Even a limited nuclear exchange could trigger a climate catastrophe.

Today’s combination of nuclear proliferation, political instability, and urban demographics poses a renewed threat of nuclear winter. The sunlight-blocking effect of smoke and soot from even a limited nuclear war could trigger a climate catastrophe. The atomic explosion shown here occurred at the Nevada Test Site northwest of Las Vegas before dawn on Feb. 6, 1951.

If those bombs exploded over the most-populated areas of the nations, more than 5 million metric tons of smoke and soot would soar into the sky. Most of those particles would stay aloft for more than 6 years, says Oman. On average, the temperature at Earth’s surface would drop around 1.25°C for up to 3 years—about four times the short-term cooling effect resulting from the 1991 eruption of Mount Pinatubo in the Philippines. After 10 years, the global average temperature would still be 0.5°C below normal.

Those temperature decreases may seem no more than a slight chill, but they’re substantial, says Alan Robock, also of Rutgers University. Temperatures in the first few years after a 100-bomb India-Pakistan war would be cooler than during a centuries-long cold spell called the Little Ice Age, which ended during the mid-1800s. Average global temperatures were at that time between 0.6°C and 0.7°C below what they are today, and glaciers advanced in mountainous regions worldwide.

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