Reassessing the evidence for tree-growth and inferred temperature change during the Common Era in Yamalia, northwest Siberia

There is generally good agreement between all chronologies as regards the major features of the decadal to centennial variability. Low tree-growth periods for which the inferred summer temperatures are approximately 2.5 °C below the 1961-90 reference are apparent in the 15-year smoothed reconstructions, centred around 1005, 1300, 1455, 1530, particularly the 1810s where the inferred cooling reaches −4 °C or even −6 °C for individual years, and the 1880s. These are superimposed on generally cool pre-20th century conditions: the long-term means of the pre-1900 reconstructed temperature anomalies range from −0.6 to −0.9 °C in our alternative reconstructions. There are numerous periods of one or two decades with relatively high growth (and inferred summer temperatures close to the 1961-1990 level) but at longer timescales only the 40-year period centred at 250 CE appears comparable with 20th century warmth. Although the central temperature estimate for this period is below that for the recent period, when we take into account the uncertainties we cannot be highly confident that recent warmth has exceeded the temperature of this earlier warm period. While there are clear warm decades either side of 1000 CE, neither TRW nor MXD data support the conclusion that temperatures were exceptionally high during medieval times. One previous version of the Polar Urals TRW chronology is shown here to be in error due to an injudicious application of RCS to non-homogeneous sample data, partly derived from root-collar samples that produce spuriously high chronology values in the 11th and 15th centuries. This biased chronology has been used in a number of recent studies aimed at reconstructing wider scale temperature histories. All of the chronologies we have produced here clearly show a generally high level of growth throughout their most recent 80 years. Allowing for chronology and reconstruction uncertainty, the mean of the last 100 years of the reconstruction is likely warmer than any century in the last 2000 years in this region.