# How likely and severe is the threat of a gamma ray burst to earth?

In the National Geographic article: http://news.nationalgeographic.com/news/2009/04/090403-gamma-ray-extinction_2.html it is suggested that a gamma ray burst likely caused a mass extinction in earth's history. How severe is the threat of a gamma ray burst to life on earth? It would be nice to see a chart that considers the threat within the next hundred years as follows (I have no idea what the actual numbers are, hence I am asking):

Severity - Likelihood in 100 years:

• Global extinction - 0.0000001%
• Major impact - 0.001%
• Moderate impact - 0.1%
• Minor impact (e.g. deplete ozone layer by 0.01%) - 10%
• Observable - 100% ("minor" gamma ray bursts are frequently detected)