Earth’s atmosphere is continually and gradually changing. Over cycles that usually last many thousands of years, the concentrations of different gases—including oxygen, carbon dioxide, and others—go up and down, as does the concentrations of tiny dust particulates such as carbon soot.

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. This increase has created a substantial greenhouse effect, according to some scientists, which could be increasing the average temperature on Earth at a much faster rate than typical ecological and geological timescales.