume oxygen and produce carbon dioxide, which, for them, is not breathable. Plants do just the opposite: they take in carbon dioxide, which they need to live, and release oxygen instead. Thanks to this system, life goes on. Plants release millions of tons of oxygen into the atmosphere every day.

Without the cooperation and balance of these two different groups of living things, our planet would be unlivable. For example, if living things only took in carbon dioxide and released oxygen, the earth's atmosphere would support combustion much more easily than it does and even a tiny spark could set off enormous fires. Similarly, if both took in oxygen and released carbon dioxide, life would eventually die out when all the oxygen had been used up.

In fact, the atmosphere is in a state of equilibrium in which risk and benefit are nicely balanced.

The perfection in the density of air

The numerical values of the atmosphere are not only necessary for us to breathe but are also essential for our Blue Planet to stay blue.

"God created the heavens and the Earth with truth. There is certainly a Sign in that for the believers." (Qur'an, 29:44)

Michael Denton comments on this:

It is clear that if either the viscosity or the density of air were much greater, the airway resistance would be prohibitive and no conceivable redesign of the respiratory system would be capable of delivering sufficient oxygen to a metabolically active air-breathing organism... By plotting all possible atmospheric pressures against all possible oxygen contents, it becomes clear that there is only one unique tiny area... where all the various conditions for life are satisfied... It is surely of enormous significance that several essential conditions are satisfied in this one tiny region in the space of all possible atmospheres.

At sea level, air density is about 1 gram/liter. Again at sea level, its viscosity is nearly 50 times that of water. You might think these numbers unimportant but they are vital for our lives because the overall composition and general character of the atmosphere–its density, viscosity, and pressure, etc-–must be very similar to what it is, particularly for air-breathing organisms.

When we breathe, our lungs use energy to overcome a force called "airway resistance". This force is the result of the resistance of air to movement. Owing to the physical properties of the atmosphere however, this resistance is weak enough that our lungs can take air in and let it out with a minimum expenditure of energy. If air resistance were higher, our lungs would be forced to work harder to enable us to breathe. This can be explained by an example. It easy to draw water into the needle of an injector but drawing honey in is much more difficult. The reason is that honey is denser than water and also more viscous.

If the density, viscosity, and pressure of air were higher, breathing would be as difficult as drawing honey into a needle. Someone might say "That's easy to fix. We'll just make the hole of the needle larger to increase the rate of flow." But if we did that in the case of the capillaries in the lungs, the result would be to reduce the size of the area in contact with air, with the result that less oxygen and carbon dioxide would be exchanged in the same amount of time and the respiratory needs of the body would not be satisfied. In other words, the individual values of air's density, viscosity and pressure must all fall within certain limits in order for it to be breathable and those of the air we breathe do exactly that.

Blue Planet

The numerical values of the atmosphere are not only necessary for us to breathe but are also essential for our Blue Planet to stay blue. If sea-level atmospheric pressure were one fifth lower than its present value, the rate of water vaporization would be much higher. Increased water in the atmosphere would have a "greenhouse effect" trapping more heat and raising the average temperature of the planet. On the other hand, if the pressure were one times higher, the rate of water vaporization would be less, turning large parts of the planet into desert.
Yet none of these possibilities actually happen, because God has created the Earth, the Solar System, and the universe around it to be flawless. He has created all the equilibriums on Earth in harmony with one another for our survival. God reveals this flawless creation in the Qur’an. It is also revealed that in return human beings must use their intellect and reflect on these examples and appreciate God’s creation:

God is He Who raised up the heavens without any support – you can see that – and then established Himself firmly on the Throne. He made the sun and moon subservient, each running for a specified term. He directs the whole affair. He makes the Signs clear so that hopefully you will be certain about the meeting with your Lord. It is He Who stretched out the Earth and placed firmly embedded mountains and rivers in it and made two types of every kind of fruit. He covers over day with night. There are Signs in that for people who reflect. In the earth there are diverse regions side by side and gardens of grapes and cultivated fields, and palm-trees sharing one root and others with individual roots, all watered with the same water. And We make some things better to eat than others. There are Signs in that for people who use their intellect. (Qur'an, 13:2-4)