Short answers, pulled from the story.
The Medieval Warm Period was a time of elevated warmth in the North Atlantic region, conventionally dated from about 950 to about 1250. It is also called the Medieval Climatic Anomaly and the Medieval Climate Optimum. It was followed by a cooler interval known as the Little Ice Age.
The Medieval Warm Period was not a globally synchronous event. The Pages-2k consortium confirmed in 2019 that the warmest 51-year period within the MWP did not occur at the same time in different regions. Some areas, such as parts of central Antarctica, experienced conditions opposite to those of the North Atlantic during the same timeframe.
Three main causes have been proposed: increased solar activity, decreased volcanic activity, and changes in ocean circulation. Modelling evidence has established that natural variability alone was insufficient to explain the warming, meaning an external forcing must have played a role. No single cause has been definitively identified.
Vikings founded the Eastern and Western Settlements in southern Greenland around 985, when the climate was warm enough for farming. As the climate cooled around 1250, the settlers' diet shifted from livestock to ocean hunting; by around 1300, seal hunting provided more than three quarters of their food. The last surviving document from the settlements dates to 1412, after which the Europeans gradually withdrew.
Global temperature records from ice cores, tree rings, and lake deposits suggest the Earth may have been slightly cooler overall during the MWP, by about 0.03 degrees Celsius, compared to the early to mid-20th century. In parts of southern Greenland, warmth during the MWP exceeded 1961-1990 baseline levels and in some regions even surpassed the 1990-2010 period. A reconstruction of tropical South America, covering a 1,600-year study window, found that peak MWP temperatures did not reach late 20th century levels.
The Medieval Warm Period gave way to a cooler interval in the North Atlantic and elsewhere known as the Little Ice Age. The IPCC First Assessment Report described the Little Ice Age as ending in the middle to late 19th century. The transition was not abrupt and varied by region.