Aerosol Characteristics and Their Impact on the Himalayan Energy Budget

The extensive work on the increasing burden of aerosols and resultant climate implications shows a matter of great concern. In this study, we investigate the aerosol optical depth (AOD) variations in the Indian Himalayan Region (IHR) between its plains and alpine regions and the corresponding consequences on the energy balance on the Himalayan glaciers. For this purpose, AOD data from Moderate Resolution Imaging Spectroradiometer (MODIS, MOD-L3), Aerosol Robotic Network (AERONET), India, and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) were analyzed. Aerosol radiative forcing (ARF) was assessed using the atmospheric radiation transfer model (RTM) integrated into AERONET inversion code based on the Discrete Ordinate Radiative Transfer (DISORT) module. Further, air mass trajectory over the entire IHR was analyzed using a hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. We estimated that between 2001 and 2015, the monthly average ARF at the surface (ARFSFC), top of the atmosphere (ARFTOA), and atmosphere (ARFATM) were −89.6 ± 18.6 Wm−2, −25.2 ± 6.8 Wm−2, and +64.4 ± 16.5 Wm−2, respectively. We observed that during dust aerosol transport days, the ARFSFC and TOA changed by −112.2 and −40.7 Wm−2, respectively, compared with low aerosol loading days, thereby accounting for the decrease in the solar radiation by 207% reaching the surface. This substantial decrease in the solar radiation reaching the Earth’s surface increases the heating rate in the atmosphere by 3.1-fold, thereby acting as an additional forcing factor for accelerated melting of the snow and glacier resources of the IHR.

Measurement of Light absorbing particles in surface snow of central and western Himalayan glaciers: spatial variability, radiative impacts, and potential source regions

We collected surface snow samples from three different glaciers: Yala, Thana, and Sachin in the central and western Himalayas to understand the spatial variability and radiative impacts of light-absorbing particles. The Yala and Thana glaciers in Nepal and Bhutan, respectively, were selected to represent the central Himalayas. The Sachin glacier in Pakistan was selected to represent the western Himalayas. The samples were collected during the pre-and post-monsoon seasons of the year 2016. The samples were analysed for black carbon (BC) and water-insoluble organic carbon (OC) through the thermal optical method. The average mass concentrations (BC 2381.39 ng g−1; OC 3896.00 ng g−1; dust 101.05 µg g−1) in the western Himalaya (Sachin glacier) were quite higher compared to the mass concentrations (BC 357.93 ng g−1, OC 903.86 ng g−1, dust 21.95 µg g−1) at the central Himalaya (Yala glacier). The difference in mass concentration may be due to the difference in elevation, snow age, local pollution sources, and difference in meteorological conditions. BC in surface snow was also estimated through WRF-Chem simulations at the three glacier sites during the sampling periods. Simulations reasonably capture the spatial and seasonal patterns of the observed BC in snow but with a relatively smaller magnitude. Absolute snow albedo was estimated through the Snow, Ice, and Aerosol Radiation (SNICAR) model. The absolute snow albedo reduction was ranging between 0.48 % (Thana glacier during September) to 24 % (Sachin glacier during May) due to BC and 0.13 % (Yala glacier during September) to 5 % (Sachin glacier during May) due to dust. The instantaneous radiative forcing due to BC and dust was estimated in the range of 0 to 96.48 W m−2 and 0 to 25 W m−2 respectively. The lowest and highest albedo reduction and radiative forcing were observed in central and western Himalayan glaciers, respectively. The potential source regions of the deposited pollutants were inferred using WRF-Chem tagged-tracer simulations. Selected glaciers in the western Himalayas were mostly affected by long-range transport from the Middle East and Central Asia; however, the central Himalayan glaciers were mainly affected by local and South Asia emissions (from Nepal, India, and China) especially during the pre-monsoon season. Overall, South Asia and West Asia were the main contributing source regions of pollutants.

