That graph seems to have opposing data for the global Medieval Warm Period and Little Ice Age. Medieval Warm Period (MWP) from about 900 A.D. to 1300 A.D. and the Little Ice Age (LIA) from about 1300 A.D. to 1915 A.D.
> The Medieval Warm Period (MWP) also known as the Medieval Climate Optimum, or Medieval Climatic Anomaly was a time of warm climate in the North Atlantic region lasting from c. 950 to c. 1250.[1] It was likely[2] related to warming elsewhere[3][4][5] while some other regions were colder, such as the tropical Pacific
.. which implies that Japan may have been slightly colder at the time.
(Neatly illustrates that computing global average temperature from a set of local measurements is not as simple as it sounds, because there may also be local climate phenomena)
> Schematic diagrams of global temperature variations since the Pleistocene on three time scales
> There is giowing evidence that worldwide temperatures were higher than at present during the mid-Holocene (especially 5 000-6 000 BP), at least in summer, though carbon dioxide levels appear to have been quite similar to those of the pre-mdustnal era at this time (Section 1 i Thus parts <si western Euiope China, Japan, the eastern USA were a few degrees warmer in July during the midHolocene than in recent decades (Yoshino and Urushibara, 1978, Webb ct al 1987, Huntley and Prentice, 1988, Zhang and Wang 1990) Parts of Australasia and Chile were also waimei The late tenth to early thirteenth centuries (about AD 950-1250) appear to have been exceptionally warm in western Europe, Iceland and Greenland (Alexandre 1987, Lamb, 1988) This period is known as the Medieval Climatic Optimum China was, however, cold at this time (mainly in winter) but South Japan was warm (Yoshino, 1978) This period of widespread warmth is notable in that there is no evidence that it was accompanied by an increase of greenhouse gases
Sorry about the spelling issues in the copied text, the PDF's OCR isn't the best.
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Also here in another report, graph b) on page 2/16 shows the same MWP and little ice age as "Global temperature trend for millennium"
The graph c) also shows that in the past 25,000 years, it first used to be much colder, then got much hotter and then cooler and then warmer. It is pretty much impossible for it to be a local phenomenon if the variation was that much in that many areas.
Dr. David Deming, geologist and geophysicist, College of Earth and Energy, University of Oklahoma, senate testimony from December 6, 2006:
> I had another interesting experience around the time my paper in Science was published. I received an astonishing email from a major researcher in the area of climate change. He said, "We have to get rid of the Medieval Warm Period." The Medieval Warm Period (MWP) was a time of unusually warm weather that began around 1000 AD and persisted until a cold period known as the "Little Ice Age" took hold in the 14th century. Warmer climate brought a remarkable flowering of prosperity, knowledge, and art to Europe during the High Middle Ages. The existence of the MWP had been recognized in the scientific literature for decades. But now it was a major embarrassment to those maintaining that the 20th century warming was truly anomalous. It had to be "gotten rid of."
> The first phase of the LIA began around the thirteenth century in all the regions for
which there is evidence. The glacial phase preceding the MWP seems to have begun
between the seventh and ninth centuries A.D. but is generally less securely dated
and not dated at all in Canada. There are at least some indications of fluctuations
in ice position in the course of the MWP in Norway, Alaska, and perhaps in
extratropical South America and New Zealand, indicating that recession may have
been interrupted by advances, perhaps of limited extent, as in the European Alps.
The available evidence suggests that the MWP was global in extent and not uniform
climatically. The glacial data needs to be considered in relation to that from other
sources, but is of value in obtaining a more complete understanding of both the
environment in the later medieval period and the possible causes of climatic change
on the century time scale.
> The available evidence suggests that the MWP was global in extent and not uniform
climatically.
> The first phase of the LIA began around the thirteenth century in all the regions for
which there is evidence. The glacial phase preceding the MWP seems to have begun
between the seventh and ninth centuries A.D. but is generally less securely dated
and not dated at all in Canada. There are at least some indications of fluctuations
in ice position in the course of the MWP in Norway, Alaska, and perhaps in
extratropical South America and New Zealand, indicating that recession may have
been interrupted by advances, perhaps of limited extent, as in the European Alps.
The available evidence suggests that the MWP was global in extent and not uniform
climatically. The glacial data needs to be considered in relation to that from other
sources, but is of value in obtaining a more complete understanding of both the
environment in the later medieval period and the possible causes of climatic change
on the century time scale.
> The available evidence suggests that the MWP was global in extent and not uniform
climatically.
> Dating of organic material closely associated with moraines in many montane regions has reached the point where it is possible to survey available information concerning the timing of the medieval warm period. The results suggest that it was a global event occurring between about 900 and 1250 A.D., possibly interrupted by a minor readvance of ice between about 1050 and 1150 A.D.
> The findings support the view that the Holocene Thermal Maximum, the Medieval Warm Period, and the Little Ice Age were global events, and they provide a long-term perspective for evaluating the role of ocean heat content in various warming scenarios for the future.
And yes, there have been reversals in some years. But overall you can see the industrialisation "hockey stick" quite clearly in that calendar data.