Fossil soils constrain ancient climate sensitivity

• carbon dioxide
• paleoclimate
• paleosol

Global temperatures have covaried with atmospheric carbon dioxide (CO₂) over the last 450 million years of Earth’s history (1). Critically, ancient greenhouse periods provide some of the most pertinent information for anticipating how the Earth will respond to the current anthropogenic loading of greenhouse gases. Paleo-CO₂ can be inferred either by proxy or by the modeling of the long-term carbon cycle. For much of the geologic past, estimates of CO₂ are consistent across methods (1). One exception is the paleosol carbonate proxy, whose CO₂ estimates are often more than twice as high as coeval estimates from other methods (1). This discrepancy has led some to question the validity of the other methods and has hindered attempts to understand the linkages between paleo-CO₂ and other parts of the Earth system. In this issue of PNAS, Breecker and colleagues (2) break important new ground for resolving this conflict.

The paleosol carbonate proxy for atmospheric CO₂ is based on the analysis of carbonate nodules that precipitate in soils in seasonally dry to dry climates. These nodules incorporate carbon from two sources: atmospheric CO₂ that diffuses directly into the soil and in situ CO₂ from biological respiration. Because the stable carbon isotopic composition of these two sources is distinct, the concentration of atmospheric CO₂ can be inferred if the concentration of soil CO₂ and the isotopic compositions of the two sources are known …

¹E-mail: droyer{at}wesleyan.edu