Title: 638 years of summer temperature variability over the Bhutanese Himalaya reconstructed from tree rings: a record for placing recent temperature change and glacier retreat in a long-term perspective
We have conducted tree-ring research in the Kingdom of Bhutan since 2002, principally for the study of climate change there, but also for use in practical forestry applications. From this long-term effort, we evaluated a small network of mid-to-high elevation tree-ring chronologies and selected a subset of climatically sensitive Himalayan spruce (Picea spinulosa Griff.) to produce a 638 year summer, June - August (JJA), temperature reconstruction covering the period 1376-2013. This reconstruction has been carefully validated against withheld instrumental temperature data not used for calibration and found to have significant skill in its estimates. It has also been examined for spatial correlation with larger-scale gridded summer temperatures over monsoon Asia and found to have the most skill over Bhutan as well. Relative to the 1950-2013 mean of the instrumental temperature data used for calibrating the tree rings, two prominent cold periods stand out, the earliest in the mid 15th century, lasting about 10 years, and the second beginning in the late 17th century lasting nearly 25 years. These cold periods are separated by episodes of above-average summer warmth. However, the warmest period in the entire reconstruction is occurring now in the first decade of the 21st century, a time of rapid glacier retreat in Bhutan, with no sign of immediate abatement. It is, however, not yet statistically unprecedented compared to the past when reconstruction uncertainties are taken into account. This new eastern Himalayan temperature reconstruction shows some evidence for cooling associated with explosive volcanism, while the aforementioned cold periods are coincident with known solar irradiance minima during the 15th, late 17th and early 19th centuries, thereby suggesting that solar variability over this region has had an influence on decadal time scale temperature variability over the Himalaya as well. Greenhouse gas forcing is also likely to be a contributor to the most recent warming trend as well.
Edward R. Cook
Lamont-Doherty Earth Observatory
of Columbia University, New York, USA