The atmosphere is the gaseous envelope surrounding the Earth retained by the Earth’s gravity.
Much of the early atmosphere was created by volcanic activity and consisted mainly of carbon dioxide with little or no oxygen. Smaller amounts of water vapor, ammonia and methane were also present. As the earth cooled down the water vapor condensed to form the Earth’s oceans.
The primary atmosphere of the Earth was composed of gases captured form the solar nebula. The captured gas was mainly hydrogen the most abundant element in the universe, along with water vapor, methane and ammonia (NH3).
In the secondary atmosphere that followed, the atmosphere and oceans were produced by chemical reactions, such as volcanic eruptions, that released water and gases from condense materials on Earth. At this stage the atmosphere consisted of carbon dioxide and water vapor, with methane, carbon monoxide, hydrogen sulphide (H2S) and ammonia and little or no oxygen – similar to the gases produced by volcanoes today. The exact composition of this early stage of the atmosphere’s evolution is still a matter of debate.
So how did we get from the early high-carbon dioxide/low oxygen original atmosphere to the present high-oxygen pre-industrial composition? This is again a vast topic and we will only take a brief look to help us understand the significance of current changes in the atmosphere and climate that are taking place.
Many components of the Earth have played – and and still playing – a role in changing the climate from these early days to the time of the industrial revolution.
These components (ecosystems) are the atmosphere, the oceans, the land mass, the cryosphere (ice regions) and the biosphere (plants, animals and humans). We will consider each of these ecosystems in The Pre-Industrial Climate section.
Origin of atmospheric oxygen
Much of the story of the Earth’s atmosphere and its biosphere relates to oxygen. Until about 2.4 billion years ago there was a negligible amount of oxygen in the atmosphere.
This situation relates to the pre-biotic state of the Earth, that is before life began and started to make major changes to the atmosphere by producing oxygen in large quantities by the biological process of photosynthesis.
We will consider photosynthesis in detail in a later section when we look at the vital role that microorganisms have played in creating the environment.
On other planets the photochemical decomposition of gaseous oxides is a significant source of oxygen. For example, carbon dioxide can be broken down into carbon monoxide (CO) and oxygen. On Earth, the major inorganic source of oxygen is the photolysis (decomposition by light) of water into hydrogen and oxygen. However, it has been estimated that less than 3% of oxygen present in today’s atmosphere has been generated by this mechanism.
Having covered the characteristics of the early atmosphere and climate we can now look at the climate up to the time of the industrial revolution (from the late 1700s) when things began to change quickly.