The Changing Atmosphere
The early atmosphere
Scientists believe that the Earth was formed about 4.5 billion years ago. Its early
atmosphere was probably formed from the gases given out by volcanoes. It is
believed that there was intense volcanic activity for the first billion years of the
Earth's existence.
The early atmosphere was probably mostly carbon dioxide with little or no
oxygen. There were smaller proportions of water vapour, ammonia and
methane. As the Earth cooled down, most of the water vapour condensed and
formed the oceans.
Mars and Venus today
It is thought that the atmospheres of Mars and Venus today, which contain
mostly carbon dioxide, are similar to the early atmosphere of the Earth.
The table shows the proportions of the main gases in their atmospheres.
Gas
Mars today Venus today
carbon dioxide
95.3
96.5
nitrogen
2.7
3.5
argon
1.6
trace
oxygen, water vapour and other gases trace
trace
Oxygen and carbon dioxide
The Earth’s early atmosphere is believed to have been mainly carbon dioxide
with little or no oxygen gas. The Earth’s atmosphere today contains around 21
percent oxygen and about 0.04 percent carbon dioxide. So how did the
proportion of carbon dioxide in the atmosphere go down, and the proportion of
oxygen go up?
Increasing oxygen
Plants and algae can carry out photosynthesis. This process uses carbon dioxide
from the atmosphere (with water and sunlight) to produce oxygen (and glucose).
The appearance of plants and algae caused the production of oxygen, which is
why the proportion of oxygen went up.
The Changing Atmosphere
Decreasing carbon dioxide
Photosynthesis by plants and algae used carbon dioxide from the atmosphere,
but this is not the only reason why the proportion of carbon dioxide went down.
These processes also absorb carbon dioxide from the atmosphere:



dissolving in the oceans
the production of sedimentary rocks such as limestone
the production of fossil fuels from the remains of dead plants and animals
Today, the burning of fossil fuels (coal and oil) is adding carbon dioxide to the
atmosphere faster than it can be removed. This means that the level of carbon
dioxide in the atmosphere is increasing, contributing to global warming. It also
means that the oceans are becoming more acidic as they dissolve increasing
amounts of carbon dioxide. This has an impact on the marine environment, for
example making the shells of sea creatures thinner than normal.
Star Task
Draw a pie chart to show the proportions of gases in the atmosphere when earth
was very young (e.g. 4 billion years ago). You will need to use a protractor to
make sure each segment is the right size. Colour each segment and cut out /
stick in your pie chart with a key to explain which segment represents which
type of gas.
Draw another pie chart to show the proportions of gases in the atmosphere
today, colour, stick in and label.
Gas
Nitrogen
Oxygen
Carbon dioxide
Argon and others
Total
Early Earth
%
degrees
80
0
18
2
100
360
Modern Earth
%
degrees
78
21
0.04
0.96
100
0.144
3.456
360
NB. You may not be able to show a segment for carbon dioxide on your
modern earth chart because the percentage is so small. Make that clear in your
book.