Nice Weather For Ducks: The Asian monsoon (Part II)

In my previous post, I discussed some of the key factors controlling the distribution and extent of the Asian monsoon. A review of the literature demonstrated substantial variability in the monsoon depending on surface and atmospheric temperatures, snow cover and other meso-scale factors. Overall, studies noted a weakening in Asian monsoon from the 1920s and Kripalini et al., (2003) observed a de-linking of the winter snow-monsoon relationship from a negative to positive correlation. This post will focus on how a weakening of the monsoon may impact the sensitivity of Himalayan glaciers to climate change and suggests, like many other factors, there is substantial complexity affecting general trends.

SNOWFALL, GLACIAL FORMATION AND CHANGING CLIMATE.

Snowfall is fundamental to the formation and preservation of glaciers. As snow accumulates it is compressed and compacted by the overlying layers. Over time, the density of the snow increases as air bubbles are removed forming glacial ice at 850kg cubic metres (Figure 1).

Figure 1: Video describing how glaciers are formed. 

Therefore changes in snowfall will have a substantial impact on the glacial mass balance of glaciers in the Himalayas. The high altitude location of most glacial catchments means that the rising summer temperatures are unlikely to significantly alter the proportion of precipitation that falls as snow (Benn and Owen, 1998). Thus, in this post, changes in snowfall are assumed to be due to declines in precipitation.

In addition to monsoon advecting snow moisture-rich air masses to the Himalayas, generally, more snow is expected to fall in a warmer atmosphere due to increases evaporation (Shekhar et al., 2010). This is due to greater cloud formation and hence more snowfall at high altitudes. However, a recent study by Dimri and Kumar (2008) shows a reduction in snowfall over the western Himalayas despite the warming climate. Conducting research over four different mountain ranges, overall snowfall declined by 280cm between 1988/89 and 2007/08 over the region (Dimri and Kumar, 2008). This trend was not uniform, but suggests that a weakening monsoon may be counteracting rising evaporation rates, as the air masses are contain relatively less moisture and are not extending as far into the Himalayan mountain range.

Shekhar et al., (2010) observed a decreasing number of snowfall days in the western Himalayas during the peak snowfall months (June-March) although no significant trends were observed in changes to the annual frequency of western disturbances. This supports the idea that changes to snowfall may be partly attributed to a decline in precipitation due to the weakening monsoon.

SPATIAL COMPLEXITY IN GLACIAL RESPONSE

Orographic and other mesoscale influences cause regional variability in the distribution of monsoon precipitation across the Himalayas. Thus a weakening monsoon is likely to cause different responses in different spatial regions (Benn and Owen, 1998). Summer precipitation falls sharply from south to north across the Himalayas and is much higher in Nepal than Karakoram and the western Tibetan Plateau (Benn and Owen, 1998). Between 1901 and 1995, summer precipitation has decreased by 19%, 9% and 6% in Nepal, Bangladesh and North-east India respectively, indicating a decline in precipitation due to the Asian monsoon (Duan et al., 2006). While glaciers in these regions have generally been decreasing during the 20^th century, glaciers in the Karakoram have remained relatively static or have advanced (Scherler et al., 2011). Relating this regional disparity to the observation by Benn and Owen (1998) the decrease in the influence of the Asian monsoon may not be as significant in Karakoram which is dominated by western midlatitute systems (Figure 2).

Figure 2. Map of the Himalayan mountain range, showing the location of the advancing or static glaciers in the Karakoram region in the north and retreating glaciers in Nepal and the southern Himalayan chain.

OTHER FACTORS

An additional forcing complicating the impact of the Asian monsoon on glacial mass balance is the affect of a weakening monsoon on the duration that ABCs are present over Asia (Bonsani et al., 2010). The onset of the summer monsoon removes aerosols from the atmosphere. As discussed in an earlier post, aerosols can have two counteracting impacts on glacial mass balance causing global dimming when aerosols are present in the atmosphere and surface darkening once black carbon has been added to the surface layer (Flanner et al., 2009). As the South Asian monsoon cycle determines atmospheric, a delay and weakening of the monsoon season may increase the time that atmospheric aerosols contribute to global dimming, counteracting global warming.