Contrasting surface velocities between lake- and land-terminating glaciers in the Himalayan region

Meltwater from Himalayan glaciers sustains the flow of rivers such as the Ganges and Brahmaputra on which over half a billion people depend for day-to-day needs. Upstream areas are likely to be affected substantially by climate change, and changes in the magnitude and timing of meltwater supply are expected to occur in coming decades. About 10 % of the Himalayan glacier population terminates into proglacial lakes, and such lake-terminating glaciers are known to exhibit higher-than-average total mass losses. However, relatively little is known about the mechanisms driving exacerbated ice loss from lake-terminating glaciers in the Himalaya. Here we examine a composite (2017–2019) glacier surface velocity dataset, derived from Sentinel 2 imagery, covering central and eastern Himalayan glaciers larger than 3 km2. We find that centre flow line velocities of lake-terminating glaciers (N = 70; umedian: 18.83 m yr−1; IQR – interquartile range – uncertainty estimate: 18.55–19.06 m yr−1) are on average more than double those of land-terminating glaciers (N = 249; umedian: 8.24 m yr−1; IQR uncertainty estimate: 8.17–8.35 m yr−1) and show substantially more heterogeneity than land-terminating glaciers around glacier termini. We attribute this large heterogeneity to the varying influence of lakes on glacier dynamics, resulting in differential rates of dynamic thinning, which causes about half of the lake-terminating glacier population to accelerate towards the glacier termini. Numerical ice-flow model experiments show that changes in the force balance at the glacier termini are likely to play a key role in accelerating the glacier flow at the front, with variations in basal friction only being of modest importance. The expansion of current glacial lakes and the formation of new meltwater bodies will influence the dynamics of an increasing number of Himalayan glaciers in the future, and these factors should be carefully considered in regional projections.

Accelerated mass loss of Himalayan glaciers since the Little Ice Age

Himalayan glaciers are undergoing rapid mass loss but rates of contemporary change lack long-term (centennial-scale) context. Here, we reconstruct the extent and surfaces of 14,798 Himalayan glaciers during the Little Ice Age (LIA), 400 to 700 years ago. We show that they have lost at least 40 % of their LIA area and between 390 and 586 km3 of ice; 0.92 to 1.38 mm Sea Level Equivalent. The long-term rate of ice mass loss since the LIA has been between − 0.011 and − 0.020 m w.e./year, which is an order of magnitude lower than contemporary rates reported in the literature. Rates of mass loss depend on monsoon influence and orographic effects, with the fastest losses measured in East Nepal and in Bhutan north of the main divide. Locally, rates of loss were enhanced with the presence of surface debris cover (by 2 times vs clean-ice) and/or a proglacial lake (by 2.5 times vs land-terminating). The ten-fold acceleration in ice loss we have observed across the Himalaya far exceeds any centennial-scale rates of change that have been recorded elsewhere in the world.

Hydro-meteorological Correlations of Himalayan Glaciers: A Review

This review manuscript addresses hydro-meteorological correlations of various glaciers situated in the Himalayan region. Meteorological parameters influence the discharge pattern of the glacier. A strong correlation has been observed between discharge and air temperature of the studied Himalayan glaciers. Whereas, other meteorological parameters such as wind speed and wind direction etc. were not significantly correlated with the meltwater runoff of different glaciers in this region. In general, variability (Cv) in discharge from the various Himalayan glaciers such as Chhota Shigri and Gangotri glaciers follow the variability (Cv) in the temperature of these glaciers. Maximum variability (Cv) in meltwater runoff from the Chhota Shigri glacier has been reported in the month of September, which might be due to the fast decline in stream runoff and air temperature of the study area during the month of September. A strong relationship has been observed between suspended sediment concentration and temperature of the majority of studied Himalayan glaciers. Such type of result shows that the suspended sediment concentration in the glacial meltwater has increased with rising air temperature in this region.

