The IPCC on a heterogeneous Medieval Warm Period

Abstract

In their 2007 report, IPCC working group 1 refers to an increased heterogeneity of climate during medieval times about 1000 years ago. This conclusion would be of relevance, as it implies a contrast in the spatial signature and forcing of current warmth to that during the Medieval Warm Period. Our analysis of the data displayed in the IPCC report, however, shows no indication of an increased spread between long-term proxy records. We emphasize the relevance of sample replication issues, and argue that an estimation of long-term spatial homogeneity changes is premature based on the smattering of data currently available.

The extra-tropical Northern Hemisphere temperature in the last two millennia: reconstructions of low-frequency variability

“…However, applying a F-variance test shows that the standard deviation of 950-1049 AD is only significantly different from other periods when the standard deviation of these periods is less than 0.65 • C. Therefore, this analysis gives only weak indications that the MWP was unusually spatially variable. This is in accordance with results from Esper and Frank (2009) and Ljungqvist et al (2012) (see also Sect. 6).…”

“…It is still not fully established if the temperature anomalies in these periods are fully temporally synchroneous througout the globe, and therefore there exist no universally accepted chronological definitions for the start and end of either period. In particular, the spatio-temporal homogeneity of the MWP on a global or hemispheric scale is still debated (Bradley et al, 2003;Broecker, 2001;Esper and Frank, 2009;Diaz et al, 2011), whereas a homogeneously cold LIA recently seems to be a less contested issue (Juckes et al, 2007;Matthews and Briffa, 2005;National Research Council, 2006;Wanner et al, 2008;Ljungqvist et al, 2012).…”

Filling the Eastern European gap in millennium-long temperature reconstructions

“…Less distinct positive deviations with respect to the 1961-1990 reference period are centered at ∼1050, ∼1140, ∼1530, between ∼1580 and 1610, in the 18th century, ∼1900, and during the second half of the 20th century. Increased uncertainty associated with the early portion of the record, however, likely precludes definitive evidence of a relatively warmer MCA in comparison with the ARW (2,8). Pronounced cold spells associated with the LIA between ∼1350 and 1850 characterize most of the 14th century, the late 15th century, the 17th century, and the first half of the 19th century.…”

“…Precise evaluation of the absolute timing and amplitude of the Medieval Climate Anomaly (MCA; ∼950-1250), relative to the Little Ice Age (LIA; ∼1250-1850) and the Anthropogenic Recent Warmth (ARW; ∼1850-present), however, remains challenging (8). The detection and attribution of external climate forcing factors including orbital changes, solar fluxes, volcanic eruptions, greenhouse gases, and a combination thereof (9,10), as well as the comparison of past climate variability with human history (5,(11)(12)(13)(14), represent fundamental interdisciplinary challenges.…”

Influence of solar variability, CO<sub>2</sub> and orbital forcing between 1000 and 1850 AD in the IPSLCM4 model

“…It appears that those episodes described in Europe are not always present in all temperature reconstructions around the globe. The warmest and coldest periods of two time series can present asynchronous occurrence and differ in amplitude when comparing one region to another (Jones and Mann, 2004), although those differences can depend on the reconstruction methods (Esper and Frank, 2009). Nevertheless, at hemispherical scale the different reconstructions seem to converge toward a relatively warm period at the beginning of the last millennium, between 900 and 1300 AD, followed by a slow decrease in temperature reaching a minimum between 1600 and 1900 AD.…”