What would happen if the Earth was hit by a gamma-ray burst

Radiation from a gamma-ray burst would ionize Earth’s atmosphere and destroy its ozone layer, causing an electromagnetic explosion.

The resulting shock wave would generate enough heat to dissociate nitrogen molecules N2 into atomic nitrogen, which would in turn react with the dissociated oxygen leading to the formation of nitrogen monoxide NO and later to the formation of nitrogen dioxide NO2. The resulting smog would cause something very similar to a nuclear winter, which would impair photosynthesis on earth and greatly lower global average temperatures. Nitrogen dioxide would react with water vapor to trigger rains of hydrogen nitrate, which would not harm plants but would be bad for animal life.

Despite the severity of these events, they would be eclipsed by the sudden lack of the ozone layer. Without it, a constant and powerful stream of ultraviolet rays from the sun would damage the DNA of the vast majority of living creatures, given their great ionizing power and penetrative capacity. All life on earth would have to start from scratch, or nearly so.

It is also probable that at least one such event has already occurred in the last billion years. It is possible that the first of the great mass extinctions on the planet 450 million years ago was actually caused by a gamma-ray burst from a supernova located a few thousand light-years away from us, and that it is the cause of the loss of about 85% of the then existing species