A medium-sized asteroid plunging into the ocean would destroy much of the ozone layer, leaving the Earth exposed to dangerous levels of ultraviolet radiation, it was claimed today.
The impact from a space rock 500 metres to one kilometre in diameter would send vast amounts of water into the atmosphere, according to US expert Dr Elisabetta Pierazzo.
Seawater chemicals such as chloride and bromide would strip away significant amounts of ozone, which provides a shield against harmful sun rays.
The result would be a huge spike in ultraviolet (UV) radiation levels at the Earth's surface.
People with fair skins would find their skin burning after just a few minutes of sun exposure.
Farmers would have difficulty growing crops, and rates of skin cancer and cataracts would be likely to rise.
Previous research looking at the effects of an oceanic asteroid impact has focused on the danger of tsunamis.
Dr Pierazzo's new work, published in the journal Earth and Planetary Science Letters, used computer simulations to model the effects on atmospheric ozone.
She tested two impact scenarios, involving a 500 metre and kilometre diameter asteroid.
"The results suggest that mid-latitude oceanic impact of one kilometre asteroids can produce significant global perturbation of upper atmospheric chemistry, including multi-year global ozone depletion comparable to record ozone holes recorded in the mid 1990s," said Dr Pierazzo, from the Planetary Science Institute in Tucson, Arizona.
"The removal of a significant amount of ozone in the upper atmosphere for an extended period of time can have important biological repercussions at the Earth's surface as a consequence of increase in surface UV-B irradiance. These include increased incidence of erythema (skin reddening), cortical cataracts, changes in plant growth and changes in molecular DNA."
People may be forced to avoid direct sunlight to protect themselves against the harmful UV rays, she added.
UV intensity is measured by the ultraviolet index (UVI), with levels of 10 or above assumed to be dangerous.
The highest UVI recorded on Earth so far has been 20, said Dr Pierazzo. But a 500-metre asteroid crashing into an ocean could see UVI jump to values above 20 for several months in the northern subtropics.
A one kilometre impact would see UVI soar to 56, rising above 20 for around two years in both the northern and southern hemispheres.
"A level of 56 has never been recorded before, so we are not sure what it is going to do," said Dr Pierazzo. "It would be producing major sunburn. We could stay inside to protect ourselves, but if you go outside during daylight hours you would burn. You would have to go outside at night, after sunset, to avoid major damage."
Assuming there is enough warning, farmers could reduce the effects of the impact by planting crops with a higher UV tolerance, she said.
Food could also be stored to ensure supplies during a few years of poor productivity.