26/10/2017

Reconstruction of the Himalaya’s glacier history


A recently published study, (Himalayan glaciers experienced significant mass loss during later phases of little ice age), co-authored by Anshuman Bhardwaj, Javier Martín-Torres and María Paz Zorzano, from the Atmospheric Science Group, has contributed relevant data to fill the existing gaps in the records of glacier’s mass balance in the Western Himalaya region. The study, based on an unprecedented dendrochronological sampling, casts light on the comprehension of the climate sensitive subtropical glacier systems evolution, and dates back to 1615, encompassing the latter period of the Little Ice Age.

Himalayan glacier. Credit: Raphael Worni.

Subtropical Himalayan glaciers harbour some 50% of the glaciers outside the polar regions, which are very sensitive to climate and are part of hydrological systems sustaining 1,300 million people. Therefore, it is highly interesting to investigate the mechanisms driving their evolution, so that it is possible to understand the relation with climate and to foresee the effects of this latter’s change in the availability of natural resources. It was difficult to achieve a proper comprehension of the state of glaciers and their evolution over time given the lack of data about them, with very scarce and spatially fragmented glaciometeorological records from the region. This novel study has taken advantage of dendrochronological data (information extracted from the analysis of trees’ growth rings width), to retrieve the mass balance time series of some 10,000 glaciers in the region from 1615 on. Mass balance (that is to say, the difference between snow/ice accumulation and loss), is the most significant parameter to assess the state of glaciers, and its progression over time provides a reliable way to diagnose their health. Fortunately, this parameter depends on the same factors driving the yearly growth of the trees rings, so the method applied overcomes the difficulty of dealing with the complex combination of climatic factors determining it. The correlation of the tree rings with the mass balance has been solidly established by comparing the retrieved data with regional scale estimations performed by means of geodetic methods, showing a high agreement with them for regional-wide and long term spatial and temporal scales respectively.

Studied glaciers mass balance trend.
Credit: Anshuman Bhardwaj

Three different analysis have been performed for the Indian states of Jammu and Kashmir, Himachal Pradesh and Uttarakhand, which are representatives of different climates and meteorological regimes. The first objective was to visualize the Himalayan glaciers mass balance in this regions throughout the Little Ice Age (LIA, a period which is itself controversial regarding its duration, driving factors, climatological characteristic etcetera), with relation to the main assumed determinant factors, namely, El Niño-Southern Oscillation (ENSO), and the Total Solar Irradiance (TSI), as well as to the anthropogenic climate change, though the extent of this latter with relation to the natural climate variability determinants is difficult to assess with the available records so far.
As a result, it has been clearly found out that the studied Himalayan glaciers started suffering a significant decay even during the latter half of the LIA, a period whose effects in the region were thus considerably weaker and briefer than in the Arctic and subarctic regions, due to the complex climatic controls they are submitted to. For instance, they have a weak dependence on El Niño-Southern Oscillation and the Total Solar Irradiance individually considered, but a strong influence of the combined action of both. Some differential mechanisms have been identified, not only determining the diverse behaviour of the Himalayan glaciers with respect to the higher latitudes ones, but also those which mean differences between glacier progress in the three studied zones.
The study means a step forward in the establishment of a complete chronology of the Himalayan glaciers, providing a valuable new data set for the scientific community to perform other kind of investigations and, remarkably, for supporting hydroclimatic studies in a climate sensitive and densely populated area, in which the effects of the climate change can have serious consequences.