Gamma‐Ray Bursts and the Earth: Exploration of Atmospheric, Biological, Climatic, and Biogeochemical Effects

Gamma-Ray Bursts (GRBs) are likely to have made a number of significantimpacts on the Earth during the last billion years. We have used atwo-dimensional atmospheric model to investigate the effects on the Earth'satmosphere of GRBs delivering a range of fluences, at various latitudes, at theequinoxes and solstices, and at different times of day. We have estimated DNAdamage levels caused by increased solar UVB radiation, reduction in solarvisible light due to $\mathrm{NO_2}$ opacity, and deposition of nitratesthrough rainout of $\mathrm{HNO_3}$. For the ``typical'' nearest burst in thelast billion years, we find globally averaged ozone depletion up to 38%.Localized depletion reaches as much as 74%. Significant global depletion (atleast 10%) persists up to about 7 years after the burst. Our results dependstrongly on time of year and latitude over which the burst occurs. We find DNAdamage of up to 16 times the normal annual global average, well above lethallevels for simple life forms such as phytoplankton. The greatest damage occursat low to mid latitudes. We find reductions in visible sunlight of a fewpercent, primarily in the polar regions. Nitrate deposition similar to orslightly greater than that currently caused by lightning is also observed,lasting several years. We discuss how these results support the hypothesis thatthe Late Ordovician mass extinction may have been initiated by a GRB.Comment: Minor revisions, includes more discussion of reaction rates; 1 new figure. Accepted for publication in ApJ. 68 pages, 21 figures, 12 table