Using a computer model developed by NASA, scientists have linked the dramatic eruption in 1783 of the Laki volcano (photo), on southern Iceland, which killed 9000 islanders, to a famine in Egypt that decreased the population of the Nile valley by a sixth.

This the first study to firmly link high-latitude eruptions and rainfall in tropics, like North Africa. "Our findings may help us improve predictions of climate response following the next strong high-latitude eruption, specifically concerning changes in temperature and precipitation," said Luke Oman from Rutgers' department of environmental sciences in 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 and save lives."

Eruptions of tropical volcanoes were already known to produce warmer winters in higher latitudes, but the novelty of this study is the proof that volcanoes from north can also influence weather in lower latitudes. "New, strong evidence, from both observations and climate model simulations" prove that the Laki eruption have perturbed northern hemisphere atmospheric circulation deep into the tropics in the following summer.

The eruption in June 1783 of the Laki volcano is regarded as the largest at high-latitude in the last millennium: it produced three cubic miles ( 8 cubic kilometers) of lava and over 100 million tons of sulfur dioxide and other toxic gases, destroying vegetation, livestock and people.

The drought across northern Africa that followed the eruption led to a very low flow of the Nile River. "The [annual Nile] inundation of 1783 was not sufficient, great part of the lands therefore could not be sown for want of being watered, and another part was in the same predicament for want of seed. In 1784, the Nile again did not rise to the favorable height, and the dearth immediately became excessive. Soon after the end of November, the famine carried off, at Cairo, nearly as many as the plague," wrote French scholar Constantin Volney.

Tree ring analysis showed that, in the northern hemisphere, the summer of 1783 was the coldest in more than five centuries due to sulfate aerosols that reflected the sunlight, impeding the warmth of the Earth's surface. Laki is also linked to a weak monsoon (annual rainfall) in southern Asia. The cold in the North decreased the temperature contrast between the land and the oceans, so wet air from Indian Ocean was not sought over Asian land masses. But the computer models also showed significant warming across Africa to the southern Arabian Peninsula and on to India during the same summer.

Little or no monsoon means the collapse of agriculture and famine.