ABSTRACT

The extinction that marks the end of the Cretaceous period and the beginning of the Paleogene, 66 Ma, is one of the five great extinctions of the Phanerozoic eon. It is estimated that 75-80% of all species became extinct, including all marine dinosaurs and almost all terrestrial dinosaurs. In 1980, Luis Alvarez an colleagues proposed that the extinction was caused by the impact of an asteroid with a diameter of 10 km; in 1981, Glen Penfield and Antonio Camargo identified the impact crater in the vicinity of the town of Chicxulub, in the Yucatán Peninsula. We present the results of numerical simulations of the consequences of the impact, assuming as an initial condition that the impact produced global fires that injected ~15,000 Tg of carbon soot into the upper troposphere and lower stratosphere. The results show that a small part of this soot is entrained into the circulation of the stratosphere and mesosphere, where it remains suspended for a prolonged period, absorbing solar radiation and blocking sunlight at the surface. As a consequence, the Earth’s surface experiences an “asteroid night” lasting 1-2 years, followed by a period of reduced illumination (5-50% of normal) for another 4-5 years. The suppression of sunlight at the surface causes global cooling of about 15°C (10-12° in the oceans and 25-30°C on land). At the same time, the soot in heats the stratosphere, which warms by as much as 100°C. Stratospheric heating affects the chemical composition and photochemistry, such that the ozone layer is reduced globally by 80% of normal, which allows high levels of UV radiation to reach the surface. The combination of darkness, cold, and enhanced UV flux appears to be able to account for the widespread pattern of extinction of Cretaceous flora and fauna.

About the Speaker:

Rolando R. Garcia is Senior Scientist in the Atmospheric Chemistry Observations and Modeling Laboratory of the National Center for Atmospheric Research (NCAR). He is a graduate of New York University (Physics) and the University of Miami’s Rosenstiel School of Marine and Atmospheric Sciences (Atmospheric Science). His research interests include observational, theoretical and numerical studies of ozone and other minor atmospheric constituents; the effects of waves on the circulation and transport of chemical constituents in the middle atmosphere; excitation mechanisms and propagation of atmospheric waves; and numerical modeling of the global atmospheric circulation and chemistry-climate interactions. He has served as guest editor of the Journal of Atmospheric and Solar-Terrestrial Physics, and Editor of the Journal of the Atmospheric Sciences. He was chair of the Middle Atmosphere Committee of the American Meteorological Society, and a member of the Committee on Solar Terrestrial Research of the U.S. National Research Council. He is the author or co-author of over 150 refereed papers, and the recipient of the NCAR Publication Award (1990), the NOAA/ERL Distinguished Publication Award (1987, 1998), and the JGR Editor’s Citation for Excellence in Refereeing (1992, 1993, 2009). He was named Haurwitz Memorial Lecturer for 2010 by the American Meteorological Society, and is a Fellow of the American Geophysical Union and the American Meteorological Society.