Estimation of Past and Future Mass Balance of Glaciers of Sikkim Himalaya using Energy Balance Modelling Approach and Regional Climatic Projections

In this study, the mass balance of Sikkim Himalayan glaciers is computed by the energy balance modeling approach using REMO and APHRODITE data. According to the present work, the glaciers show a mass balance of ~0, +0.31 and –0.32 m w. e. yr–1 for time periods 1981-1990, 1991-2000 and 2001-2005. To investigate the possible changes in the near future (2006-2049) and far future (2070-2099), REMO data under different representation concentration pathway scenarios 2.6, 4.5 and 8.5 are also analysed. For the time period 2006–2100, RCP2.6, RCP4.5 and RCP8.5 give an average mass balance of -0.75 m w. e. yr–1, -1.04 m w. e. yr–1 and -1.4 m w. e. yr–1, respectively. The results are comparable to other studies. This study is one of the few studies carried out to estimate the mass balance of glaciers using only climate model data.

An Exigency for Ice Core Studies to Determine Spatio-temporal Variability in Moisture Sources and Impact of Black Carbon – Mineral Aerosols on the Himalayan Glaciers

Himalayan glaciers‒ the store house of fresh water outside the polar region contributes ~45% of the total river flow by glacial melt in the Indus, Ganga and Brahmaputra watersheds which supports the livelihood of ~500 million people . The sustainability of these rivers is being questioned because of the growing evidences of accelerated glacier retreat in the recent decades, which is expected to have cascading effects on the mountainous areas and their surrounding lowlands. The rapid melting of Himalayan glaciers reveals their sensitivity to ongoing changes in climate dynamics, and if the current trend continues, rivers that rely heavily on snow/ice melt are expected to suffer hydrological disruptions to the point where some of the most populous areas may ‘run out of water’ during the dry season. Therefore, efforts are being made to study the glacier mass balance trends in order to understand the patterns and causes of recent recessional trend. Despite their importance, the absence of long-term mass-balance and remote sensing data restricts our knowledge of the Himalayan glaciers’ sensitivity/ response to climate change. Furthermore, such studies may be insufficient unless are compared to long-term glacier fluctuations (millennial and multi-millennial time scales), which aid in better understanding the natural trends of and human impacts on climate change, as well as assessing the causes and possible future of contemporary shrinking glaciers. This will also improve our understanding of past glacier behaviour in the context of primary causes of glacier change, which is critical for water resource management and understanding climate variability in high alpine areas where alternative proxy climate archives are typically scarce. Therefore, it is pertinent to pool our scientific resources and energy (i) towards understanding the Himalayan glaciers’ feeders (precipitation sources) and how they changed over time (geological and historical), as well as the causes of glaciers recession, one of which has been identified as (ii) black soot (carbon) in aerosol pollution.

Annual and seasonal glaciological mass balance of Patsio Glacier, western Himalaya (India) from 2010 to 2017

Improving the knowledge on Himalayan glaciers mass balance is a key to understand the present and past annual atmospheric variations and future water availability in the region. Here, we present glaciological mass balance for Patsio Glacier, located in Himachal Pradesh (India), western Himalaya. Annual glacier-wide mass balance was measured for 7 consecutive years (2010/11 to 2016/17) and winter mass balance for 6 years (2011/12 to 2016/17). The cumulative mass balance over this period was −2.35 ± 0.37 m w.e. The corresponding mean mass balance was −0.34 m w.e. a−1. The mean annual ablation gradient excluding the debris-covered area was 0.47 m w.e. (100 m)−1. The annual ablation over the debris-covered area is reduced by an average of −1.0 m w.e. compared to the clean ice surface. Winter mass balance was consistently positive with a maximum of 1.34 m w.e. in 2014/15 and a minimum of 0.88 m w.e. in 2011/12. Multiple regression analysis between annual mass balance versus annual and winter precipitation of the Lahaul-Spiti region shows a significant positive correlation. Our results highlight the importance of monitoring seasonal mass balance and consideration of non-climatic parameters (debris and aspect) while estimating the glacier-wide mass balance.