Oliver Milman “Vikings were not spurred to Greenland by warm weather, research shows”http://www.theguardian.com/world/2015/dec/04/vikings-greenland-warm-weather-history
これまでは、ヴァイキングたちは中世中期の温暖化に伴ってグリーンランドに移住し、その後の温暖化の終わりと寒冷化（「小氷河期」の訪れ）に適応できずに撤退したというのが通説となっていた*1。しかし、米国コロンビア大学のNicolas Young氏*2らがグリーンランドのかつて氷河に覆われていた岩を分析した結果、ヴァイキングたちが移住した時期、「中世温暖期」*3のグリーンランドの気温には目立った変動が見られなかった。つまり、ヴァイキングたちは、温暖化に誘われて移住し寒冷化に追われて撤退したのではないことになり、別の動機・要因を考えなければならないことになる。また、中世の温暖化は（例えばヨーロッパに）限定的な気候現象であり、地球規模のものではなかったことになる。
別の研究；
But new analysis of glaciers in Greenland shows that there was no significant change in their extent during the medieval warm period, suggesting that it remained relatively cold throughout the Viking colonisation of Greenland.US geologists measured an isotope found in rocks once covered by glaciers to determine how long they have been exposed to sunlight. The results from western Greenland and Baffin Island show there was no major fluctuation during the medieval warm period, with other records from sites closer to Viking settlements showing the same.
While it takes some time for glaciers to retreat due to warming temperatures, scientists say there would have had to have been a huge increase in precipitation, of around 50%, to have held the Greenland glaciers steady. As there is no evidence of such an increase in rainfall, the research suggests that the medieval warm period was confined to certain areas, such as Europe, and did not stretch to all corners of the world.
「中世温暖期」は大体950〜1250年頃まで続いたという。日本史でいうと、平安時代から鎌倉時代初期頃に対応する。日本における軍事貴族（武士）の勃興、院政、或いは源平の抗争などを気候温暖化との関係で説明するというのはあるのだろうか。
Previous research has painted a complex picture of the climate of Greenland during Norse occupation. Study of an ice core extracted in 2011 shows temperatures dropped as Vikings arrived, with warming intervals during their stay*4. A separate study suggests it may have been warmer when the Norse arrived but then the climate started cooling in 1160, before the start of the little ice age*5.
それから、現在進行中の温暖化が地球規模の現象であることはいうまでもない*6。グリーンランドでも何処でも、「氷河の後退」の規模は「未曽有」なのだ*7。
*1:ヴァイキングのグリーンランド移住と撤退については、http://d.hatena.ne.jp/sumita-m/20100530/1275185190で言及している。
*2:See eg. http://www.ldeo.columbia.edu/user/nicolasy
*3:See eg. “The "Medieval Warm Period"” https://www.ncdc.noaa.gov/paleo/globalwarming/medieval.html
*4:Takuro Kobashi,Kenji Kawamura,Jeffrey P. Severinghaus,Jean-Marc Barnola,Toshiyuki Nakaegawa,Bo M. Vinther,Sigfús J. Johnsen,and Jason E. Box "High variability of Greenland surface temperature over the past 4000 years estimated from trapped air in an ice core" Geophysical Research Letters 38-21, November 2011 http://onlinelibrary.wiley.com/doi/10.1029/2011GL049444/abstract
*5:William J. D’Andrea,Yongsong Huang,Sherilyn C. Fritzb, and N. John Andersonc "Abrupt Holocene climate change as an important factor for human migration in West Greenland" Proceedings of the National Academy of Sciences of the United States of America 108-24, June 2011 http://www.pnas.org/content/108/24/9765.abstract?sid=cf79de8b-9908-4d58-aa62-50ce6671c9b4
*6:See eg. http://d.hatena.ne.jp/sumita-m/20151126/1448553862
*7:See Tim Radford “Greenland's fastest glacier picks up pace” http://climatenewsnetwork.net/greenlands-fastest-glacier-picks-up-pace/ Tim Radford “Speed of glacier retreat worldwide 'historically unprecedented', says report” http://www.theguardian.com/environment/2015/aug/04/speed-glacier-retreat-worldwide-historically-unprecedented-climate-change
