Papers on the MWP as Global Event

This list contains papers on the medieval warm period (MWP) with emphasis on global analysis. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

UPDATE (February 9, 2012): Goosse et al. (2012) added.
UPDATE (January 6, 2012): Zhou et al. (2011) added.
UPDATE (September 20, 2011): Koch & Clague (2011) added. Thanks to Barry for pointing it out, see the comment section below.
UPDATE (June 17, 2011): Graham et al. (2010) added.
UPDATE (August 19, 2010): Ljungqvist (2009) added, thanks to Darius for pointing it out (see the comment section below).
UPDATE (May 17, 2010): Trouet et al. (2009) added.
UPDATE (January 20, 2010): Osborn & Briffa (2006) added.

The role of forcing and internal dynamics in explaining the “Medieval Climate Anomaly” – Goosse et al. (2012) “Proxy reconstructions suggest that peak global temperature during the past warm interval known as the Medieval Climate Anomaly (MCA, roughly 950–1250 AD) has been exceeded only during the most recent decades. To better understand the origin of this warm period, we use model simulations constrained by data assimilation establishing the spatial pattern of temperature changes that is most consistent with forcing estimates, model physics and the empirical information contained in paleoclimate proxy records. These numerical experiments demonstrate that the reconstructed spatial temperature pattern of the MCA can be explained by a simple thermodynamical response of the climate system to relatively weak changes in radiative forcing combined with a modification of the atmospheric circulation, displaying some similarities with the positive phase of the so-called Arctic Oscillation, and with northward shifts in the position of the Gulf Stream and Kuroshio currents. The mechanisms underlying the MCA are thus quite different from anthropogenic mechanisms responsible for modern global warming.” Hugues Goosse, Elisabeth Crespin, Svetlana Dubinkina, Marie-France Loutre, Michael E. Mann, Hans Renssen, Yoann Sallaz-Damaz and Drew Shindell, Climate Dynamics, DOI: 10.1007/s00382-012-1297-0.

A comparison of the Medieval Warm Period, Little Ice Age and 20th century warming simulated by the FGOALS climate system model – Zhou et al. (2011) “To compare differences among the Medieval Warm Period (MWP), Little Ice Age (LIA), and 20th century global warming (20CW), six sets of transient and equilibrium simulations were generated using the climate system model FGOALS_gl. This model was developed by the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences. The results indicate that MWP warming is evident on a global scale, except for at mid-latitudes of the North Pacific. However, the magnitude of the warming is weaker than that in the 20th century. The warming in the high latitudes of the Northern Hemisphere is stronger than that in the Southern Hemisphere. The LIA cooling is also evident on a global scale, with a strong cooling over the high Eurasian continent, while the cooling center is over the Arctic domain. Both the MWP and the 20CW experiments exhibit the strongest warming anomalies in the middle troposphere around 200–300 hPa, but the cooling center of the LIA experiment is seen in the polar surface of the Northern Hemisphere. A comparison of model simulation against the reconstruction indicates that model’s performance in simulating the surface air temperature changes during the warm periods is better than that during the cold periods. The consistencies between model and reconstruction in lower latitudes are better than those in high latitudes. Comparison of the inter-annual variability mode of East Asian summer monsoon (EASM) rainfall during the MWP, LIA and 20CW reveals a similar rainfall anomalies pattern. However, the time spectra of the principal component during the three typical periods of the last millennium are different, and the quasi-biannual oscillation is more evident during the two warm periods. At a centennial time scale, the external mode of the EASM variability driven by the changes of effective solar radiation is determined by the changes of large scale land-sea thermal contrast. The rainfall anomalies over the east of 110°E exhibit a meridional homogeneous change pattern, which is different from the meridional out-of-phase change of rainfall anomalies associated with the internal mode.” TianJun Zhou, Bo Li, WenMin Man, LiXia Zhang and Jie Zhang, Chinese Science Bulletin, Volume 56, Numbers 28-29, 3028-3041, DOI: 10.1007/s11434-011-4641-6. [Full text]

Extensive glaciers in northwest North America during Medieval time – Koch & Clague (2011) “The Medieval Warm Period is an interval of purportedly warm climate during the early part of the past millennium. The duration, areal extent, and even existence of the Medieval Warm Period have been debated; in some areas the climate of this interval appears to have been affected more by changes in precipitation than in temperature. Here, we provide new evidence showing that several glaciers in western North America advanced during Medieval time and that some glaciers achieved extents similar to those at the peak of the Little Ice Age, many hundred years later. The advances cannot be reconciled with a climate similar to that of the twentieth century, which has been argued to be an analog, and likely were the result of increased winter precipitation due to prolonged La Niña-like conditions that, in turn, may be linked to elevated solar activity. Changes in solar output may initiate a response in the tropical Pacific that directly impacts the El Niño/Southern Oscillation and associated North Pacific teleconnections.” Johannes Koch and John J. Clague, Climatic Change, Volume 107, Numbers 3-4, 593-613, DOI: 10.1007/s10584-010-0016-2.

