Medeltida värmeperioden

Den medeltida värmeperioden var en period på medeltiden mellan ungefär år 950 och 1250 då medeltemperaturen åtminstone i Europa^[1], på Grönland^[2], i Japan^[3], i Kina^[4], i Sibirien^[5] och på Nya Zeeland^[6] var högre än historiskt. Begreppet infördes 1965 av den brittiske klimatforskaren Hubert Horace Lamb (1913–1997)^[7], som uppskattade att temperaturen 1100–1200 e.Kr. varit 1–2 °C varmare än årsmedeltemperaturen 1931–1960. Lambs resultat gällde dock endast västra Europa. I den nyare forskningen är det omdiskuterat huruvida den medeltida värmeperioden var global och hur temperaturen var i förhållande till dagens.

Många studier har visat att temperaturen varierade i olika regioner och att skillnaden i global temperatur var mycket liten (och något kallare) jämfört med nu.^[8] Andra studier har istället visat att den medeltida värmperioden hade temperaturer som var högre än dagens.^[9] Kunskapen om en eventuell medeltida värmeperiod på södra halvklotet är fortfarande mycket bristfällig. Flera nyare studier antyder dock att en medeltida värmperiod även förekom på södra halvklotet.^[10]

Den medeltida värmeperioden följdes av Lilla istiden cirka 1350–1860 som var den kallaste klimatperioden sedan senaste istidens slut. I västra Nordamerika tycks den Lilla istiden först ha tagit slut omkring 1920.

Referenser

[1]
Linderholm, H.W. and Gunnarson, B.E. 2005. Summer temperature variability in central Scandinavia during the last 3600 years. Geografiska Annaler 87A: 231-241; Grudd, Håkan. 2006. Tree Rings as Sensitive Proxies of Past Climate, Dissertations from the Department of Physical Geography and Quaternary Geology. Stockholm; Change Mangini, A., Spotl, C. and Verdes, P. 2005. Reconstruction of temperature in the Central Alps during the past 2000 yr from a δ18O stalagmite record. Earth and Planetary Science Letters 235: 741-751.; Abrantes, F., Lebreiro, S., Rodrigues, T., Gil, I., Bartels-Jónsdóttir, H., Oliveira, P., Kissel, C. and Grimalt, J.O. 2005. Shallow-marine sediment cores record climate variability and earthquake activity off Lisbon (Portugal) for the last 2000 years. Quaternary Science Reviews 24: 2477-2494; Martinez-Cortizas, A., Pontevedra-Pombal, X., Garcia-Rodeja, E., Novoa-Muñoz, J.C. and Shotyk, W. 1999. Mercury in a Spanish peat bog: Archive of climate change and atmospheric metal deposition. Science 284: 939-942.
[2]
Dahl-Jensen, D. Mosegaard, K., Gundestrup, N., Clow, G. D., Johnsen, S. J., Hansen, A. W. & Balling, N., Past Temperatures Directly from the Greenland Ice Sheet, Science 282 (1998): 268–271; Jensen, Karin G., Kuijpers, Antoon, Nalân Koç & Heinemeier, Jan, Diatom evidence of hydrografhic changes and ice conditions in Igaliku Fjord, South Greenland, during the past 1500 years, The Holocene 14(2) (2004): 152–164; Kaplan, Michael R., Wolfe, Alexander P. & Miller, Gifford H., Holocene Environmental Variability in South-ern Greenland Inferred from Lake Sediments, Quaternary Research 58 (2002): 149–159; Lassen, Susanne J., Kuijpers, Antoon, Kunzendorf, Helmar, Hoffmann-Wieck, Gerd, Mikkelsen, Naja & Kon-radi, Peter, Late-Holocene Atlantic bottom-water variability in Igaliku Fjord, South Greenland, recon-structed from foraminifera faunas, The Holocene 14 (2004): 165–171; Roncaglia, Lucia & Kuijpers, Antoon, Palynofacies analysis and organic-walled dinoflagellate cysts in late-Holocene sediments from Igaliku Fjord, South Greenland, The Holocene 14 (2004): 172–184.
[3]
Goto, S., Hamamoto, H. and Yamano, M. 2005. Climatic and environmental changes at southeastern coast of Lake Biwa over past 3000 years, inferred from borehole temperature data. Physics of the Earth and Planetary Interiors 152: 314-325.
[4]
Ji, J., Shen, J., Balsam, W., Chen, J., Liu, L. and Liu, X. 2005. Asian monsoon oscillations in the northeastern Qinghai-Tibet Plateau since the late glacial as interpreted from visible reflectance of Qinghai Lake sediments. Earth and Planetary Science Letters 233: 61-70; Chu, G., Liu, J., Sun, Q., Lu, H., Gu, Z., Wang, W. and Liu, T. 2002. The 'Mediaeval Warm Period' drought recorded in Lake Huguangyan, tropical South China. The Holocene 12: 511-516; Qian, W. and Zhu, Y. 2002. Little Ice Age climate near Beijing, China, inferred from historical and stalagmite records. Quaternary Research 57: 109-119
[5]
Mackay, A.W., Ryves, D.B., Battarbee, R.W., Flower, R.J., Jewson, D., Rioual, P. and Sturm, M. 2005. 1000 years of climate variability in central Asia: assessing the evidence using Lake Baikal (Russia) diatom assemblages and the application of a diatom-inferred model of snow cover on the lake. Global and Planetary Change 46: 281-297;
[6]
Wilson, A.T., Hendy, C.H. and Reynolds, C.P. 1979. Short-term climate change and New Zealand temperatures during the last millennium. Nature 279: 315-317; Williams, P.W., King, D.N.T., Zhao, J.-X. and Collerson, K.D. 2004. Speleothem master chronologies: combined Holocene 18O and 13C records from the North Island of New Zealand and their palaeoenvironmental interpretation. The Holocene 14: 194-208.
[7]
H. H. Lamb, Palaeogr. Palaeoclimatol. Palaeoecol. 1, 13 (1965).
[8]
R. S. Bradley, M. K. Hughes och H. F. Diaz, Science 302, 404-405 (2003).
[9]
D'Arrigo, R., Wilson, R. and Jacoby, G. 2006. On the long-term context for late twentieth century warming. Journal of Geophysical Research 111:10.
[10]
Thompson, L.G., Mosley-Thompson, E., Davis, M.E., Lin, P.-N., Henderson, K. and Mashiotta, T.A. 2003. Tropical glacier and ice core evidence of climate change on annual to millennial time scales. Climatic Change 59: 137-155; Rein B., Luckge, A. and Sirocko, F. 2004. A major Holocene ENSO anomaly during the Medieval period, Geophysical Research Letters 31:10; Holmgren, K., Tyson, P.D., Moberg, A. and Svanered, O. 2001. A preliminary 3000-year regional temperature reconstruction for South Africa. South African Journal of Science 97: 49-51.