Spatial and Temporal Characteristics of Climate in Medieval Times Revisited

Abstract

The increase in high-resolution proxy records over expanding areas of the globe helps deepen understanding of the unusual climate patterns-and the forcing mechanisms responsible for them-during the years 950 to 1400. C limate in medieval times, a period usually understood to extend from A.D. ~950 to ~1400, is of considerable interest to students of modern and future climate. This is because the period differs from recent centuries mainly by predating the Industrial Revolution with its associated changes in the composition of the atmosphere and oceans and the nature of the land surface (Hughes and Diaz 1994; Crowley and Lowery 2000; Bradley et al. 2003a,b). In this sense it represents an appealing, but imperfect "control case" for the unintended global climate experiment that has resulted from industrial and agricultural development and population growth.In order to stimulate a synthesis of recent work on these topics, we convened a meeting of international experts to consider the following issues: 1) What were some of the key regional patterns of climatic anomalies during medieval times derived from the proxy climate records and from model simulations, and how do they compare with the twentieth-century patterns? 2) Recognizing that multiple proxy climate records are needed to constrain as much as possible the spatial and temporal variability of climate, what do recent studies using powerful statistical methods tell us about the timing and geographic coverage of the major features of climate in medieval times? 3) A number of numerical simulations using climate models of varying complexity and external forcing histories have been completed in the past few years. What do the latest model results tell us about geographic patterns and temporal characteristics of the simulated climate of this period?

Recent burning of boreal forests exceeds fire regime limits of the past 10,000 years

“…suggests increased biomass burning during the Medieval Climate Anomaly (MCA). The MCA is known to have been a warm and drought-prone period in many regions globally (27). In Alaska, lake sediment analyses suggest near-modern temperature during the MCA (28,29) accompanied by relatively arid conditions (29,30), perhaps analogous to the recent trend of diminishing water availability driven by increased evapotranspiration because of warming (31).…”

“…Although the timing and magnitude of the MCA were spatially heterogeneous, temperatures during the period approached 20 th century values across much of the Northern Hemisphere, and many areas experienced exceptional hydroclimatic variability (27). Paleorecords of the period thus offer an opportunity to assess the ecosystem impacts of ongoing and future climate change (37).…”

“…Two factors may have contributed to this recent fire regime change. First, growing evidence suggests that climatic conditions of recent decades exceeded the range of variability during the MCA (27), which has probably favored forest burning (38). Second, the LIA fire regime was characterized by comparatively low fire frequency and severity, and exceptionally low biomass burning (Fig.…”

Paleoclimate Data–Model Comparison and the Role of Climate Forcings over the Past 1500 Years*

“…The past 1500 years have been characterized by significant changes in the global climate (e.g., Jansen et al 2007;Jones et al 2009;Mann et al 2009;Diaz et al 2011;Graham et al 2011). The Northern Hemisphere (NH) experienced a relatively warm period known as the Medieval Climate Anomaly [MCA; ;950-1250 CE (Common Era)], during which the hemispheric-scale temperature was similar to twentieth-century levels.…”

“…The cause of the MCA is unclear, with climate models unable to reproduce the reconstructed warmth of this period (e.g., Mann et al 2009;Fern andez-Donado et al 2013). When combined with proxy evidence, this suggests that the MCA may have arisen from an internal reorganization of the climate system (Diaz et al 2011;Graham et al 2011). While detection and attribution studies have found a significant role of volcanic eruptions in driving the preindustrial Northern Hemisphere climate, the availability of proxy data has restricted these studies to the extratropics and/or periods shorter than 1000 years (Hegerl et al 2003(Hegerl et al , 2007a.…”