Upchurch, Garland , Kiehl, Jeff , Shields, Christine , Scotese, Christopher .
Plants, clouds, and climate models: Understanding warm climates of the latest Cretaceous.
The nature of past warm climates has been a major area of disagreement between paleobotanists and climate modelers. Paleobotanical proxies for temperature such as foliar physiognomy indicate equable conditions and reduced latitudinal temperature gradients for warm intervals such as the Late Cretaceous, while climate models typically produce cold conditions and steep latitudinal temperature gradients. Paleobotanical proxies for atmospheric pCO2, such as Stomatal Index, indicate only moderate increases in atmospheric pCO2 for times such as the Late Cretaceous, while climate models generally require significant increases in pCO2 to simulate the warmth indicated by paleobotany and geochemistry. Fully coupled simulations using the low-resolution version of the Community Climate System Model, version 3 (CCSM3) demonstrate high congruence between paleobotanical and geochemical temperature indicators and climate model output for the latest Cretaceous (Maastrichtian). In these simulations we explore the combined roles of greenhouse gases, vegetation cover, and the properties of liquid clouds in creating warm and equable conditions. Our model successfully reproduces warm polar temperatures and the latitudinal gradient of Mean Annual Temperature without overheating the tropics, using paleobotanically realistic levels of atmospheric pCO2. The best fit for Mean Annual Temperature is a simulation that prescribes 560 ppm pCO2 (2x preindustrial), 2000 ppb CH4, realistic vegetation, and liquid cloud properties that most likely reflect pre-anthropogenic levels of cloud condensation nuclei. If Late Cretaceous pCO2 levels were lower than 560 ppm, as suggested by stomatal index studies of fossil Lauraceae, additional warming mechanisms are probably needed. One good candidate is reduced rates of transpiration in Late Cretaceous vegetation, which would have resulted from the evolution of vein density in Cretaceous angiosperms and reduced fractional cover of angiosperms during the Cretaceous relative to the Cenozoic.
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1 - University of Texas, Department of Earth and Environmental Sciences, Box 19049, Arlington, TX, 76019, USA
2 - National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder, CO, 80307, USA
Presentation Type: Oral Paper:Papers for Sections
Location: Forsyth Room/Chase Park Plaza
Date: Wednesday, July 13th, 2011
Time: 9:30 AM