Support for global climate reorganization during the “Medieval Climate Anomaly” – Graham et al. (2010) “Widely distributed proxy records indicate that the Medieval Climate Anomaly (MCA; ~900–1350 AD) was characterized by coherent shifts in large-scale Northern Hemisphere atmospheric circulation patterns. Although cooler sea surface temperatures in the central and eastern equatorial Pacific can explain some aspects of medieval circulation changes, they are not sufficient to account for other notable features, including widespread aridity through the Eurasian sub-tropics, stronger winter westerlies across the North Atlantic and Western Europe, and shifts in monsoon rainfall patterns across Africa and South Asia. We present results from a full-physics coupled climate model showing that a slight warming of the tropical Indian and western Pacific Oceans relative to the other tropical ocean basins can induce a broad range of the medieval circulation and climate changes indicated by proxy data, including many of those not explained by a cooler tropical Pacific alone. Important aspects of the results resemble those from previous simulations examining the climatic response to the rapid Indian Ocean warming during the late twentieth century, and to results from climate warming simulations—especially in indicating an expansion of the Northern Hemisphere Hadley circulation. Notably, the pattern of tropical Indo-Pacific sea surface temperature (SST) change responsible for producing the proxy-model similarity in our results agrees well with MCA-LIA SST differences obtained in a recent proxy-based climate field reconstruction. Though much remains unclear, our results indicate that the MCA was characterized by an enhanced zonal Indo-Pacific SST gradient with resulting changes in Northern Hemisphere tropical and extra-tropical circulation patterns and hydroclimate regimes, linkages that may explain the coherent regional climate shifts indicated by proxy records from across the planet. The findings provide new perspectives on the nature and possible causes of the MCA—a remarkable, yet incompletely understood episode of Late Holocene climatic change.” N. E. Graham, C. M. Ammann, D. Fleitmann, K. M. Cobb and J. Luterbacher, Climate Dynamics, DOI: 10.1007/s00382-010-0914-z. [Full text]

Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly – Mann et al. (2009) “Global temperatures are known to have varied over the past 1500 years, but the spatial patterns have remained poorly defined. We used a global climate proxy network to reconstruct surface temperature patterns over this interval. The Medieval period is found to display warmth that matches or exceeds that of the past decade in some regions, but which falls well below recent levels globally.” [Link to PDF]

Centennial Variations of the Global Monsoon Precipitation in the Last Millennium: Results from ECHO-G Model – Liu et al. (2009) “The authors investigate how the global monsoon (GM) precipitation responds to the external and anthropogenic forcing in the last millennium by analyzing a pair of control and forced millennium simulations with the ECHAM and the global Hamburg Ocean Primitive Equation (ECHO-G) coupled ocean–atmosphere model. … Conversely, strong GM was simulated during the model Medieval Warm Period (ca. 1030–1240). … The simulated change of GM in the last 30 yr has a spatial pattern that differs from that during the Medieval Warm Period, suggesting that global warming that arises from the increases of greenhouse gases and the input solar forcing may have different effects on the characteristics of GM precipitation.”

Persistent Positive North Atlantic Oscillation Mode Dominated the Medieval Climate Anomaly – Trouet et al. (2009) “The Medieval Climate Anomaly (MCA) was the most recent pre-industrial era warm interval of European climate, yet its driving mechanisms remain uncertain. We present here a 947-year-long multidecadal North Atlantic Oscillation (NAO) reconstruction and find a persistent positive NAO during the MCA. Supplementary reconstructions based on climate model results and proxy data indicate a clear shift to weaker NAO conditions into the Little Ice Age (LIA). Globally distributed proxy data suggest that this NAO shift is one aspect of a global MCA-LIA climate transition that probably was coupled to prevailing La Niña–like conditions amplified by an intensified Atlantic meridional overturning circulation during the MCA.” [Full text]

Temperature proxy records covering the last two millenia: a tabular and visual overview – Ljungqvist (2009) “Here, the first systematic survey is presented, with graphic representations, of most quantitative temperature proxy data records covering the last two millennia that have been published in the peer-reviewed literature. In total, 71 series are presented together with basic essential information on each record. This overview will hopefully assist future palaeoclimatic research by facilitating an orientation among available palaeotemperature records and thus reduce the risk of missing less well-known proxy series. The records show an amplitude between maximum and minimum temperatures during the past two millennia on centennial timescales ranging from c. 0.5 to 4°C and averaging c. 1.5–2°C for both high and low latitudes, although these variations are not always occurring synchronous. Both the Medieval Warm Period, the Little Ice Age and the 20th century warming are clearly visible in most records, whereas the Roman Warm Period and the Dark Age Cold Period are less clearly discernible.”

