Fossil soils constrain ancient climate sensitivity.

G lobal 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 (3). Atmospheric CO2 estimates scale directly with soil CO2 concentration: If the soil term is wrong by a factor of two, the inferred atmospheric CO2 will be off by a factor of two. Estimates of soil CO2 concentration for fossil soils have been based on measurements taken during the growing season in equivalent living soils. However, Breecker et al. (2, 4) demonstrate convincingly that the window of active carbonate formation is restricted to the warmer and dryer parts of the growing season. Carbonate formation is simply not thermodynamically favorable during cooler and wetter seasons. Critically, biological productivity and respiration are low during these dry periods. As a result, soil CO2 concentration during the critical window of active carbonate formation has been overestimated in most soils by a factor of two or more (2). What does this mean? CO2 estimates from the paleosol carbonate proxy can be cut in half (or more). Doing so snaps the paleosol-based estimates in line with most other approaches (2) (Fig. 1B) and produces the most precise view to date of Earth’s CO2 history. We are now better equipped to answer some important, basic questions. For example, what is the quantitative relationship between CO2 and temperature? That is, for every doubling of CO2, what is the long-term (10 3 –10 years) equilibrium response of global temperature (termed here climate sensitivity)? Most assessments of climate sensitivity for the present day hover around 3°C per CO2 doubling (5), although if the longterm waxing and waning of continental ice sheets are considered it is probably closer to 6°C (6). Less is known about climate sensitivity during ancient greenhouse periods, simply because having poles draped in forest instead of ice represents a profound rearrangement of climate feedbacks. Records of CO2 and temperature are now sufficiently robust for placing firm Time (Mya) 40 60 80 100 120 0 300 600 90