The 1783--1784 Laki flood lava eruption in Iceland emitted ~122 megatons (Mt) SO2 into the atmosphere and maintained a sulfuric aerosol veil that hung over the Northern Hemisphere for >5 months. The eruption columns extended to 9--13 km and released ~95 Mt SO2 into the upper troposphere/lower stratosphere (i.e., the polar jet stream), enforcing a net eastward dispersion of the plumes which reacted with atmospheric moisture to produce ~200 Mt of H2SO4 aerosols. Away from source, the Laki aerosols were delivered to the surface by subsiding air masses within anticyclones. We show that ~175 Mt of H2SO4 aerosols were removed as acid precipitation and caused the extreme volcanic pollution (i.e., dry fog) that effected Europe and other regions in 1783. The remaining ~25 Mt stayed aloft at tropopause level for >1 year. The summer of 1783 was characterized by extreme and unusual weather, including an unusually hot July in western Europe, most likely caused by perseverance of southerly air currents. The following winter was one of the most severe winters on record in Europe and North America. In these regions, the annual mean surface cooling that followed the Laki eruption was about -1.3¿C and lasted for 2--3 years. We propose that the upper troposphere/lower stratosphere aerosols from Laki disrupted the thermal balance of the Arctic regions for two summers and were the main mechanism for the associated climate perturbations. Eruptions of Laki magnitude have occurred in the recent past in Iceland and will occur again. If such an eruption were to occur today, one of the most likely immediate consequences would be disruption to air traffic over large portions of the Northern Hemisphere.