The current composition of the atmosphere
Today, dry atmosphere consists of:
- nitrogen (N2 – 78.1% by volume)
- oxygen (O2 – 20.9%)
- argon (Ar – 0.93%)
- greenhouse gases such as carbon dioxide (0.04%), methane (CH4 – 0.00018%)
- minute traces of neon (Ne), helium (He), krypton (Kr), xenon (Xe), hydrogen (H2), ozone (O3)
In addition, the atmosphere contains water vapor (variable, but typically c. 0.4-2% ). Air pressure and density decrease with altitude. The variation of temperature with altitude is more complicated.
We will consider the post-industrial increases in the greenhouse gases – carbon dioxide, methane and nitrous oxide – in a later section.
This current composition of the atmosphere is vastly different from the earlier atmosphere we looked at earlier. The changes – at least up to the start of the Industrial Revolution – were caused by ‘natural’ factors rather than human activities. As we will see later, the biosphere has played a major part in these changes.
Structure of the atmosphere
The current Earth’s atmosphere can be divided into 5 layers (strata):
- Exosphere – 700 to 10,000 km. This outermost layer is composed mainly of very low concentrations of hydrogen, helium, nitrogen, oxygen, and carbon dioxide. These molecules continually escape into space.
- Thermosphere – 80 to 700 km. The altitude of this layer varies with solar activity. Temperatures within this layer increase with height. The layer contains no water vapor. The aurora borealis and aurora australis occur in the ionosphere – part of this layer. The International Space Station orbits in this layer.
- Mesosphere – 50 to 80 km. Temperatures in this layer decrease with height. This is the highest layer that can contain clouds.
- Stratosphere – 12 to 50 km. The air pressure in this layer is c. 0.001% of pressure at sea level. It contains the ozone layer. Temperatures in this layer increase with altitude, caused by the absorption of ultraviolet radiation by the ozone layer. The ozone layer contains 2 to 8 ppm of ozone. The stratosphere is almost completely free of clouds.
- Troposphere – 0 to 12 km. Within this layer temperature normally decreases with increasing altitude. The layer contains c. 80% of the mass of the Earth’s atmosphere. 50% of the mass of the Earth’s atmosphere occurs in the lowest 5.6 km. This layer contains almost all of the atmospheric water vapor and clouds and therefore displays all of the weather characteristics.
Evolution of the atmosphere to pre-industrial times
As we will see later, much of the current concern about climate change relates to anthropogenic (man-(person?)-made) activities that essentially started at the beginning of the industrial revolution in the late 1700s.
Next, we will look at the ocean ecosystem.