Scientists who are concerned about increased carbon dioxide in the atmosphere believe global warming is caused by

B : the elevations on a map based on a given value.

Explanation:

Contour lines generally denotes the topography of an area. Each value on a line corresponds to elevation of a place above a reference surface, most times the mean sea level.

It could be quite tedious labeling each contour line on a map. In order to avoid this, contour lines are labelled with their elevations at intervals. For example, if 11 contour lines are drawn on a map, the first one having an elevation of 100m, if the contour interval is set at 10, the 11th contour line would have a value of 200m. Instead of labeling each contour, we just make the first and 11th line thicker. We put 100 and 200 on the first and last line respectively. Any geographer can easily read this off.