One of the craters of the Laki volcano chain, Iceland. An eruption here in the late 18th century caused widespread famine in Egypt.

Credit: Juhász Péter

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SYDNEY: Volcanic eruptions near the poles can cause extreme weather as far away as the tropics, a U.S. study has shown for the first time.

A massive volcanic explosion in Iceland in 1783-84 caused a drought and famine over 5,200 kilometres away in Egypt, which reduced the population of the Nile Valley by a sixth, said researchers at Rutgers University, New Brunswick.

"Given the sensitivity of these arid regions to reductions in rainfall, our predictions may ultimately allow society time to plan for the consequences [of eruptions] and save lives," said Luke Oman, lead author of the paper published in Geophysical Research Letters.

Iceland's Laki volcano erupted more than ten times in 1783-84, killing 9,000 Icelanders (a quarter of the population), and releasing gases into the atmosphere that caused temperature changes.

While the northern landmasses, such as Europe, experienced cooling of up to three degrees Celsius after the eruption, closer to the equator in Africa, Arabia and India there was warming of one to two degrees Celsius.

By comparison, global temperatures dropped by 0.5 degrees Celsius on average after the 1991 eruption of Mt Pinatubo, in the Phillipines, and by 1.2 degrees Celsius on average after the infamous 1883 eruption of Krakatoa, in Indonesia.

"While considerable research has shown that eruptions in the tropics influence climate in the Northern Hemisphere winter, this study indicates that eruptions in high-latitudes produce changes in atmospheric circulation in the Northern Hemisphere summer," said Oman.

The Laki eruption had a significant impact on the climate because it released large amounts of sulfur dioxide gas into the atmosphere. When combined with water vapour, the gas formed into tiny particles called aerosols that reduced incoming sunlight, cooling the average global temperature.

The unusually cool weather reduced the temperature difference between the Eurasian continent and the Indian Ocean, and Africa and the Atlantic Ocean. Without a significant temperature contrast between the land and the ocean, onshore winds are thought to have weakened, reducing the rainfall in the region.

The researchers believe the weak monsoon reduced the cloud cover over northern Africa and India, allowing more of the sun's energy to reach the Earth's surface, raising temperatures and further worsening drought conditions.

The researchers' climate computer model, developed by the U.S. space agency NASA, was checked against historical records of the Nile dating back to 622 AD.

Extremely low Nile water levels, indicating reduced rainfall, coincided with an eruption in 1912 at Mount Katmai, Alaska, and an eruption in 939 at Eldgjá, Iceland, confirming the model's predictions.

Iceland has over 30 active volcanoes, with the most recent eruption in 2004 of the Grímsvötn volcano sending an ash cloud over Europe.