New simulations show that even a limited nuclear exchange (such as might occur in a war between India and Pakistan) could have truly catastrophic effects for the entire world.

New climatological simulations show 100 Hiroshima-sized nuclear bombs — relatively small warheads, compared to the arsenals military superpowers stow today — detonated by neighboring countries would destroy more than a quarter of the Earth’s ozone layer in about two years.

Regions closer to the poles would see even more precipitous drops in the protective gas, which absorbs harmful ultraviolet radiation from the sun. New York and Sydney, for example, would see declines rivaling the perpetual hole in the ozone layer above Antarctica. And it may take more than six years for the ozone layer to reach half of its former levels.

Researchers described the results during a panel Feb. 18 at the annual meeting of the American Association for the Advancement of Science, calling it “a real bummer” that such a localized nuclear war could bring the modern world to its knees.

“This is tremendously dangerous,” said environmental scientist Alan Robock of Rutgers University, one of the climate scientists presenting at the meeting. “The climate change would be unprecedented in human history, and you can imagine the world … would just shut down.”