A gamma ray burst hitting Earth would seriously harm the plankton that are responsible for a significant fraction of photosynthesis. That would cause a massive spike in carbon dioxide levels, potentially causing a mass extinction...and it's quite possibly already happened.
Gamma ray bursts (or GRBs) are the brightest electromagnetic events in the universe, creating unimaginable amounts of gamma radiation in narrow beams over just a few seconds. The most common type of burst is thought to occur during the final collapse of supernovas. We've yet to see such a burst in our galaxy, but the ones we've seen in more distant locales are some of the most dramatic cosmic phenomena ever observed.
So what if a gamma ray burst managed to hit the Earth? It's an unlikely proposition, to be sure, but it's effects would be absolutely devastating, according to a new study led by Rolando Cárdenas of the Cuban Central University of Las Villas:
"Our wish was to link astrophysics with environmental science, which is quite an unexplored area. We wanted to know how stellar explosions might affect the evolution of life on Earth."
The team modeled what would happen if a gamma ray burst from 6000 light-years away hit the Earth. It isn't actually the gamma rays themselves that would be devastating, but instead what they would do to particles in the atmosphere. The initial arrival of the rays would rip electrons from gas molecules, exciting the entire atmosphere and causing the emission of ultraviolet energy.
This UV energy could reach up to 250 feet below the ocean surface, affecting all the plankton in its wake and damaging a crucial enzyme that allows them to perform photosynthesis. The plankton would also have to reduce the energy expenditure on photosynthesis so as to have a great chance of repairing their damaged DNA.
That massive reduction in total photosynthesis - one particular plankton species is responsible for 20% of the entire planet's photosynthesis - would throw the balance between carbon dioxide and oxygen dangerously out of whack, and the likely death of much of the plankton would also remove the foundation of the ocean food chain. At that point, a major extinction event would be a very likely outcome, and in fact scientists speculate a GRB was the cause of the Ordovician mass extinction 450 million years ago.
Earth does have one good natural defense against gamma ray bursts - we chose our galaxy wisely. Because the Milky Way is unusually high in metals and other elements heavier than helium, gamma ray bursts are far rarer here than they are in other galaxies. That still doesn't mean that there's no danger over the lifetime of our planet, explains researcher Andrew Levan:
"GRBs are likely to happen in our galaxy around every 10 million years or so. To affect the Earth it would have to be lined up with us and not too far away. However, it is plausible that over the Earth's 4.5 billion year history we could be affected by a GRB."