Potential climatic effects of meteoric smoke in the Earth's paleo-atmosphere

Abstract

Modelling of the growth of meteoric smoke in the Earth's atmosphere, by assuming the formation of either simple spherical (compact) particles or, more realistically, fractal (porous) aggregates, highlights important differences in the predicted atmospheric size distributions as a function of altitude. The calculated UV extinction and direct radiative forcing (DRF) of these types of particles is also quite different. It is shown that, with regard to (a), forming a UV barrier before the presence of significant ozone levels in the atmosphere and (b), triggering 'snowball Earth' glaciations by negative DRF, fractal smoke particles are unlikely to have been important even if the flux of interplanetary dust into the atmosphere was 3 orders of magnitude higher than the present day. However, if these particles are effective ice nuclei, then subsequent indirect forcing through ice cloud formation could have made a more significant contribution to the onset of ancient glaciation episodes. Copyright 2007 by the American Geophysical Union.