The idea of a medieval warm period was formulated for the first time in 1965 by the English climatologist Hubert H. Lamb . Lamb, who founded the UK Climate Research Unit (CRU) in 1971, saw the peak of the warming period from 1000 to 1300, i.e. in the High Middle Ages. He estimated that temperatures then were 1-2 ° C above the normal period of 1931-1960. In the high North, it was even up to 4 degrees warmer. The regular voyages of the Vikings between Iceland and Greenland were rarely hindered by ice, and many burial places of the Vikings in Greenland still lie in the permafrost.
Glaciers were smaller than today
Also the global retreat of glaciers that occurred in the period between about 900 to 1300  speaks for the existence of the Medieval Warm Period. An interesting detail is that many glaciers pulling back since 1850 reveal plant remnants from the Middle Ages, which is a clear proof that the extent of the glaciers at that time was lower than today .
Furthermore, historical traditions show evidence of unusual warmth at this time. Years around 1180 brought the warmest winter decade ever known. In January 1186/87, the trees were in bloom near Strasbourg. And even earlier you come across a longer heat phase, roughly between 1021 and 1040. The summer of 1130 was so dry that you could wade through the river Rhine. In 1135, the Danube flow was so low that people could cross it on foot. This fact has been exploited to create foundation stones for the bridge in Regensburg this year .
Clear evidence of the warm phase of the Middle Ages can also be found in the limits of crop cultivation. The treeline in the Alps climbed to 2000 meters, higher than current levels are . Winery was possible in Germany at the Rhine and Mosel up to 200 meters above the present limits, in Pomerania, East Prussia, England and southern Scotland, and in southern Norway, therefore, much farther north than is the case today . On the basis of pollen record there is evidence that during the Middle Ages, right up to Trondheim in Norway, wheat was grown and until nearly the 70th parallel/latitude barley was cultivated. In many parts of the UK arable land reached heights that were never reached again later.
Also in Asia historical sources report that the margin of cultivation of citrus fruits was never as far north as in the 13th century. Accordingly, it must have been warmer at the time about 1 ° C than today .
Archeology and history confirm interglacial
Insects can also be used as historical markers for climate. The cold sensitive beetle Heterogaster urticae was detected during the Roman Optimum and during the Norman High Middle Age in York. Despite the warming of the 20th century, this beetle is found today only in sunny locations in the south of England .
During the medieval climate optimum, the population of Europe reached hitherto unknown highs. Many cities were founded at this very time with high-altitude valleys, high pastures and cultivated areas, which were at the beginning of the Little Ice Age again largely abandoned .
The Middle Ages was the era of high culture of the Vikings. In this period their expansion occurred into present-day Russia and the settlement of Iceland, Greenland and parts of Canada and Newfoundland. In Greenland even cereals were grown about this time.. With the end of the Medieval Warm Period the heyday of the Vikings ended. The settlements in Greenland had to be abandoned as well as in the home country of Norway, during this time, many northern communities located at higher altitudes . The history of the Vikings also corresponds very well to the temperature reconstructions from Greenland, which were carried out using ice cores. According to the reconstructions, Greenland was at the time of the Vikings at least one degree warmer than in the modern warming period .
Climate scientists want to eliminate contradictions
Until about the mid-90s of last century the Medieval Warm Period was for climate researchers an undisputed fact. Therefore in the first progress report of the IPCC from 1990 on page 202, there was the graphics 7c , in which the Medieval Warm Period was portrayed as clearly warmer than the present. However, the existence of this warm period became quickly a thorn in the side for the scientists responsible. When in 12th century without human influence the climate has been even warmer than at the height of industrialization, why should the current warming have non-natural causes?
Thus, the Medieval Warm Period was soon declared an odious affair. Meanwhile, an e-mail is legendary, which was sent to a U.S. climate researcher David Deming  in 1995. This scientist published an article in the prestigious journal Science in which he had presented research on climate change in North America based on cores .
With this publication, he was immediately known among climate researchers, and some of them obviously thought that he was toeing their line [13, 15]:
Meanwhile, the climate machinery for the eradication of the Medieval Warm Period has already started. In 1995, the English climatologist Keith Briffa published in the journal Nature a study with sensational results. According to his studies of tree rings in the Siberian Polar-Ural, there had never been a Medieval Warm Period and the 20th century, suddenly appeared as the warmest of the last 1000 years . The real breakthrough was the thesis of 20th Century experience as the warmest of the millennium, but not until three years later, and that with the release of Michael Mann’s infamous Hockeystick [17, 18].
Warm period is extinguished
In this diagram that became the icon of human-induced global warming in the 3rd IPCC Assessment Report, the Medieval Warm Period has now been completely eradicated[/B][/COLOR]. [COLOR="Navy"][B]However, this curve was quickly under attack, mainly because the Canadian mathematician Steven McIntyre had serious doubts about the correctness of the representation and those pursued with the meticulousness of an auditor . McIntyre showed not only that Mann had used an algorithm that resulted in 90 percent of the cases to a hockey stick, but found also serious errors in the selection of the data and the location of places, as well as the use of incorrect data.
Of course, the Mann’s gang could not let these allegations unanswered. In response, Realclimate.com was founded, a name intended to suggest the truth, but somehow reminiscent of the Real Ghostbusters, a poorly made copy of the genuine, which in contrast to the original only pretends to be the right thing. This webpage was henceforth used for accusations and slanders against the non-”believers” . It took also increasingly care not to call McIntyre, in the meantime identified as the main enemy, by his name.
Following the publication of Michel Mann’s hockey stick and the criticism, whole series of further studies was published to demonstrate that the results of Mann’s actually represented the real temperatures over the last 1000 years. The highpoint of the debate was the forced disclosure of the raw data from tree ring studies long held under lock and key, which served as one of the principal witnesses for the correctness of the thesis of the unusually warm 20th century. It turned out that clearly the data were selected intently to get the desired result .
This survey shows one thing quite clearly. At the time of the Middle Ages, that is, from 1000 to 1300 it was almost everywhere in the world warmer than today. There have been periods of warming, that exceeded 0.6 degree Celsius rise in
temperature in the 20th century and totally without the man-made increased emissions of the supposed “climate killer” of CO2. The statements, that there has not been any Medieval Warm Period, or it was merely a localized phenomenon, can safely be regarded as untenable.
It is therefore not surprising that there are influences on the climate, which can by far exceed the CO2 as a driver of climate variability. This hypothesis is massively supported by the observations made during the last 10 years
. Finally, we have been experiencing no increase since 2002, the temperatures have dropped slightly . And that even though the emissions of CO2 from fossil fuels in exactly the same period increased to previously unmatched dimensions.