FIGURE 2. Operating curve for the climate/ice-sheet model.
From the following article:
Neoproterozoic 'snowball Earth' simulations with a coupled climate/ice-sheet model
William T. Hyde, Thomas J. Crowley, Steven K. Baum and W. Richard Peltier
Nature 405, 425-429(25 May 2000)
doi:10.1038/35013005
Ice volume and global sea level temperature are shown as a function of
CO2 level and of infrared forcing. A bifurcation occurs at a cooling
of 
5 W m-2, beyond which all land surfaces
are ice-covered and global temperature drops considerably: the snowball Earth.
