The climate has changed over the last millennium. This has been recognized since the pioneering work of English meteorologist Hubert Lamb, founder of the Climate Research Unit at the University of East Anglia, and French historian Emmanuel Le Roy Ladurie, holder of the History of Modern Civilization chair at the Collège de France (1973-1999). Direct thermometric measurements have made it possible to reconstruct series going back as far as the 17thcentury , particularly in England and the Netherlands. However, these series present numerous problems that need to be corrected before any interpretation can be made, as illustrated by the multi-century temperature series for the city of Bologna.
To go back before the invention of the thermometer, it is possible to exploit indirect climatic data, such as the qualitative perception of temperature, wind, rain and clouds, as well as the observation of extreme events such as storms, river levels and flooding, and the seasonal duration of frost and snow cover. In addition to these meteorological indications, it is also useful to look at other indicators, such as the seasonal cycles of plants and animals, crop types and harvest dates and yields.
Lamb's first published millennial climate curve focused on England. Based on instrumental temperatures and indirect indicators, it introduced the notions of medieval climatic optimum (MCO) and Little Ice Age (LIA).
