On the long‐term context for late twentieth century warming

Abstract

[1] Previous tree-ring-based Northern Hemisphere temperature reconstructions portray a varying amplitude range between the ''Medieval Warm Period'' (MWP), ''Little Ice Age'' (LIA) and present. We describe a new reconstruction, developed using largely different methodologies and additional new data compared to previous efforts. Unlike earlier studies, we quantify differences between more traditional (STD) and Regional Curve Standardization (RCS) methodologies, concluding that RCS is superior for retention of low-frequency trends. Continental North American versus Eurasian RCS series developed prior to merging to the hemispheric scale cohere surprisingly well, suggesting common forcing, although there are notable deviations (e.g., fifteenth to sixteenth century). Results indicate clear MWP (warm), LIA (cool), and recent (warm) episodes. Direct interpretation of the RCS reconstruction suggests that MWP temperatures were nearly 0.7°C cooler than in the late twentieth century, with an amplitude difference of 1.14°C from the coldest (1600-1609) to warmest (1937)(1938)(1939)(1940)(1941)(1942)(1943)(1944)(1945)(1946) decades. However, we advise caution with this analysis. Although we conclude, as found elsewhere, that recent warming has been substantial relative to natural fluctuations of the past millennium, we also note that owing to the spatially heterogeneous nature of the MWP, and its different timing within different regions, present palaeoclimatic methodologies will likely ''flatten out'' estimates for this period relative to twentieth century warming, which expresses a more homogenous global ''fingerprint.'' Therefore we stress that presently available paleoclimatic reconstructions are inadequate for making specific inferences, at hemispheric scales, about MWP warmth relative to the present anthropogenic period and that such comparisons can only still be made at the local/regional scale.

The Medieval Climate Anomaly and Little Ice Age in Chesapeake Bay and the North Atlantic Ocean

“…In addition, the inception of cooling that culminated in the LIA began earlier in the western North Atlantic (Chesapeake) than in eastern regions (Nordic Seas). Such spatial variability has also been observed in terrestrial temperature records from tree rings (D'Arrigo et al, 2006) and in ocean records near Iceland, Norway, Scotland and the Iberian Peninsula (Eiríksson et al, 2006).…”

Evidence for a weakening relationship between interannual temperature variability and northern vegetation activity

“…18), but we also note that these TRENDY models did not capture the observed change of R NDVI-GT during the last three decades at NH scale. Analysis of longer time series of proxies, such as tree ring growth index 32 , should provide further insights on this question. We also found compelling indications that change in soil moisture is an important process that modulates the temperature-productivity correlations, although the models used in this study may be presently less trusted to fully describe this.…”

Climatic response of ring width and maximum latewood density of Larix sibirica in the Altay Mountains, reveals recent warming trends

“…4). However, due to chronology length limit, old tree-ring data of the Altay Mountains did not track the most recent abnormal warming Mann et al 1999;Briffa 2000;Esper et al 2002;D'Arrigo et al 2006). In the Altay Mountains, new tree-ring data (Kelan River and Zuolesa) show the warming trend in the recent decadal years.…”