LATE CRETACEOUS CLIMATE EVOLUTION FROM A MODELING PERSPECTIVE

The Late Cretaceous was a transitional period in Earth’s climate. From peak thermal conditions in the Cenomanian-Turonian, the climate experienced a long-term (~25 myr) but discontinuous cooling to warm conditions of the Late Maastrichtian. Sea surface and deep marine temperature proxies indicate that total cooling may have been ~10 and ~20 °C, respectively. Following this long-term decline, Late Maastrichtian climate experienced further disruptions from Deccan Trap volcanic degassing prior to Chicxulub bolide impact. Causes of the long-term Late Cretaceous thermal decline are uncertain and have been variably attributed to reduction in greenhouse forcing due to atmospheric CO₂ drawdown and changes in meridional heat transport through tectonic restructuring of ocean basins.

Here, we report results from new Late Cretaceous experiments simulated using the NCAR Community Climate System Model (CCSM4), with a 1.9x2.5° atmosphere/land-surface grid and ~1° ocean/sea-ice grid, to investigate the role of atmospheric CO₂ and continental drift on the Late Cretaceous thermal decline. We simulated three time slices, the Cenomanian, Campanian, and Maastrichtian, with CO₂ concentrations set to 2x or 4x preindustrial levels (560 or 1120 ppm), values roughly representative of proxy-reconstructed averages, and age appropriate solar irradiance values. In addition, we are developing and hope to present results from a Maastrichtian simulation that incorporates Deccan Trap volcanic outgassing.

CCSM4 Late Cretaceous simulations indicate that atmospheric CO₂ and geography were important to regional climate change, and led to changes in the source and flux of intermediate and deep waters. Further, the decline in atmospheric CO₂ may have primed regions of Antarctica for ice sheet development. However, neither of these factors, either independently or in combination, fully explains Late Cretaceous cooling observed by proxies. Global mean-annual surface temperatures decrease by only 3.1 °C in response to a 560ppm CO₂ reduction and increase by 0.1 °C in response to geographic changes. Given that the Maastrichtian 2x simulation shows good agreement with proxy data, identifying causes of extreme warmth in the Cenomanian-Turonian remains the largest obstacle to understanding Late Cretaceous climate evolution.