Checking two decades causes data of the earlier decade to be
subtracted from that of the later decade.
Checking two Model climate variables causes data of the first
variable to be subtracted from that of the second variable.
Checking a Model climate variable and an observation cause the
observation to be subtracted from the Model variable.
If JavaScripts are enabled by your browser, the status line will
provide more information about climate variables or observations
when the mouse pointer is over one of these quantities or its
checkbox to its left.
