Commentary on “The Anthropogenic Greenhouse Era Began Thousands of Years Ago”

Bill Ruddiman (Climatic Change, 61, 261–293, 2003) recently suggested that early civilisations could have saved us from an ice age because land management over substantial areas caused an increase in atmospheric CO₂ concentration. Ruddiman suggests a decreasing “natural course” of the Holocene greenhouse gases concentrations and sea-level by referring to analogous situations in the past, namely the last three interglacials. An examination of marine isotopic stage 11 would perhaps make Ruddiman’s argument even more thought-challenging. Yet, the hypothesis of a natural lowering of CO₂ during the Holocene contradicts recent numerical simulations of the Earth carbon cycle during this period. We think that the only way to resolve this conflict is to properly assimilate the palæoclimate information in numerical climate models. As a general rule, models are insufficiently tested with respect to the wide range of climate situations that succeeded during the Pleistocene. In this comment, we present three definitions of palæoclimate information assimilation with relevant examples. We also present original results with the Louvain-la-Neuve climate-ice sheet model suggesting that if, indeed, the Holocene atmospheric CO₂ increase is anthropogenic, a late Holocene glacial inception is plausible, but not certain, depending on the exact time evolution of the atmospheric CO₂ concentration during this period.