It takes more than a moment to fully realize this is Earth . . . People have increased the use of air, water, and other natural resources by a factor of 10 in the last 200 years. This activity has in turn affected the atmosphere, the water cycle, and the climate, and has altered ecological systems. Scientists have begun to monitor these effects both from the ground and from space, to identify problems, as well as to predict the future health of the planet.
Earth is surrounded by a delicate envelope of air, part and product of life on the planet.
Human beings have changed the composition of this atmosphere. Tons of carbon dioxide and methane, among other compounds, are added annually to the atmosphere from the burning of fossil fuels. These and other chemical pollutants raise concerns about the effects a changing atmosphere may have on life. Green plants cover much of Earth's land area, and microscopic plants known as phytoplankton inhabit its waters. More than 35 percent of the planet's surface is used, at least indirectly, for harvesting food and other materials.