Climate Sensitivity during the Phanerozoic: Lessons for the Future
Understanding the link between the level of atmospheric CO2 and global surface temperature is profoundly important. Almost all estimates of climate sensitivity (typically defined as the warming caused by a CO2 doubling) have come from studying records spanning the last ~20 kyr Most of these studies find a modal climate sensitivity of ~3 °C, with a long probability tail at the high end. These studies have been vital for informing climate change issues, but a limitation is that they are calibrated to a present-day or cooler-than-present-day Earth; investigation of deep-time records (older than 2 Myr ago) are required to understand the dynamics of a globally-warm Earth.
Ancient levels of CO2 can be estimated from long-term carbon cycle models, which quantitatively track the major sources and sinks of atmospheric CO2 over these timescales, or by proxy indicators. A synthesis of CO2 estimates from carbon cycle models and from proxies shows a strong, first-order fit between CO2 and geologic indicators of temperature: continental icesheets are common when CO2 drops below 500 ppm and absent when CO2 exceeds 1000 ppm.
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 and temperature, and we should expect a climate sensitivity of 3 °C or more in the near future.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009