Observation series and models can be geographically restricted to Europe. A model-data comparison over the last millennium still shows a correspondence with the OCM and PAG periods, superimposed on which are secular variations of around 0.5°C in amplitude. The consequences are particularly visible in the Alps, where glacier advances and retreats are consistent, as are episodes of rapid sedimentation in mountain lakes. High-resolution records from the Alps and coastal areas from Morocco to Scandinavia show rapid variations attributed to modulation of the North Atlantic Oscillation (NAO). This atmospheric oscillation leads to variations in the intensity and location of lows crossing the Atlantic from west to east. Series measured on Greenland ice, stalagmites and trees at various sites have enabled us to construct a paleoclimatic NAO index for the last millennium. This research has led to the hypothesis of a NAO oscillation locked in a positive phase during the CMO, but recent compilations and modelling show fairly weak effects in terms of amplitude.
Climatic variations in the North Atlantic to the Arctic zone are studied using ocean sediments and marine organisms with calcareous skeletons and annual rings (Icelandic meadowsweet, encrusting red algae). The compilation of marine series shows the classic succession of OCM and PAG, with an impact on pack-ice extension.
These climatic and hydrological fluctuations may well have favored Viking sea voyages during the CMO, including the colonization of Iceland and Greenland, and the exploration of the North American coast. The climatic deterioration of the PAG clearly impacted Greenland's Viking settlements, which adapted to their environment, contrary to certain preconceived ideas. This emblematic example shows the complexity of the influences on these human societies, which cannot be reduced to the direct impact of temperature. We can cite the conflictual relations with the Inuit and the decline in the walrus ivory trade, combined with the drop in contacts with Europe ravaged by the Black Death. Recent studies have shown that plague contamination is modulated by the climate in Central Asia, which affects large gerbils and fleas, the vectors of the bacterium. Climatic variations during the PAG even seem to be at the root of multiple reintroductions since the Black Death.
