The Earth’s atmosphere

The Earth’s atmosphere is 99.9% nitrogen, oxygen and argon, of the remaining 0.1%, roughly half (0.04%) is accounted for by the three transition gases CO2, Ne and He.

The remaining minute fraction of the atmosphere (0.66%) is composed of a complex mixture of hundreds of trace gases, the most abundant being methane at 1.7 ppm. Many gases at the parts per trillion levels (10-12v/v), are entirely man-made but may still have significant effect on the environment.

Some trace gases control or affect the Earth’s climate and stability. Long lived gases that are increasing at substantial rates because of human activities are of particular current interest since they lead eventually to stratospheric ozone depletion, global warming, and disturbances in atmosphere chemistry that many believe will be harmful to human life.

As per the present estimate the composition of the atmospheric gases are presented in Table 10.1. Substantial effects of human activities on the global cycles of trace gases have arisen only during the last century.

Since 1800 the human population has increased from 1 to 7 billion, bringing with it fast changes in the surface of the earth and disturbing the global balance of the trace gases, changing in atmospheric composition that have occurred are unprecedented during the last several thousand years.

The rate of change is much faster now than can occur from natural climatic cycles, although it is hard to prove that there were no times when trace gas concentrations did not rise rapidly and later fall. The changes that are occurring now have their origin in the Industrial Revolution (since 1760) and the expansion of technology.

These changes are directly related to human events and activities. There is a prevalent belief that the changes man is causing to the atmospheric environment are unnatural and unhealthy.

There were various types of natural processes like volcanic, earthquake, decomposition and climatic transformations that led to the changes in global atmospheric composition in addition to man-made activities like industrialisation, urbanisation and modernisation of agricultural activities.