Laki

Laki () or Lakagígar (, "Craters of Laki") is a volcanic fissure in the western part of Vatnajökull National Park, Iceland, not far from the volcanic fissure of Eldgjá and the small village of Kirkjubæjarklaustur. The fissure is properly referred to as Lakagígar, while Laki is a mountain that the fissure bisects. Lakagígar is part of a volcanic system centered on the volcano Grímsvötn and including the volcano Thordarhyrna. It lies between the glaciers of Mýrdalsjökull and Vatnajökull, in an area of fissures that run in a southwest to northeast direction.
The system erupted violently over an eight-month period between June 1783 and February 1784 from the Laki fissure and the adjoining volcano Grímsvötn, pouring out an estimated 42 billion tons or of basalt lava and clouds of poisonous hydrofluoric acid and sulfur dioxide compounds that contaminated the soil, leading to the death of over 50% of Iceland's livestock population, and the destruction of the vast majority of all crops. This led to a famine which then killed approximately 25% of the island's human population. The lava flows also destroyed 20 villages.
The Laki eruption and its aftermath caused a drop in global temperatures, as 120 million tons of sulfur dioxide was spewed into the Northern Hemisphere. This caused crop failures in Europe and may have caused droughts in North Africa and India.
1783 eruption.
On 8 June 1783, a 25 km (15.5 mi) long fissure with 130 craters opened with phreatomagmatic explosions because of the groundwater interacting with the rising basalt magma. Over a few days the eruptions became less explosive, Strombolian, and later Hawaiian in character, with high rates of lava effusion. This event is rated as 4 on the Volcanic Explosivity Index, but the eight-month emission of sulfuric aerosols resulted in one of the most important climatic and socially repercussive events of the last millennium.
The eruption, also known as the Skaftáreldar (, "Skaftá fires") or Síðueldur produced an estimated of basalt lava, and the total volume of tephra emitted was . Lava fountains were estimated to have reached heights of . The gases were carried by the convective eruption column to altitudes of about .
The eruption continued until 7 February 1784, but most of the lava was ejected in the first five months. One study states that the event "occurred as ten pulses of activity, each starting with a short-lived explosive phase followed by a long-lived period of fire-fountaining". Grímsvötn volcano, from which the Laki fissure extends, was also erupting at the time, from 1783 until 1785. The outpouring of gases, including an estimated 8 million tons of hydrogen fluoride and an estimated 120 million tons of sulfur dioxide, gave rise to what has since become known as the "Laki haze" across Europe.
Consequences in Iceland.
The consequences for Iceland, known as the or "Mist Hardships", were disastrous. An estimated 20–25% of the population died in the famine and fluoride poisoning after the fissure eruptions ensued. (Some sources specify a death toll of 9,000 people.) Approximately 80% of sheep, 50% of cattle and 50% of horses died because of dental fluorosis and skeletal fluorosis from the 8 million tons of hydrogen fluoride that were released. The livestock deaths were primarily caused by eating the contaminated grass; the subsequent famine claimed many of the human lives that were lost.
The parish priest and dean of Vestur-Skaftafellssýsla, Jón Steingrímsson (1728–1791), grew famous because of the "eldmessa" ("fire sermon") that he delivered on 20 July 1783. The people of the small settlement of Kirkjubæjarklaustur were worshipping while the village was endangered by a lava stream, which ceased to flow not far from town, with the townsfolk still in church.
Consequences in monsoon regions.
There is evidence that the Laki eruption weakened African and Indian monsoon circulations, leading to between less daily precipitation than normal over the Sahel of Africa, resulting in, among other effects, low flow in the River Nile. The resulting famine that afflicted Egypt in 1784 cost it roughly one-sixth of its population. The eruption was also found to have affected the southern Arabian Peninsula and India.
Consequences in East Asia.
In Japan the Great Tenmei famine that was already underway was undoubtedly worsened and prolonged.
Consequences in Europe.
An estimated 120,000,000 long tons of sulfur dioxide was emitted, about three times the total annual European industrial output in 2006 (but delivered to higher altitudes, hence more persistent), and equivalent to six times the total 1991 Mount Pinatubo eruption. This outpouring of sulfur dioxide during unusual weather conditions caused a thick haze to spread across western Europe, resulting in many thousands of deaths throughout the remainder of 1783 and the winter of 1784.
The summer of 1783 was the hottest on record and a rare high-pressure zone over Iceland caused the winds to blow to the south-east. The poisonous cloud drifted to Bergen in Denmark–Norway, then spread to Prague in the Kingdom of Bohemia (now the Czech Republic) by 17 June, Berlin by 18 June, Paris by 20 June, Le Havre by 22 June, and Great Britain by 23 June. The fog was so thick that boats stayed in port, unable to navigate, and the sun was described as "blood coloured". 
Inhaling sulfur dioxide gas causes victims to choke as their internal soft tissue swells – the gas reacts with the moisture in lungs and produces sulfurous acid. The local death rate in Chartres was up by 5% during August and September, with more than 40 dead. In Great Britain, the east of England was most affected. The records show that the additional deaths were among outdoor workers; the death rate in Bedfordshire, Lincolnshire and the east coast was perhaps two or three times the normal rate. It has been estimated that 23,000 British people died from the poisoning.
The weather became very hot, causing severe thunderstorms with large hailstones that were reported to have killed cattle, until the haze dissipated in the autumn. The winter of 1783–1784 was very severe; the naturalist Gilbert White in Selborne, Hampshire, reported 28 days of continuous frost. The extreme winter is estimated to have caused 8,000 additional deaths in the UK. During the spring thaw, Germany and Central Europe reported severe flood damage.
The meteorological impact of Laki continued, contributing significantly to several years of extreme weather in Europe. In France, the sequence of extreme weather events included a surplus harvest in 1785 that caused poverty for rural workers, as well as droughts, bad winters and summers. These events contributed significantly to an increase in poverty and famine that may have contributed to the French Revolution in 1789. Laki was only one factor in a decade of climatic disruption, as Grímsvötn was erupting from 1783 to 1785, and there may have been an unusually strong El Niño effect from 1789 to 1793.
Consequences in North America.
In North America, the winter of 1784 was the longest and one of the coldest on record. It was the longest period of below-zero temperatures in New England, with the largest accumulation of snow in New Jersey, and the longest freezing over of Chesapeake Bay, where Annapolis, Maryland is, then the capital of the United States; the weather delayed Congressmen in coming to Annapolis to vote for the Treaty of Paris, which formally ended the American Revolutionary War. A huge snowstorm hit the South; the Mississippi River froze at New Orleans and there were reports of ice floes in the Gulf of Mexico.
Contemporary reports.
Gilbert White recorded his perceptions of the event at Selborne, Hampshire, England:
Benjamin Franklin recorded his observations in America in a 1784 lecture:
According to contemporary records, Hekla did not erupt in 1783; its previous eruption was in 1766. The Laki fissure eruption was to the east and the Grímsvötn volcano was erupting about north east. Katla, only south east, was still renowned after its spectacular eruption 28 years earlier in 1755.
Sir John Cullum of Bury St Edmunds, Suffolk, England, recorded his observations on 23 June 1783 (the same date on which Gilbert White noted the onset of the unusual atmospheric phenomena), in a letter to Sir Joseph Banks, then President of the Royal Society:
Sir John goes on to describe the effect of this "frost" on trees and crops: