section_path: External impact/Effect on Earth
section_name: Effect on Earth
paragraph_text: Effect on Earth
  
  A near-Earth supernova is a supernova close enough to the Earth to have noticeable effects on its biosphere. Depending upon the type and energy of the supernova, it could be as far as 3,000 light-years away.In 1996 it was theorised that traces of past supernovae might be detectable on Earth in the form of metal isotope signatures in rock strata. Iron-60 enrichment was later reported in deep-sea rock of the Pacific Ocean. In 2009, elevated levels of nitrate ions were found in Antarctic ice, which coincided with the 1006 and 1054 supernovae. Gamma rays from these supernovae could have boosted atmospheric levels of nitrogen oxides, which became trapped in the ice.
  
  Historically, nearby supernovae may have influenced the biodiversity of life on the planet. Geological records suggest that nearby supernova events have led to an increase in cosmic rays, which in turn produced a cooler climate. A greater temperature difference between the poles and the equator created stronger winds, increased ocean mixing, and resulted in the transport of nutrients to shallow waters along the continental shelves. This led to greater biodiversity.
  
  Type Ia supernovae are thought to be potentially the most dangerous if they occur close enough to the Earth. Because these supernovae arise from dim, common white dwarf stars in binary systems, it is likely that a supernova that can affect the Earth will occur unpredictably and in a star system that is not well studied. The closest-known candidate is IK Pegasi (HR 8210), about 150 light-years away, but observations suggest it could be as long as 1.9 billion years before the white dwarf can accrete the critical mass required to become a type Ia supernova.
  
  According to a 2003 estimate, a type II supernova would have to be closer than eight parsecs (26 light-years) to destroy half of the Earth's ozone layer, and there are no such candidates closer than about 500 light-years.