One parameter in most long-term carbon cycle models is climate sensitivity because the weathering of Ca and Mg-rich silicate rocks, which serves as a long-term sink for CO2, is sensitive to temperature. Thus, it is possible to estimate long-term climate sensitivity by adjusting the climate sensitivity parameter in the long-term carbon cycle models until the model estimates of CO2 best match the independent proxy estimates of CO2. Over the past 420 Myr, the modal climate sensitivity is ~3 °C; a sensitivity of < 1.5 °C is highly improbable, while sensitivities of 6+ °C cannot be excluded. Deep-time climate sensitivity thus matches the present-day, despite the two approaches capturing fundamentally different carbon cycle processes; a climate sensitivity of around 3 °C appears to be a robust feature of the Earth system, independent of temporal scaling. The geologic record generally supports a positive link between CO2 & temperature, & we should expect a climate sensitivity of 3 °C or more.
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