Over the past 800,000 years, an unidentified regulatory mechanism has prevented atmospheric CO2 concentrations from falling below a level that would trigger extreme cooling, reports a study published online in Nature Geoscience this week. The study suggests the mechanism may involve the biosphere, as plants and plankton struggle to grow and take up carbon under low CO2 levels.

Atmospheric CO2 concentrations vary by up to 100 ppmv (parts per million by volume) between glacial and interglacial periods. The exact processes behind this variation are unclear, but changes in the uptake of carbon by plants and ocean plankton, and the subsequent transfer of that carbon to soils and the deep ocean are thought to play an important role.

Eric Galbraith and Sarah Eggleston found that atmospheric CO2 concentrations - as recorded in ice cores - hovered near 190 ppmv during much of the past 800,000 years, but very rarely fell any lower. They therefore argue that one or more feedbacks must stabilize CO2 concentrations at this level, preventing runaway cooling. They suggest that it is the biosphere that maintains habitable temperatures - at very low CO2 levels, plant and phytoplankton photosynthesis is limited. The restriction of the growth of these organisms decreases the amount of carbon transferred from the atmosphere to soils and the deep ocean, and thus prevents CO2 concentrations from falling further, preventing extreme cooling.