This is the area-weighted IV (sum of area times average IV for each
county) change that is averaged for all 5 GCM scenarios. This is one number
that summarizes the potential fate of the species under a doubled CO2
climate. A number of -100 would mean that all 5 GCMs are indicating that
the species would be extirpated from the US while a number of 400 would
indicate a four-fold increase.