Model-data comparisons for the last millennium are in agreement for the long OCM-PAG trend, as well as for brief cooling events corresponding to major volcanic eruptions, sometimes close in time (e.g. 1230-1258, 1809-1815 AD) on which variations in solar activity are superimposed. The OCM-PAG transition is marked by the two stratospheric eruptions of volcanoes Samalas in 1257 and Kuwae in 1458 AD. The SO2 flux from the 1257 eruption was responsible for the greatest radiative disturbance in the historical period. The direct stigmata are still clearly visible on the island of Lombok, which was completely covered in volcanic ash. The climatic impact has been demonstrated as far afield as Europe, particularly in the year following the eruption, with lower temperatures, abnormal rainfall and flooding, leading to reduced harvests, higher grain prices and, in some cases, famine. The same scenario must have been repeated for the other major eruptions, notably that of 1458, which began the PAG period.
The most precise data concern one of the last major PAG eruptions: in 1815, pyroclastic flows and ash from the Tambora volcano killed more than 10,000 people on the island of Sumbawa. This eruption also triggered a global radiative disturbance that led to the famous "year without summer", well documented in the meteorological archives of 1816, with a marked drop in temperatures and an increase in precipitation in Western Europe.
As the meteorological and paleoclimatic data for the periods surrounding these major eruptions, as well as for the last smaller eruption of this type (Mount Pinatubo in 1991), show, the impact is not limited to cooling through increased reflection of incident solar radiation by volcanic aerosols. Complex temperature distributions are revealed in winter, as are changes in atmospheric dynamics linked to contrasting effects depending on altitude and latitude. Stratospheric" eruptions appear to disrupt the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO).
