Climate Forcing of Impact-released S-bearing gases, water vapor and CO2 injected in the Stratosphere

32nd LPSC (2001) Abst. #1196

Introduction

The stratigraphy at the Chicxulub structure suggests that in addition to the direct short-term effects the K/T boundary impact event may have caused a longer-lasting strong and abrupt climate shift, and be responsible for the end-Cretaceous mass extinction. In particular, the release of large amounts of S-bearing gases produced a large climate forcing, as suggested by Pope et al. (1997). This work assesses the climate forcing associated with the stratospheric production of sulfate aerosols from the reaction of various loads of impact-released S-bearing gases and water vapor.

The climate forcing due to stratospheric S-injection is investigated using the Column Radiation Model (CRM) of the National Center for Atmospheric Research (NCAR), modified to include a Sulfate Aerosol Model (SAM) developed specifically for this work. The model has been tested adn tuned using the well-recorded and studies Pinatubo eruption of June 1991.
SAM: The microphysical model for the formation and evolution of the stratospheric aerosols follows that described in (Turco et al., J. Atmo. Sci. 36, 699, 1979). The aerosols are continuously formed by combining of SO₂ with H₂O, and their evolution is described by processes like coagulation and growth, gravitational settling and diffusion, for various particle sizes. The model starts from the relevant impact-produced gases, namely SO₂, SO₃, and H₂, injected in the stratosphere by the impact event. The initial SO₃ is transformed right away into sulfate aerosols (to account for an initial aerosol formation in the expansion plume). As H₂O and SO₂ react, small sulfate aerosol particles are formed, which then grow in size by coagulation while settling through the stratosphere. The smallest particle radius modeled is 0.5 microns, and the aerosol growth is followed up to a radius of 4 microns. As the aerosol particles leave the lowermost stratospheric layer and enter the troposphere, they are removed from the model (under the assumption that they are rained out from the troposphere in a matter of days). Aerosols size (radius, R) affects the type of forcing produced by aerosols: for R<2 microns the net effect at the Earths surface is negative and cooling occurs; for R>2microns the net effect is positive, suggesting warming.
CRM: The CRM is a standalone version of the radiation model used in CCM3 (www.cgd.ucar.edu/cmr/ccm3), NCAR's General Circulation Model (GCM). The atmosphere is modeled by 18 levels, ranging from the surface to about 48 km. The top 7 levels represent the stratosphere. The incoming Short-Wave (solar) and outgoing Long-Wave (terrestrial) radiation are treated separately. The SW radiation is resolved by 19 spectral bands covering the range 0.2-5 micron, while the LW radiation is resolved by 8 bands in the 6.5-20 micron region. The CRM is particularly useful for studying the Earth's energy budget and the radiative forcing of greenhouse gases and aerosols.

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 gases and sulfate aerosols are initially distributed in the uppermost 3 stratospheric layers.

The direct climate forcing of the sulfate aerosols depends both on the aerosols radiative properties and concentration. The aerosols single scattering albedo and asymmetry parameter are evaluated from standard Mie's theory, while the aerosols specific extinction for a given wavelength depends also on the aerosols size distribution. The concentration of aerosols depends on the S injected in the stratosphere. The amount of aerosols initially injected into the stratosphere depends on the amount of sulfur released as SO₃. Experimental studies suggest that about 20% of the evaporitic sulfur is released as SO₃, and quickly reacts with H₂O in the expansion plume to form H₂SO₄.

Left: Climate forcing for various loads of sulfur (20% SO₃ and 80% SO₂). The lower loads (2 and 10 Gt, shown in red in the figure) correspond to the S injection from the Chicuxlub projectile alone, as estimated by Kring et al. (EPSL 140, 201, 1996). The upper load (300 Gt, in yellow) is close to the maximum S-release from the sedimentary layer. Dashed and dotted lines represent the climate forcing associated with volcanic injections of SO₂ from the Pinatubo (June 1991, 20 Mt) and Toba (about 71 kyr ago, 1 Gt) eruptions, respectively. For comparison, the climate forcing associated with the largest CO₂ injection (for a 100 km transient crater) is only around -2 W/m² (i.e., warming with the adopted convention). Right Visible transmission associated with the climate forcing on the right.