Session H7: Astrobiology and Planetary Effects

Astrophysical radiation sources and terrestrial biodiversity

Extrasolar events such as gamma-ray bursts and supernovae have long been recognized as threats to the terrestrial biosphere, and possibly a driver of the mutation rate. It has recently become possible to quantify the severity of some of these effects and their expected incidence rate, with the dominant stress likely to be increased UVB at the surface due to ozone depletion, leading to protein and DNA damage to exposed organisms. Substantial fluctuations in the cosmic ray rate are also likely, and supported by terrestrial data. I will summarize results to date, and argue that biodiversity oscillations evident in the fossil record may be understood as a consequence of solar motion within the galaxy.   

Cosmic Rays, CO$_2$ Runaway, Sea Level Rise and Severe Global Flooding

In CO$_2$ runaway, the ocean surface layers (which in 1999AD contain, in each 100 m of depth, more CO$_2$ than the entire atmosphere) can suddenly become a continuous strong source of CO$_2$ as surface temperatures rise, because the solubility of CO$_2$ decreases 3 percent per degree Celsius. The evolving CO$_2$ increases atmospheric greenhouse longwave opacity providing positive feedback accelerating the CO$_2$ release (without the long time scales of deep ocean processes). Because the present atmospheric CO$_2$ concentration, 380ppmv, is already so much higher than the 270 ppmv thought to be the preindustrial Pleistocene maximum, it now presents a climate warming forcing so strong that orbital and other climate forcings, which in the past have brought about periodic ice ages, are not able to restore the glaciating mode.   

The Cosmogenic Origin of the 62 Myr Biodiversity Oscillation

Temporal variation of the diversity of terrestrial organisms has recently been put on the firm statistical basis. Rohde and Muller (Nature, 434, 208, 2005) have shown, using the de-trended data from Sepkoski’s compendium, that the number of marine genera varies periodically with time with approximately $62 \pm 3$-million-year period. No satisfactory explanation has been put forward so far. For instance, ``traditional'' explanations of the cycle by the Solar system passing through the galactic spiral arms or the galactic plane do not reproduce the correct phase and/or period. Here we argue that the modulated cosmic ray flux can naturally explain the observed periodicity in the fossil record. We present a physical theory, which based on the cosmic ray propagation in the galactic interstellar medium magnetic turbulence and the Sun's motion in the Milky Way.