Black Carbon and Himalaya

Black Carbon (BC) has a noticeable history over the past 150 years in Hindu-Kush-Himalayan (HKH) and Tibetan Plateau (TP),  from pre-industry to modern periods. An analysis on Mt. Everest ice core during 1860–2000 and based on Single Particle Soot Photometer demonstrates that BC concentration from 1975-2000 shows about 3-fold increase compared to the background level of 1860-1975, indicating BC from anthropogenic source has been introduced and transported to highland of Himalayas.  

What is black carbon

BC is derived from incomplete combustion of organic matter from both natural and anthropogenic sources, in the form of carbonaceous aerosols, heating the aerosol-planetary boundary layer with a net warming effect by absorbing and scattering sunlight. BC can also be deposited onto snow and ice, darkening the surfaces and reducing their albedo feedback. Both mechanisms are believed to have strong implication for glacier melting. 

Picture source: http://www.aces.su.se/new-study-reveals-the-origin-of-soot-emissions-that-melt-himalayan-glaciers/

Source and Pathway

A handful approaches have attempted to detect the source of BC responsible for the concentration in HKH and TP, mainly using bottom-up inventories, climate circulation modeling, and back-trajectory modeling. Based on different emissions inventories, BC concentrations are largely affected by emissions in the regions flanking the Himalayas, i.e. South Asia and East Asia. Local emission from HKH and TP might also be an attribute of concentration but there are large uncertainties and disagreement among different inventories to discern precisely from ambient emission, which merits further examination. 

The contribution of different source regions relatively varies with season, BC in HKH and TP is generally low during monsoon season, and reaching high during winter dry season. Indian and China are two largest countries corresponding to the overwhelming majority of BC in HKH and TP in all season. As shown in Figure 1 fossil fuel (46±11%) and biomass (54±11%) combustion are equally contributing to Himalayas BC concentration, consistent with sources from IGP in India, whilst BC in remote TP is dominantly from fossil fuel combustion (66±16%), associated with BC sources record in China. Rapid urbanisation, industrial expansion and economic development in Asia are expected to continue in the next decades, it is crucial for policy-makers to take more effective and science basis actions for pollutants mitigation and sustainable development. 

Figure 1. Green, black and brown bars represent biomass, liquid fossil fuels and coal combustion, respectively. Black and brown arrows denote the transport of BC from East Asia, black and green arrows indicate the transport of BC from South Asia, and green arrows represent BC emissions from local/domestic activities in the TP. Source: Sources of black carbon to the Himalayan–Tibetan Plateau glaciers

BC and the Himalayan Glaciers

In general, the change to the Himalayan glacier mass balance due to BC concentration can be determined by 1) BC and precipitation change which impacts the accumulation, and 2) BC and surface energy budget which affects the ablation. As aforementioned, BC could impact the surface energy budget through two mechanismsof warming atmosphere and changing the surface albedo on snow and ice. Gertler et al., 2016 in their table 4 & 5 has summarised the results from a number of studies attemping to qunatify the climate forcing of BC in high Himalayas.  

This review also identified the uncertainty regarding the BC effects on snow and ice in the Himalayas due to lack of 1) BC observation data, 2) agreement of emission inventories, and 3) high spatial resolution, which limit our understanding of BC deposition process and magnitude. In addtion, there is observation indicating a semi-direct effect over the Indian Himalaya from atmospheric BC on clouds and precipitation in the Himalayas:  increased (decreased)  precipitation before and during monsoon may lead to increased (reduced) snow cover in the high elevation of Himalayas, which covers (exposes) BC deposited on 
the surface and increase (decrease) glacier albedo. However, the full effects of these simulations are constrained by the same uncertainties.

In summary, it is of critical importance to deepening our understanding of the correlation between BC concentration and meteorological data such as the precipitation, monsoon variability, natural and anthropogenic sources, on both global and local scale.To fully contextualising these, there is impressing need of increased observation data collection of snow extent, improved modeling of both atmosphere and cryosphere and in-depth analysis of BC variability through time.