(a): estimated CET between 1200 & 1300 AD ~ 10.4degC (* see below);
(b): calculated CET 1950 - 1999 = 9.6degC
(c): calculated CET 1990 - 1999 = 10.1degC
Generally regarded as the 'Medieval Warm Period'. Characterised by world-wide warmth(+ see below), most notably (compared with before and after) at mid and high latitudes * winters probably mild (& wet) * occasional severe / cold winters * summers often dry & warm * warming 1000 - 1200AD at an irregular rate * melting of pack / drift ice in arctic regions * founding of Norse colonies around sub-arctic margin & possibly North America (Newfoundland etc.) * reduced North Atlantic storminess / jetstream strength? * 11th century a high number of disastrous floods along the English east coast: implies increased winter-half storminess, but also reflection of increased sea-levels generally * 12th & much of 13th century regarded as a 'golden age' for Scotland * increase in North Sea storminess during 13th century [Lamb] after lull earlier.
[ Thought to be the last period in history when most parts of the world were at least as warm as the late 20th century. (and thus a possible clue to what may happen in our current warming phase. )
(*): but, remember that earlier temperature record not known to a great degree of accuracy.
(**): specifically, Hubert Lamb suggested that the 'High Summer' months (roughly July & August) were probably warmer at this time than the latter part of the 20th century, but the mid-winter period (roughly December & January) is now as warm, or warmer than those of the early Middle Ages.
(+): some climatologists now challenge this and suggest the warming was a 'regional-scale' event only.]