[Music]
in this video we will trace the history
of the earth's atmosphere from its early
beginnings with no oxygen to the present
day atmosphere polluted by mankind
by examining rocks laid down over earth
history we can piece together a story of
how our atmosphere has changed
four and a half billion years ago when
the earth had cooled enough for the
surface rocks to solidify we believe the
atmosphere was made mostly of nitrogen
with some carbon dioxide water vapor and
hydrogen compounds such as methane and
ammonia
within 800 million years life evolved in
the oceans and by 3.2 billion years ago
photosynthesis had evolved
with the first oxygen-producing
cyanobacteria
similar to today's blue-green algae
at first the oxygen given off simply
reacted with the rocks but gradually it
built up in the oceans and started to
bubble out into the atmosphere
to start with it was a toxic gas harmful
to life
but soon life evolved to make use of the
oxygen through aerobic
respiration by two billion years ago
about halfway through the earth's
history there was oxygen in the
atmosphere
when scientists look at stars for signs
of life on planets that orbit them the
presence of oxygen in the atmosphere
would be a sure sign that life had taken
hold
with the oxygen in place ozone could
form in the upper atmosphere so
preventing dangerous ultraviolet light
from the sun reaching the land
before then life could only exist under
the water where the uv could not reach
however life did not invade the land
immediately the ozone was in place it
was a big evolutionary step for plants
which needed water to give them support
about 400 million years ago a hundred
million years
after the so-called cambodian explosion
life finally learnt to live on land the
atmosphere remained much the same until
the last century mostly nitrogen with 21
oxygen and just a tiny amount of carbon
dioxide but enough to give plants their
carbon during photosynthesis
however a hundred years ago or so
another living organism started to
change the atmosphere
this organism found seams of coal and
pockets of oil and natural gas
underground
and began extracting them and reacting
them with oxygen in the atmosphere
increasing the carbon dioxide
concentration
mankind for that was the name of the
organism also began to throw away other
unwanted material into the atmosphere
and you can learn all about these in our
related videos
for carbon dioxide see global warming
how it happens and global warming the
effects on the environment
for the acidic oxides we throw away up
our chimneys and in our vehicle exhausts
see acid rain and reducing acid rain or
its effects
for the cfcs which propelled our
aerosols but destroyed the ozone layer
see the hole in the ozone layer
and for the soot and other particles we
throw away into the atmosphere
see
the effect on health of air pollutants
and reducing particulates soot
to summarize it's the presence of life
on this planet that has caused the
atmosphere to change
first the evolution of oxygen two
billion years ago by cyanobacteria
then very recently mankind is changing
the atmosphere as we learned to combine
fossil fuels with oxygen to provide
energy for our evolving lifestyle