In my last post, I reviewed the impact of ABCs on regional climate in the Himalayas. One of the aerosol present in ABCs is black carbon (BC). Produced by the incomplete combustion of carbonaceous material (mainly fossil fuel and biomass), several studies have indicated an additional forcing by this particle through reducing the snow/ice albedo (Hansen and Nazarenko, 2003; Jacobson, 2004). Referred to as ‘surface darkening’ by Flanner et al., (2009), this forcing was not included in the IPCC (2007) evaluations despite estimates that the snow/ice albedo forcing causes twice as much global warming as carbon dioxide of the same magnitude (efficiency forcing ~ 2) (Hansen and Nazerenko, 2003). The aim of this post will be to review the potential impact of this snow/ice albedo forcing on glaciers within the Himalayas.
INTRODUCTION OF BLACK CARBON INTO THE SNOW/ICE LAYER
The primary mechanism for the removal of BC from the atmosphere to the surface is by wet deposition and accounts for 98% of BC fallout (Hansen and Nazarenko, 2003). This consists of two main processes, nucleation scavenging (leads to rainout) and impaction scavenging (leads to washout) (Figure 1) (Jacobson, 2004).
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Figure 1: The movement of black carbon from the atmosphere to the surface by wet deposition. |
Impaction scavenging is the process whereby aerosols coagulate with, then enter the liquid, ice and graupel hydrometeors (Jacobson, 2004). If these particles become large enough to overcome the air viscosity, a similar process occurs as nucleation scavenging causing the particles to fall to the surface.
HOW DOES BLACK CARBON AFFECT SNOW ALBEDO?
Black carbon affects regional climate by reducing the albedo in snow and ice-covered areas. The deposited particles darken the snow/ice surface decreasing the amount of insolation that is reflected by the snow surface enhancing warming of the surface as more radiation is absorbed. The warming of the surface acts as a positive feedback, accelerating snow melt causing a further reduction in the snow/ice albedo (Flanner et al., (2009). In addition to this, the melting snow tends to retain aerosols, thus the greatest relative darkening (and therefore the highest albedo reduction) occurs during late winter and spring when the sun is high in the sky and is most efficient, lengthening the duration of the melt season (Hansen and Nazerenko, 2003).
BLACK CARBON FORCING
The influence of BC on the snow/ice albedo is highly variable in space and time. It depends fundamentally on the concentration of BC in the snow and changes seasonally, reflecting the hydrological regime, with peak concentrations during the wet season (Ming et al., 2009). Hansen and Nazerenko (2003) estimated an albedo reduction due to BC on land areas in the northern hemisphere at approximately 3%, accounting for around 0.17°C (over 25%) of the recent warming since the 1880s. A more recent study by Jacobson (2004) calculated slightly lower values of <3% over most high-latitude areas in the Northern Hemisphere with an average of 1%. However, what both studies concur is that anthropogenic soot is a significant global and regional forcing.
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Figure 2: The presence of black carbon and other aerosols over South Asia (Source: NASA, 2008) |
BLACK CARBON FORCING IN THE HIMALAYAS.
The dynamic non-linear and indirect feedbacks associated with BC forcing means there is significant uncertainty of the strength of the darkening forcing with some estimates indicated in Table 1. A study by Ming et al., (2008) on the East Rongbuk Glacier, on the northeast saddle of Mt. Everest estimated atmospheric in the region at around 80ngm-3 during 1951-2001, exceeding 50μg kg-1 in the summer of 2001. Using the values provided in table 1, this would suggest a reduction in surface albedo between 3.3-4.6%. Ming et al., (2008) calculated that the radiative forcing in the summer of 2001 was approximately 4.5Wm-2. Taking account for reductions in the proportion of incoming insolation due to scattering in the atmosphere, this value seems to support this range.
Table 1: Estimated reductions in snow/ice albedo attributed to black carbon present in the snow layer.
Concentration of black carbon (ng/g) | Reduction in snow/ice albedo (%) | Source |
15 | 1.0 | |
25 | 2.3 | Jacobson (2004) |
Ming et al., (2008) study also demonstrated temporal variability of BC influence on radiative forcing, with fluctuations in the radiative forcing between 1951 to 2001 changing in correspondence to variations in the concentration of BC present in the ice core layer (Table 2). Overall, there is an increasing trend in the concentration of BC present in snow in the Himalayas. This is supported by Bond et al., (2007) that calculated South Asia as the largest emitters of BC in recent years, emitting 563.9Gg of BC in the 1990s, accounting for a total of 12.3% of BC emitted throughout the world.
Table 2: Temporal variations in black carbon concentration in the snow and changes in radiative forcing on the East Rongbuk Glacier, Mt. Everest between 1951 to 2002.
Year | Concentration of BC in the snow (μg kg-1) | Radiative forcing (Wm-2) |
1951-76 | 16±10.8 | 2 |
1977-1994 | 11.7±5.4 | 1 |
1995-2002 | 20.3±9.2 | 4.5 (by summer of 2001). |
(Source Ming et al., (2009)
CONCLUSIONS
These studies suggest that BC acts a regional forcing in glacial environments such as the Himalayan mountain range, reducing snow/ice albedo rate and consequently causing acceleration of ice melt as more insolation is absorbed enhancing the warming of the surface. This forcing acts a positive feedback as lower albedo rates induce further melting of the snow which further reduces the snow/ice albedo. Thus, this would suggest that increased BC concentration in the snow layer will act to in conjunction with global warming to enhance glacial retreat. Flanner et al., (2009) have also suggested that the ‘darkening’ forcing (including BC and other aerosols not discussed in this post), may exceed surface dimming caused by ABCs in glacial environments. This reinforces the point that the control of aerosol emissions needs to occur alongside reduced carbon dioxide to control the rise in global temperatures. To date the IPCC have not included surface darkening in their evaluations of climate change, however as the literature in this post shows, it is an important global and regional forcing with the potential to be twice as efficient as carbon dioxide at causing changes to the world’s climate.
Reference:
Bond, T., C.E. Bhardwaj, R. Dong, R. Jogani, S. Jung, C. Roden, D.G. Streets and N.M. Trautmann (2007) ‘Historical emissions of black and organic carbon aerosol from energy-related combustion, 1850-2000’, Global Biogeochemical Cycles, 21, GB2018, 1-16.
Flanner, M.G., C.S. Zender, P.G. Hess, N.M. Mahowald, J.H. Painter, V. Ramanathan and P.J. Rasch (2009) ‘Springtime warming and reduced snow cover from carbonaceous particles’, Atmospheric Chemistry and Physics, 9: 2481-2497.
Hansen, J. and L. Nazerenko (2003) ‘Soot climate forcing via snow and ice albedos’, Proceedings of the National Academy of Sicences of the United States of America, 101, 2: 423-428.
Jacobson, M.Z. (2004) ‘Climate response of fossil fuel and biofuel soot, accounting for soot’s feedback to snow and sea ice albedo and emissivity’, Journal of Geophysical Research, 109, D21201.
Ming, J., H. Cachier, C. Xiao, D. Qui, S. Kang, S. Hou, and J. Xu (2008) ‘Black carbon record based on a shallow Himalayan ice core and its climate implications’, Atmospsheric Chemistry and Physics, 8: 1342-1352.
Warren, S.G. and W.J. Wiscombe (1980) ‘A model for spectral albedo of snow II: Snow containing atmospheric aerosols’, Journal of Atmospheric Sciences, 37: 2734- 2745.
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