The atmosphere, and how it became livable
Our atmosphere consists mostly of nitrogen and oxygen. (They are
about 78% and 21% of it by volume.) Nitrogen is a building block
of all life forms. Free oxygen was poisonous to early forms of life
but is essential to animal life we breathe it.
The next most abundant constituent
is water vapor. Then argon (an inert element which essentially does
nothing; its Greek name means no work). Then carbon
dioxide. Then various other elements (such as neon and helium) and
compounds (mainly methane, nitrous oxide, carbon monoxide, ozone),
and dust and sea spray and pollen and man-made pollutants.
The smaller constituents are usually
measured by volume in parts per million; one ppm is 0.0001%. Carbon
dioxide is now about 390 ppm. A millionth of the atmosphere may
sound like a small quantity; but the atmosphere covers the 200 million
square miles of the earth's surface to a depth of fifty miles or
In the far past the atmosphere was
very different. It had no oxygen gas, but abundant carbon dioxide.
From somewhere around 2,500,000,000 years ago, cyanobacteria began
a slow and massive transformation, which algae and plants continue.
They take in water and carbon dioxide,
and use the energy of sunlight to reshuffle these into carbohydrate
and excess oxygen. They expel the excess oxygen as a waste product,
and build themselves with the carbohydrate, which thus not only
stores the carbon but becomes the basic foodstuff of almost all
other living things.
This is the magnificent recipe called
photosynthesis, putting together with light. It created
the new oxygen-rich atmosphere in which animals can live. Plants
take carbon dioxide from the air and put oxygen into it, animals
breathe oxygen from the air and exhale carbon dioxide into it.
There are some other processes: Carbon
dioxide is also removed from the air into rocks and into the ocean.
Carbon dioxide is put back into the air not only by animals but
by plants as they decompose, by fire, and by volcanoes. And by human
farming and industry. There is, or was, a steady state: the rate
at which carbon dioxide escaped into the air was balanced by the
processes that capture it back.