Estimates of Atmospheric CO2 For the Past 420 Million Years and Its Relationship to Temperature

Temperature and atmospheric CO₂ are arguably the two most critical parameters for understanding ancient climate change. Many proxies exist for reconstructing temperature in the geologic past, and these proxies have been used to generate thousands of paleotemperature estimates. In contrast, fewer techniques are available for reconstructing CO₂. One of the more popular methods involves the measurement of the δ¹³C of pedogenic carbonate nodules; the carbon in these nodules represents a mixture of isotopically depleted CO₂ from biological respiration and isotopically enriched atmospheric CO₂. Other CO₂ methods include the δ¹³C of carbonate-secreting algae, the distribution of stomatal pores on fossil leaves, and the δ¹¹B of marine boron. Application of these methods to geological materials results in a CO₂ history that is broadly in-step with temperature records: CO₂ is high (>1000 ppm) when the earth is ice-free and low (<500 ppm) when continental ice sheets are present. Several recent studies have sought to calculate the quantitative relationship between CO₂ and temperature (climate sensitivity) in deep time. One approach, developed by us, involves minimizing the misfit between proxy records of CO₂ and calculations of CO₂ from a global model of the long-term carbon cycle. We tune the simulated carbon cycle by adjusting a temperature-sensitivity parameter that is embedded within the model. This approach suggests an ancient climate sensitivity that is equal to or greater than calculations for the present day (~3 °C warming per CO₂ doubling), depending on the choices for other model parameters. In this contribution, we present an updated CO₂ record for the past 420 million years and new calculations of climate sensitivity that focus on isolated time intervals within the geologic record.