Medieval Warm Period

Medieval Warm Period
source
Name	Medieval Warm Period
Period	ca. 9th–14th centuries
Also known as	Medieval Climate Anomaly
Region	Northern Hemisphere, Atlantic, Pacific, Arctic, parts of Asia, Africa
Preceding	Dark Ages Cold Period
Following	Little Ice Age

Medieval Warm Period The Medieval Warm Period was an interval of relatively elevated Northern Hemisphere temperatures roughly between the 9th and 14th centuries, documented in paleoclimate archives and historical records. It features in debates among climatologists, historians, dendrochronologists and glaciologists about regional climate variability, solar forcing, volcanic activity, ocean circulation and societal response during the High Middle Ages. Research draws on data from tree rings, ice cores, speleothems, marine sediments and documentary sources across Europe, Greenland, Asia and the Americas.

Overview and definition

The term emerged in climatology and historical climatology literature during the 20th century and has been used by researchers in National Oceanic and Atmospheric Administration, Intergovernmental Panel on Climate Change, University of Cambridge, Harvard University and other institutions to denote a multi-century interval of warmth in parts of the Northern Hemisphere. Chronologists and paleoclimatologists often contrast it with the subsequent Little Ice Age and with earlier events like the Roman Warm Period. Prominent scientists such as Michael E. Mann, Phil Jones, Keith Briffa, Camille Parmesan and Gifford Miller have contributed reconstructions that shape definitions used in syntheses by the IPCC. Usage varies between disciplines including historical geography, paleobotany and glaciology.

Regional evidence and proxy records

Proxy networks combining data from Greenland Ice Sheet Project, GISP2, North Greenland Ice Core Project, Dome C, Kilimanjaro glaciers, Himalayan glaciers, Andes, Alaska, Tree-ring International Tree-Ring Data Bank, Speleothem records, Cariaco Basin, Santa Barbara Basin and Sargasso Sea provide spatially heterogeneous signals. European documentary sources such as Anglo-Saxon Chronicle, Icelandic sagas, Domesday Book and monastic chronicles complement paleolimnology from Lake Baikal, Lake Suigetsu, Loch Ness and Lake Malawi. Isotope ratios (δ18O, δ13C) from cores used by researchers at Lamont–Doherty Earth Observatory and Swiss Federal Institute for Forest, Snow and Landscape Research are integrated with dendroclimatology datasets from the International Tree-Ring Data Bank to infer temperature and hydroclimate anomalies. Regional heterogeneity is evident in comparisons between data from North Atlantic Oscillation-influenced sites, Pacific Decadal Oscillation-influenced sites and continental records across Eurasia, North America, Africa and Mesoamerica.

Causes and climate mechanisms

Hypotheses invoke changes in solar irradiance estimated from Steinhilber reconstruction and cosmogenic isotopes (14C, 10Be) recorded in Greenland ice cores and Antarctic Vostok. Reduced volcanic aerosol loading inferred from sulfate layers in GISP2 and EPICA cores is another factor, as is variability in ocean circulation such as shifts in the Atlantic Meridional Overturning Circulation and modes like the North Atlantic Oscillation and El Niño–Southern Oscillation. Climate model experiments by groups at National Center for Atmospheric Research, Hadley Centre, Max Planck Institute for Meteorology and Lawrence Livermore National Laboratory explore combined forcing from solar, volcanic and internal variability. Feedbacks involving sea-ice extent around Iceland, Arctic amplification over Barents Sea and land surface changes from agricultural expansion documented in Anglo-Saxon Chronicle and Iberian chronicles are also considered.

Societal and ecological impacts

Warmer intervals are associated in some regions with agricultural expansion, population growth and exploration such as Norse voyages to Greenland and temporary settlements in Vinland noted in Icelandic sagas and Saga of Erik the Red. In parts of Iberia and England viticulture and crop yields improved, a pattern discussed by historians at University of Oxford and University of Cambridge. Conversely, climatic shifts contributed to droughts affecting Ancestral Puebloans, Maya, and pastoral societies in Central Asia, influencing migration, conflict and institutional responses recorded in sources from Song dynasty officials and Abbasid Caliphate annals. Ecological effects include glacier retreat in Alps documented by Rudolf Hauer-era studies, tree-line changes in Scandinavia and altered marine productivity observed in sediment cores from the North Sea and Gulf of Maine.

Temporal and spatial extent debate

Scholars continue to debate whether the interval was globally synchronous or a set of regional anomalies; syntheses by IPCC and reconstructions by Mann et al. emphasize hemispheric-scale reconstructions while regional studies from Icelandic sagas, Chinese chronicles, Andean ice cores and Lake sediment analyses reveal asynchronous timing and magnitude. Controversies have involved public and political discourse, with contributions from commentators at The Wall Street Journal, Nature and Science and critiques by researchers at Berkeley Earth. Methodological issues include proxy calibration, chronological uncertainties, seasonal biases in proxies, and spatial sampling influenced by archives held at British Library, Biblioteca Nacional de España, Royal Irish Academy and other repositories.

Comparison with modern warming

Instrumental records maintained by Met Office Hadley Centre, NOAA, NASA Goddard Institute for Space Studies and University of East Anglia show 20th–21st century warming that is more rapid and globally pervasive than the medieval interval in most multiproxy syntheses. Attribution studies published by IPCC, WMO and research teams at MIT, Princeton University and Columbia University attribute recent warming primarily to increased greenhouse gas concentrations linked to Keeling Curve measurements, contrasting with medieval forcing scenarios dominated by solar and volcanic variability. Modern impacts on sea-level rise recorded by Permanent Service for Mean Sea Level and cryospheric loss observed at Greenland Ice Sheet Project exceed many regional medieval changes in rate and scope.

Category:Climate history