In a recent Forum piece that discusses climate variability (Eos, December 16, 1997), S. F. Singer claims that “a higher CO2 level may be less dangerous to the climate system than a lower one.” The basis for this reasoning is the observation that during the last Ice Age, CO2 levels (∼200 ppmV) were lower than pre-Industrial Revolution Holocene levels (∼280 ppmV), yet the climate system was apparently more unstable than during the Holocene.

Singer neglects to add that, in addition to CO2, other Ice Age global boundary conditions were very different from today's, notably larger continental ice sheets in the Northern Hemisphere, higher levels of aerosols, and colder sea-surface temperatures. Climatic variations associated with these boundary conditions are not completely understood, but clearly singling out CO2 grossly oversimplifies the complexity of the climate system and the underlying causes of climate variability.