Naulier, M., Savard, M.M., Bégin, C., Gennaretti, F., Arseneault, D., Marion, J., Nicault, A. and Bégin, Y. 2015. A millennial summer temperature reconstruction for northeastern Canada using oxygen isotopes in subfossil trees. Climate of the Past 11: 1153-1164.
According to Naulier et al. (2015), the latest IPCC assessment report shows that "northeastern Canada is poorly represented among existing millennial temperature reconstructions in the Northern Hemisphere," and as a result, they say that "better knowledge of regional past climate variations registered in natural archives is needed." Thus, it was the objective of this team of eight researchers to fill this data void by developing the "first summer temperature reconstruction for eastern Canada based on a millennial oxygen isotopic series (δ18O) from tree rings.
In accomplishing this objective, Naulier et al. examined both living and subfossil trees from a boreal lake, Lake L20 (54.94°N, 71.40°W), ultimately producing an annually resolved δ18O series that served as a proxy for summer (Jun-Aug) maximum temperatures. And what did the proxy show?
As shown in the figure below, the millennial record revealed a cooling trend over the length of the record of 0.6°C, which they report is "in the same order of magnitude as the decrease of summer-months mean temperature reconstructed using pollen in the North American tundra," citing the work of Viau et al. (2012), as well as in other northern regions of high latitudes, citing the additional works of Kaufman et al. (2009) and Esper et al. (2012). In addition, Naulier et al. report the Lake L20 data set was punctuated by two major climatic episodes -- a warm period during the 11th and 12th centuries that corresponds to the well-known Medieval Warm Period (WMP), and a cold period extending from the early 15th century through the end of the 19th century, representing the Little Ice Age (LIA).
Figure 1. Reconstructed summer (JJA) maximum temperatures for Lake L20. Red line is a 21-year moving average. Adapted from Naulier et al. (2015).
In comparing the maximal temperature of the MWP with that of the Current Warm Period -- which comparison follows the approach used in the IPCC report -- Naulier et al. determined that the running 50-year averages between 1000 and 1100 were higher (+0.2 ± 0.1 °C) than the measured temperature of the last 50 years (1959-2009). This finding suggests, therefore, that there is nothing unusual, unnatural, or unprecedented about the current level of warmth in northeastern Canada, providing no proof for climate-alarmist claims in this part of the world that rising greenhouse gases are causing exceptional warmth. Furthermore, in discussing potential causes of change across their millennial temperature series, Naulier et al. report that their data suggest that "solar radiation was the most influential forcing on Tmax changes in the studied region," further noting that "low temperature periods were always associated to low solar radiation periods (p < 0.05)."
Esper, J., Frank, D.C., Timonen, M., Zorita, E., Wilson, R.J., Luterbacher, J., Holzkämper, S., Fischer, N., Wagner, S. and Nievergelt, D. 2012. Orbital forcing of tree-ring data. Nature Climate Change 2: 862-866.
Kaufman, D.S., Schneider, D.P., McKay, N.P., Ammann, C.M., Bradley, R.S., Briffa, K.R., Miller, G.H., Otto-Bliesner, B.L., Overpeck, J.T. and Vinther, B.M. 2009. Recent warming reverses long-term Arctic cooling. Science 325: 1236-1239.
Viau, A., Ladd, M. and Gajewski, K. 2012. The climate of North America during the past 2000 years reconstructed from pollen data. Global and Planetary Change 84: 75-83.Posted 7 February 2017