Ash Thursday

Just over 200 years ago an Icelandic volcano erupted with catastrophic consequences for weather, agriculture and transport across the northern hemisphere – and helped trigger the French revolution.

The Laki volcanic fissure in southern Iceland erupted over an eight-month period from 8 June 1783 to February 1784, spewing lava and poisonous gases that devastated the island's agriculture, killing much of the livestock. It is estimated that perhaps a quarter of Iceland's population died through the ensuing famine.

Then, as now, there were more wide-ranging impacts. In Norway, the Netherlands, the British Isles, France, Germany, Italy, Spain, in North America and even Egypt, the Laki eruption had its consequences, as the haze of dust and sulphur particles thrown up by the volcano was carried over much of the northern hemisphere.

Ships moored up in many ports, effectively fogbound. Crops were affected as the fall-out from the continuing eruption coincided with an abnormally hot summer. A clergyman, the Rev Sir John Cullum, wrote to the Royal Society that barley crops "became brown and withered … as did the leaves of the oats; the rye had the appearance of being mildewed".

The British naturalist Gilbert White described that summer in his classic Natural History of Selborne as "an amazing and portentous one … the peculiar haze, or smokey fog, that prevailed for many weeks in this island, and in every part of Europe, and even beyond its limits, was a most extraordinary appearance, unlike anything known within the memory of man.

"The sun, at noon, looked as blank as a clouded moon, and shed a rust-coloured ferruginous light on the ground, and floors of rooms; but was particularly lurid and blood-coloured at rising and setting. At the same time the heat was so intense that butchers' meat could hardly be eaten on the day after it was killed; and the flies swarmed so in the lanes and hedges that they rendered the horses half frantic … the country people began to look with a superstitious awe, at the red, louring aspect of the sun."

Across the Atlantic, Benjamin Franklin wrote of "a constant fog over all Europe, and a great part of North America".

The disruption to weather patterns meant the ensuing winter was unusually harsh, with consequent spring flooding claiming more lives. In America the Mississippi reportedly froze at New Orleans.

The eruption is now thought to have disrupted the Asian monsoon cycle, prompting famine in Egypt. Environmental historians have also pointed to the disruption caused to the economies of northern Europe, where food poverty was a major factor in the build-up to the French revolution of 1789.

Volcanologists at the Open University's department of earth sciences say the impact of the Laki eruptions had profound consequences.

Dr John Murray said: "Volcanic eruptions can have significant effects on weather patterns for from two to four years, which in turn have social and economic consequences. We shouldn't discount their possible political impacts."