It was initially believed that the temperature changes were global . However, this view has been questioned; the 2001 IPCC report summarises this research, saying "…current evidence does not support globally synchronous periods of anomalous cold or warmth over this time frame, and the conventional terms of 'Little Ice Age' and 'Medieval Warm Period' appear to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries". The National Oceanic and Atmospheric Administration (NOAA) states that the "idea of a global or hemispheric "Medieval Warm Period" that was warmer than today however, has turned out to be incorrect" and that what those "records that do exist show is that there was no multi-century periods when global or hemispheric temperatures were the same or warmer than in the 20th century". Indeed, global temperature records taken from ice cores, tree rings, and lake deposits, have shown that the Earth was actually slightly cooler (by 0.03 degrees Celsius) during the 'Medieval Warm Period' than in the early- and mid-20th century.
Palaeoclimatologists developing region-specific climate reconstructions of past centuries conventionally label their coldest interval as "LIA" and their warmest interval as the "MWP". Others follow the convention and when a significant climate event is found in the "LIA" or "MWP" time frames, associate their events to the period. Some "MWP" events are thus wet events or cold events rather than strictly warm events, particularly in central Antarctica where climate patterns opposite to the North Atlantic area have been noticed.
The Vikings took advantage of ice-free seas to colonize Greenland and other outlying lands of the far north. The MWP was followed by the Little Ice Age, a period of cooling that lasted until the 19th century, and the Viking settlements eventually died out. In the Chesapeake Bay, researchers found large temperature excursions during the Medieval Warm Period (about 800–1300) and the Little Ice Age (about 1400–1850), possibly related to changes in the strength of North Atlantic thermohaline circulation. Sediments in Piermont Marsh of the lower Hudson Valley show a dry Medieval Warm period from AD 800–1300.
Prolonged droughts affected many parts of the western United States and especially eastern California and the western Great Basin. Alaska experienced three time intervals of comparable warmth: A.D. 1–300, 850–1200, and post-1800. Knowledge of the North American Medieval Warm Period has been useful in dating occupancy periods of certain Native American habitation sites, especially in arid parts of the western U.S. Review of more recent archaeological research shows that as the search for signs of unusual cultural changes during the MCA has broadened, some of these early patterns (e.g. violence and health problems) have been found to be more complicated and regionally varied than previously thought while others (e.g., settlement disruption, deterioration of long distance trade, and population movements) have been further corroborated.
An ice core from the eastern Bransfield Basin, Antarctic Peninsula, clearly identifies events of the Little Ice Age and Medieval Warm Period. The core clearly shows a distinctly cold period about AD 1000–1100, neatly illustrating the fact that "MWP" is a moveable term, and that during the "warm" period there were, regionally, periods of both warmth and cold.
Corals in the tropical Pacific Ocean suggest that relatively cool, dry conditions may have persisted early in the millennium, consistent with a La Niña-like configuration of the El Niño-Southern Oscillation patterns. Although there is an extreme scarcity of data from Australia (for both the Medieval Warm Period and Little Ice Age) evidence from wave built shingle terraces for a permanently full Lake Eyre during the ninth and tenth centuries is consistent with this La Niña-like configuration, though of itself inadequate to show how lake levels varied from year to year or what climatic conditions elsewhere in Australia were like.
Adhikari and Kumon (2001), whilst investigating sediments in Lake Nakatsuna in central Japan, verified the existence there of both the Medieval Warm Period and the Little Ice Age.
For further discussion of regional and global temperature variations see: Temperature record.