Medieval Warm Period

The notion of a medieval warm period (English Medieval Warm Period, MWP or Medieval Climate Optimum ) was mainly influenced by the British climatologist Hubert H. Lamb in the 1960s. He referred to a climate warming, which stated Lamb with 1-2 degrees and culminating he situate 1000-1300.

With the advent of the controversy surrounding global warming Lambs thesis was discussed passionately. The climate skeptics applies the MWP as evidence of the natural variability in the climate and as an argument against human-induced climate change in the present. In the Fourth Assessment Report of the IPCC from 2007 is to be read to the fact that global warming in the high Middle Ages pronounced (within 0.2 to 0.4 degrees) was far less to watch and especially only in parts of North West Europe. But she is also busy in the Alps.

Arguments against a medieval warm period (or for a little warming)

From Michael Mann, Raymond S. Bradley and Malcolm Hughes, the three authors of the hockey stick graph, the amount of temperature rise is in doubt. Thus, the one-sided terms of Lamb is criticized on north-western Europe and thus questioned the global validity of the observation. In addition, in many high mountain regions of the world grew as the Alpine regions, Canada, Patagonia and Alaska, among others the glacier during the period between 1,050 bis 1150th

Compared to today, the situation at that time by the IPCC in the Fourth Assessment Report of 2007 is described as follows:

"The evidence currently available indicates did NH mean temperatures applications falling on medieval times (950-1100) were indeed warm in a 2 - kyr context and even warmer in relation to the less sparse but still limited evidence of wide spread average cool conditions in the 17th century ( ... ). However, de evidence is not Sufficient to support a conclusion did hemispheric mean temperatures applications were as warm, or the extent of hot regions as expansive, as Those in the 20th century as a whole, falling on any period in medieval times (...). "

" The currently available evidence suggests that the average temperatures of the Northern Hemisphere during the Medieval Warm Period (950 - 1100 AD), were actually warm in comparison to the last 2000 years. This applies even more in comparison to the less incomplete, but still limited availability of products for widespread average cool conditions in the 17th century (...). Nevertheless, the evidence is not sufficient to support the conclusion that hemispheric mean temperatures were so warm at any time of the Middle Ages or the extent of warm regions was so great as has been the case throughout the 20th century (...). "

Proxy data for certain regions - for example, parts of Greenland - suggest that some areas were at least temporarily warmer than today. Temperature reconstructions derived from ice cores, tree rings and lake sediments showed that it was slightly cooler globally at the time of the medieval warm period ( by about 0.03 degrees) than during the early and middle twentieth century.

Arguments for the medieval one (stronger ) warm period

The glacier grown in many parts of the world 1050-1150 looks at Pierre Alexandre especially not as an argument against MWP. On the one hand he speaks, unlike Lamb, of a secured documentation for a heating only from the year 1200. Secondly, he postulated the theory that the longest warm periods 1261-1361 were. For a MWP but speak a variety of other arguments. Starting with the discovery of a Erdbegräbnisses in Greenland in present-day permafrost on the settlement of Iceland to an increase in the tree line in the Alps at about 2000 meters. That the warm period was not only linked to our European continent appearance, shows the "North migration " of vegetation in Asia in subtropical plants such as citrus fruits, which grew in 1264 a few hundred kilometers further north than in the 20th century. Next, it is proven that the glaciers decreased worldwide since 1200. So can still be reported that the malaria-carrying Anopheles mosquito had spread to many parts of Europe and that even plagues of locusts are assigned to the 14th century in Central Europe.

Type of evidence

The evidence of a high medieval warm period are many and varied, was both by scientific methods such as the radiocarbon method to determine the age of organic matter, sedimentation analysis or Eisbohrkerntechnik a significant rise in temperatures is confirmed. as well as the evaluation of written sources such as chronicles or weather diaries strong evidence found. Play a special role in this case nor the proxy data, for example, to result from harvest numbers, receipts or icing flood information. They are almost everywhere available. The German geographer Rüdiger Glaser created on the basis of such data, a database for the past 1000 years.

