Unexpected climate variability inferred from a 380-year tree-ring earlywood oxygen isotope record in the Karakoram, Northern Pakistan

Abstract

To provide a long-term context for understanding the “Karakoram Anomaly” in recent glacier studies, we originally aimed to develop an annually resolved multi-century winter–spring precipitation record using tree-ring earlywood cellulose δ18O (δ18OEW) of Pinus wallachina from the Karakoram, northern Pakistan. Out of expectation, winter (January–May) temperature rather than precipitation is found to be the dominant climate signal (r = 0.63, p < 0.01, 1955–2013) stored in the δ18OEW chronology. Precipitation signals mainly appear at high-frequency variations, but weaker than temperature signal. We reconstructed January–May temperature back to 1631 with an explained variance of 39.7 % during the calibration period of 1955–2013. The reconstruction reveals that the Little Ice Age (LIA) was 0.94 °C warmer during 1647–1746 than the twentieth century (1900–1999). These warmer conditions are additionally validated by ice-core δ18O data from the Kunlun Mountains of High Asia and the northern North America. The eastward Polar Vortex and enhanced mid-latitude Westerlies on the Euro-Asia continent may be a possible explanation of a spatial coherency of LIA temperature between the Karakoram and the northern North America. Although not the original aim of this study, we provide evidence that the attribution of anomalous behavior of Karakoram glaciers in a long-term context may be misled when using precipitation reconstructions derived from tree-ring oxygen isotope.