A key scientific issue is the role of terrestrial ecosystems in absorbing the excess atmospheric carbon dioxide generated by mankind since the beginning of the industrial revolution. To investigate this, a global perspective is needed.
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An example of the kind of data used for such investigations is found in this image, which depicts the state of vegetation during August 1987. Dark-blue areas represent dense vegetation, paler blues represent lighter vegetation, and pink and dark-red areas represent sparse vegetation.  

The Moderate-Resolution Imaging Spectroradiometer (MODIS) on the EOS satellites and the Enhanced Thematic Mapper Plus (ETM+) on Landsat 7 will provide such information routinely as part of the Earth Science Enterprise Program.