If a medium-sized asteroid lands in the ocean, tsunamis won't be the only worry. Water vapour and sea salt thrown up by the impact could damage the ozone layer, leading to record levels of ultraviolet radiation that could threaten humanity.
"This suggests new issues you could have with ocean impacts that people hadn't thought of before," says Brian Toon of the University of Colorado at Boulder, who was not involved in the work.
Elisabetta Pierazzo of the Planetary Science Institute in Tucson, Arizona, and colleagues used a global climate model to study how water vapour and sea salt thrown up from an impact will affect ozone levels for years after the event.
They focused on medium-sized asteroids, either 500m or 1km wide. To date, 818 asteroids that are at least 1km wide have been discovered on orbits that could take them close to Earth.
These objects are on orbits that give them a very small probability of hitting Earth in the near future. However, estimates of the asteroid population suggest dozens more have yet to be found, with unknown orbits that could intersect with the Earth.
To get a sense of how much water might be jettisoned into the atmosphere if these asteroids hit the ocean, the team modelled what would happen if they reached Earth's atmosphere at a clip of 18km per second, an average speed expected for a near-Earth object, and hit the ocean in the northern hemisphere at a 45-degree angle.
As expected, the simulations showed that the larger, 1km asteroid created the bigger splash, throwing 42 trillion kilograms of water and vapour – enough to fill 16 million Olympic-sized swimming pools – across an area more than 1000km wide and up to hundreds of kilometres above the Earth's surface.
Once in the atmosphere, the water, together with compounds containing chlorine and bromine from vaporised sea salts, destroyed ozone above the Earth's atmosphere at a much faster rate than it is naturally created.
Some simulated impacts created depletions that were still felt across the whole Earth a year later. "It will produce an ozone hole that will engulf the entire Earth," Pierazzo says.
The longest lasting and most severe depletion – a cut of more than 70 per cent in ozone levels – occurred over much of the northern hemisphere.
That's a far bigger hole than the one that was above the South Pole in 1993, when Earth's ozone layer was at its thinnest. The resulting ultraviolet-radiation levels would be higher than anywhere on Earth today, the team writes, presenting a new hazard for human civilisation.
While people may be able to protect themselves from the increased threat of sunburn, the intense UV light could also affect our food supply by damaging plants and the phytoplankton that represent the bottom of the ocean's food chain. "That is enough to really cause problems for our civilisation," Pierazzo says.
Better understanding these effects could help us prepare in the event of an impact. For example people could plant crops more resistant to UV radiation, she adds.
Toon notes that impacts on land or shallow water may ultimately do more damage by kicking up dust that could significantly darken skies and inhibit plant growth. Pierazzo is now working on a model to assess how asteroids that hit dry land would affect the atmosphere.