At least to the frontline climate scientists, it is becoming increasingly evident that the process of global warming presents a threat to human societies of essentially unprecedented proportions. They know beyond any reasonable doubt that the thermal impulse now being imparted to the earth’s ecological system by aggregate human activity is occurring at a rate greater than any that has been documented in the entire 65 million year paleoclimate record. The current rate of CO2 accumulation in the atmosphere is ten times higher than at any time over the past 400,000 years for which annual estimates have been made based upon ice core data. For earlier periods estimates are made for longer time periods, but the natural rate of CO2 accumulation that 50 million years ago drove atmospheric concentrations and deep ocean temperatures to the highest estimated levels on record was a factor of 20,000 less than the current rate. That distant process occurred over millions of years. At the higher rates currently prevailing, the inexorable process of reestablishing energy equilibrium will occur over a time span that will certainly be much shorter and will certainly affect the operating conditions of human societies, but the exact character, magnitude, timing, or location of the consequences cannot yet be determined.