Follow the medieval warm period in Europe

In the period in which the Medieval Warm Period was located, there it was in Europe a veritable population explosion. This is certainly also due to the favorable climate development, but by no means exclusively. Although it came as a result of the warmer climate in Europe to an expansion of the agricultural economy, both in much more northern than in upland areas of cereal cultivation was now possible. So grain marketing has been proven to Norway and in the mountains of Scotland, which in the subsequent Little Ice Age and the associated therewith cooling of the climate was reinstated.

Climatic conditions, however, were not the sole reasons for the rapid increase of population and the related expansion with their country's development. Wilhelm Abel cites as reasons for the agriculturally progress both in the use of technical equipment such as the collar for draft horses, as well as in land use as well as the diversification of crops. This combination made ​​it possible to feed a rapidly growing population with food. Thus it is assumed that the population has almost tripled in Europe 1100-1400. As a result, there was an interaction between population growth and the acquisition of new farmland. The population began with an expansion of the settlement area, (eg in the train of the German Ostsiedlung ) were transformed with the huge swathes of forest to farmland. In favor climate in Europe, the settlement areas altered by construction of numerous cities as new centers of trade and commerce, which shared the work with the agricultural areas.

Causes

The milder temperatures during the medieval optimum may be due to a significantly increased solar activity and global unusually low volcanic activity. The latter would cause less aerosols reflect sunlight and its cooling effect would thus be reduced.

Other theories point to periodic fluctuations (about 1,000 - 2,000 years) of the North Atlantic current as the cause. Evaporation of 0.25 × 106 m³ / s of water, which is transported into the Pacific, the salinity of the Atlantic increases. The circulation of the global conveyor belt is expected to increase approximately every 1,500 years strong, to compensate for the salt content. This is associated with variations in temperature of the sea water in the order of 4 to 5 K, which may also change the temperatures of the country.

Climate skeptics

The medieval warm period is sometimes led by climate skeptics as evidence that it was by no means certain that the current warming is caused by greenhouse gases emitted by humans. As the concentration of greenhouse gases at the time of the medieval warm period was not higher than before or after, only other causes may be responsible. They further argue that the same causes may be responsible for the warming of the 20th century.

The scientist Stefan Rahmstorf contributions to that argument, that no reputable climate scientists doubted that in addition to greenhouse gases, other factors influence the climate of the earth and these factors were also responsible for many climate fluctuations in the past. From the fact that these factors have caused a warming in the Middle Ages, that does not imply that greenhouse gases since the 20th century for the observed warming can not be the cause. Rather, it is so that, only the concentration of greenhouse gases have changed so much from all the known factors that can cause a global climate warming in the 20th century, so that the observed warming can be explained.

Famous for its climate- skeptical positions historian Wolfgang Behringer sees the observed effects in Europe of the medieval warm period as evidence that global warming is good for humanity. He argued that it was better off humanity in colder phases and worse in warmer phases.

This can be countered by knowing the impact of the medieval warm period only for some regions of the world, for many regions some are missing; in tropical southern China this was a time a pronounced, in North America, even an extreme drought. Thus, it is by no means certain, that have prevailed at that time a global view more favorable living conditions.

As the discussion about the medieval warm period began in the mid 1960s, this was a period of global cooling that lasted until the mid-1970s. A former warming to the level of the medieval warm period would have actually been beneficial in some regions at this time. Much, however, indicates that it was the end of the twentieth century already warmer than during the Medieval Warm Period in Europe.

Should be avoided massive reductions in greenhouse gas emissions, at the end of this century, with high probability the expected global average temperatures would be but higher than they were globally during the last several hundred thousand years, and perhaps even higher than they had ever been, since there are Homo sapiens. The observed at the end of the last ice age rapid global warming was a warming of about one degree per 1000 years. Even if the 2 ° C target would be reached (which is considered unlikely ), expected by the end of the 21st century global warming so would also run more than twenty times faster.

The debate about the extent and effects of the current and likely to be encountered man-made global warming therefore refers both what rate as well as what amount of heating regards to a unique historical process, lacking largely for the experience and only very sparsely found in paleoklimatologischen climate proxies.