In the past hundred years or so, human population growth and industrial activity has caused a much sharper change in the concentrations of some gases and particulates on a much shorter timescale than at any time in the past 200,000 years. The most dramatic effect has been a huge increase in the amount of carbon dioxide in the atmosphere. Scientific research has shown that this increase has contributed to Earth’s greenhouse effect, which as a result appears to be increasing the average temperature on Earth at a much faster rate than typical ecological and geological timescales.

The mesosphere is the uppermost layer of the stratosphere. Below the mesosphere, at altitudes of twenty-five to forty miles (forty to sixty-five kilometers), is a warm layer of the stratosphere that contains a high concentration of ozone molecules that block ultraviolet light.