The Atmosphere and the Weather

Early History of Earth

Scientists believe Earth began its life about 4.6 billion (4.5 billion) years ago.  Earth formed as cosmic dust lumped together to form larger and particles until 150 million years had passed.

Earth's first atmosphere was most likely hydrogen and helium as well as hydrogen compounds, such as methane and ammonia. This early atmosphere escaped into space from Earth's hot surface.

The continents probably began forming about 4.2 billion years ago as Earth continued to cool. The cooling also resulted in the release of gases from the lithosphere, much of which formed Earth's early atmosphere.

Most of Earth's early atmosphere was created in the first one million years after solidification (4.4 billion years ago). Carbon dioxide, nitrogen, and water vapor dominated this early atmosphere.

Oxygen (O₂), the second most abundant gas in today's atmosphere, probably began an extremely slow increase in concentration as energetic rays from the sun split water vapor (H₂O) into hydrogen and oxygen.

The table below describes the three major stages of development of the atmosphere.

Evolution of Earth's Atmosphere

Name of Stage	Duration of Stage (Billions of Years Ago)	Main Constituents of the Atmosphere	Dominant Processes and Features
Early Atmosphere	4.4 to 4.0	H2O, hydrogen cyanide (HCN), ammonia (NH3), methane (CH4), sulfur, iodine, bromine, chlorine, argon	Lighter gases like hydrogen and helium escaped to space. All water was held in the atmosphere as vapor because of high temperatures.
Secondary Atmosphere	4.0 to 3.3	At 4.0 billion H2O, CO2, and nitrogen (N) dominant. Cooling of the atmosphere causes precipitation and the development of the oceans. By 3.0 billion CO2, H2O, N2 dominant. O2 begins to accumulate.	Continued release of gases from the lithosphere. Water vapor clouds common in the lower atmosphere. Chemosynthetic bacteria appear on the Earth at 3.6 billion. Life begins to modify the atmosphere.
Living Atmosphere	3.3 to Present	N2 - 78%, O2 - 21%, Argon - 0.9%, CO2 - 0.036%	Development, evolution and growth of life increases the quantity of oxygen in the atmosphere from <1% to 21%. 500 million years ago concentration of atmospheric oxygen levels off. Humans begin modifying the concentrations of some gases in the atmosphere beginning around the year 1700.