Satellites orbiting the earth have provided Earth Scientists will information that is used to study many parameters important to scientific investigation of Planet Earth. Paramters such as sea level, motions of the earth, glacial ice, and others are measured. In a report, Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond issued by the National Research Council, it was indicated that many of the satellites being used to monitor the Earth are getting old, and need to be replaced over the next decade. You can go to the previous link, and read the report. Looking at the Table of Contents of the report will give you an idea of the breadth of studies that done by making use of satellites.

There was a very good summary of the issue reported in the New York Times that you might want to read: Scientists Warn of Diminished Earth Studies From Space.