How the Rate of Volcanism Initiated the Medieval Warm Period and Controlled Its Periods of Drought – Ward (2008) “During the Medieval Warm Period, most large volcanic eruptions are contemporaneous with short-term decreases in Northern Hemisphere temperature determined using high-resolution proxy data (Mann and Jones, 2003). When such large eruptions occur more frequently than every few years, however, the oxidizing capacity of the atmosphere is exceeded, greenhouse gases accumulate, and the earth warms. … Only 27% of these ice layers contained “volcanic” sulfate and there were only 3 instances where more than 6 contiguous layers contained “volcanic” sulfate: 179-140 BC (16 layers), the onset of the Roman Climate Optimum, 818-840 AD (11 layers), the onset of the Medieval Warm Period, and 1929-1984 AD (34 layers), the onset of the modern warming period caused by anthropogenic SO2.” [Presentation material 1, Presentation material 2]

Blueprints for Medieval hydroclimate – Seager et al. (2007) “A review of proxy evidence from around the world indicates that North American megadroughts were part of a global pattern of Medieval hydroclimate that was distinct from that of today. … A positive North Atlantic Oscillation (NAO) also helps to explain the Medieval hydroclimate pattern. … Tentative modeling results indicate that a multi-century La Niña-like state could have arisen as a coupled atmosphere–ocean response to high irradiance and weak volcanism during the Medieval period and that this could in turn have induced a persistently positive NAO state.” [Link to PDF]

The Spatial Extent of 20th-Century Warmth in the Context of the Past 1200 Years – Osborn & Briffa (2006) “Positive anomalies during 890 to 1170 and negative anomalies during 1580 to 1850 are consistent with the concepts of a Medieval Warm Period and a Little Ice Age, but comparison with instrumental temperatures shows the spatial extent of recent warmth to be of greater significance than that during the medieval period.” [Link to PDF]

A late medieval warm period in the Southern Ocean as a delayed response to external forcing? – Goosse et al. (2004) “On the basis of long simulations performed with a three-dimensional climate model, we propose an interhemispheric climate lag mechanism, involving the long-term memory of deepwater masses. Warm anomalies, formed in the North Atlantic when warm conditions prevail at surface, are transported by the deep ocean circulation towards the Southern Ocean. There, the heat is released because of large scale upwelling, maintaining warm conditions and inducing a lagged response of about 150 years compared to the Northern Hemisphere. Model results and observations covering the first half of the second millenium suggest a delay between the temperature evolution in the Northern Hemisphere and in the Southern Ocean. The mechanism described here provides a reasonable hypothesis to explain such an interhemipsheric lag.” [Link to PDF]

Climate in Medieval Time – Bradley et al. (2003) “although the High Medieval (1100 to 1200 A.D.) was warmer than subsequent centuries, it was not warmer than the late 20th century. Moreover, the warmest Medieval temperatures were not synchronous around the globe. Large changes in precipitation patterns are a particular characteristic of “High Medieval” time.”

Medieval Climatic Optimum – Mann (2002) “Thus, current evidence does not support the notion of a Medieval Climatic Optimum as an interval of hemispheric or global warmth comparable to the latter 20th century.” [Link to PDF]

Was the Medieval Warm Period Global? – Broecker (2001) “During the Medieval Warm Period (800 to 1200 A.D.), the Vikings colonized Greenland. In his Perspective, Broecker discusses whether this warm period was global or regional in extent. He argues that it is the last in a long series of climate fluctuations in the North Atlantic, that it was likely global, and that the present warming should be attributed in part to such an oscillation, upon which the warming due to greenhouse gases is superimposed.” [Link to PDF]

How Warm Was the Medieval Warm Period? – Crowley & Lowery (2000) “Despite clear evidence for Medieval warmth greater than present in some individual records, the new hemispheric composite supports the principal conclusion of earlier hemispheric reconstructions and, furthermore, indicates that maximum Medieval warmth was restricted to two-three 20–30 year intervals, with composite values during these times being only comparable to the mid-20 th century warm time interval. Failure to substantiate hemispheric warmth greater than the present consistently occurs in composites because there are significant offsets in timing of warmth in different regions; ignoring these offsets can lead to serious errors concerning inferences about the magnitude of Medieval warmth and its relevance to interpretation of late 20 th century warming.” [Link to PDF]

Was there a ‘medieval warm period’, and if so, where and when? – Hughes & Diaz (1994) “Our review indicates that for some areas of the globe (for example, Scandinavia, China, the Sierra Nevada in California, the Canadian Rockies and Tasmania), temperatures, particularly in summer, appear to have been higher during some parts of this period than those that were to prevail until the most recent decades of the twentieth century. These warmer regional episodes were not strongly synchronous. Evidence from other regions (for example, the Southeast United States, southern Europe along the Mediterranean, and parts of South America) indicates that the climate during that time was little different to that of later times, or that warming, if it occurred, was recorded at a later time than has been assumed. Taken together, the available evidence does not support a global Medieval Warm Period, although more support for such a phenomenon could be drawn from high-elevation records than from low-elevation records.” [Link to PDF]

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