Carbon is naturally found in the atmosphere. Carbon dioxide, or CO2, itself is not considered a pollutant. The CO2 being released from burning fossil fuels was part of the atmosphere hundreds of millions of years ago before being captured by plants and sea organisms.
The USGS has been studying glaciers in Glacier National Park since 1850. It is estimated that there were 150 glaciers in the park back then, and when the national park was established in 1910. Today only 25 glaciers remain.
Scientists go back every year to repeat photographs, as well as to examine the ice and the ecology of the landscape to see how glacial retreat is affecting plant and animal species that live there.