Climate Sensitivity of stratospheric injecitions of large amounts of S-bearing gases released from the Chicxulub impact

33rd LPSC (2002) Abst. #1269

ABSTRACT

This work uses an atmospheric single-column-model, SCCM, coupled to a sulfate aerosol model to provide an initial assessment of the climate sensitivity to stratospheric sulfate aerosols produced by the reaction of S-bearing gases and water vapor released in the Chicxulub impact event. The formation of large amounts of sulfate aerosols in the stratosphere results in a strong heating of the stratosphere accompanied by cooling (few degrees) at the Earth's surface.

The atmospheric model used for this work is the Single Column Model, SCCM, which is derived from NCAR's Community Climate Model Version 3 (CCM3) (www.cgd.ucar.edu/cmr/ccm3), CCM3 is a spectral atmospheric general circulation model with 18 vertical levels, ranging from the surface to about 48 km. The top 7 levels roughly represent the stratosphere.

The SCCM is a one-dimensional model equivalent to a single vertical column of the more complete 3D CCM3, which includes the CRM as the radiative transfer component of the model. In the SCCM the local time-rate-of-change of the large-scale state variables (e.g., temperature, moisture, momentum, etc.) depend on specified horizontal advection, a specified vertical motion field (from which the large scale vertical advection terms are evaluated), and subgrid scale sources, sinks and eddy transports. The subgrid scale contributions are determined by the physical parameterization. Because a single column model lacks the horizontal feedbacks that occur in complete three-dimensional models of the atmosphere, the governing equations are only coupled (incompletely) through the parameterized physics. While lacking the more complete feedback mechanisms available to an atmospheric column imbedded in a global model, SCCM is computationally inexpensive, providing a quick first look at the responses of the system to the introduced forcing.

The Chicxulub structure, on the Yucatan Peninsula, Mexico, was duced 65 Myr ago, in coincidence with the large Cretaceous-Tertiary mass extinction. The impact occurred on a partially submerged platform consisting of a thick (3 km) sequence of carbonates and evaporites overlying a continental crust. The extension of the evaporitic deposits in the sediments is not well constrained, ranging from about 50% to 30%, although lower limits of 10% have also been suggested. Hydrocode simulations indicate that the amount of S injected in the stratosphere ranged between about 75 and 270 Gt, depending also on projectile type and impact speed, with a lower limit of 25 Gt under the assumption that evaporites constituted only about 10% of the sedimentary layer. (The effect of the angle of impact, investigated in Pierazzo & Melosh -www.lpl.arizona.edu/~betty/chicx3d.html - suggests an uncertainty of about a factor of two for these estimates).
The SO₂, H₂O, and sulfate aerosols are assumed to be distributed uniformly over the globe. This assumption is partially justified by the fast expansion (well beyond the stratosphere) of the impact plume. Models of the ballistic distribution of impact ejecta suggest that impact-produced material would be distributed all over the globe in a matter of few hours. Since impact products re-enter the stratosphere from above, the gases and sulfate aerosols are initially distributed in the uppermost 3 stratospheric layers.

Time evolution (from January 2 equilibrium conditions) of net surface SW fluxes (as a ratio from the unperturbed experiment) subject to S-loads of 100 Gt for atmospheric columns located in North America (37^o N, 97^o W), Equatorial Pacific Ocean (1^o N, 160^o W), and South Pacific Ocean (46^o S, 160^o W). The column in North America shows steady decreases in net surface SW fluxes from about 60% (or a decrease of about 47 W/m^2) of the unperturbed case at the start of the integration to about 45% (or a decrease of 70 W/m^2) by day 30. The other two atmospheric columns show smaller and somewhat less consistent decreases in net surface SW flux.

    Time evolution (from January 2 equilibrium conditions) of the surface temperature differences from the unperturbed case for the column in North America (37^o N, 97^o W) for initial S-loads of 2,25, and 100 Gt. the impact-produced cooling that results from the reductions in net surface SW fluxes shown above is of the order of several degrees. This is at least an order of magnitude more than the cooling associated with a Pinatubo-like loading.