CONCLUSIONS

Similar to other impacts of climate change in the Himalayan region, the influence of the Asian monsoon on glacial mass balance is highly complex with substantial regional variability. Studies such as Duan et al., (2006) and Benn and Owen (1998) have suggested that a weakening monsoon has contributed to a decrease in precipitation in the southern regions of the Himalayan chain in Nepal and North-east India. However, the contribution of the summer monsoon to annual precipitation is less significant in northern regions (due to orographic effect), resulting in a less marked impact in these areas.

Additionally a delay in the onset of the monsoon may allow regional ABCs to persist for longer, counteracting global warming and the retreat of glaciers to rising temperatures.

Overall, the current weakening of the Asian monsoon is likely to accelerate the decline in glacial mass in the southern Himalayas due to decreases in precipitation. In northern regions, the relationship is less clear. The monsoon controls fluctuations in the Himalayan glaciers on millennial, decadal and inter-annual timescales (Benn and Owen, 1998). Therefore, in order to fully understand the response to the weakening monsoon and the influences on associated feedbacks (including the snow-monsoon forcing), substantially more research needs to be carried out.

Reference:

Benn, D. I. and L. A. Owen (1998) ‘The role of the Indian summer monsoon and the mid-latitude westerlies in Himalayan glaciation: review and speculative discussion’, Journal of the Geological Society’ 155: 353-363

Bonsani, P., P. Laj, A. Marinoni, M. Sprenger, F. Angerlini, J. Arduini, U. Bonafe, F. Calzolani, T. Colombo, S. Decessari, C. Di. Biagio, A. G. di Sarra, F. Evangelisti, R. Duchi, M. C. Facchini, S. Fuzzi, G. P. Gobbi, M. Maione, A. Panday, F. Roccato, K. Sellagri, H. Venza, G. P. Verza, P. Villani, E. Vuillermoz and P. Cristofanelli (2010) ‘Atmospheric brown clouds in the Himalayas: first two years of continuous observations at the Nepal Climate Obsrervatory Pyramid (5079m)’, Atmospheric Chemistry and Physics, 1- 7515-7531.

Dimri, A.P. and A. Kumar (2008) ‘Climatic variability of weather parameters over the western Himalayas: a ase study’, in P. K. Satyawali and A. Ganju (Eds). Proceedings of the National Snow Science Workshop, 11–12 January 2008, Chandigarh, Snow and Avalanche Study Establishment: Chandigarh, 167–173.

Duan, K., T. Yao and L.G. Thomspon (2006) ‘Response of monsoon precipitation in the Himalayas to global warming’, Journal of Geophysical Research, 111, D19110: 1-8.

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.

Kripalani, R.H., A. Kulkami and S.S. Sabada (2003) ‘Western Himalayan snow cover and Indian monsoon rainfall: a re-examination with INSAT and NCEP/NCAR data’, Theoretical and Applied Climatology, 74, 1: 1-18.

Scherler, D., B. Bookhagen and M. R. Stecker (2011) 'Spatially variable response of Himalayan glaciers to climate change affected by debris cover', Nature Geoscience, 4,3: 156-159

Shekhar, M. S., H. Chang, S. Kumar, K. Srinivasari, and A. Ganju (2010) ‘Climate-change studies in the western Himalayas’, Annals of Glaciology, 51, 54: 105-112.

Nice Weather For Ducks: The Asian Monsoon (Part 1)

Looking back over some of my previous posts, it became apparent that global warming and changes to the atmosphere composition are affecting the strength and circulation of the south Asian monsoon system. According to Oerlemans and Fortuin (1992), precipitation, radiation and air temperature are the most important factors determining glacier mass balance. Thus, the next two posts will focus on recent changes that have occurred to the Asian monsoon system variability and the possible consequences for Himalayan glaciers. Due to the complexity of this subject, I will split the topic into two posts, with this post focusing on recent changes to the Asian monsoon.

ASIAN MONSOON: THE BASICS

The word ‘monsoon’ is derived from an Arabic word meaning seasons (Goswani, 2005). The South Asian monsoon is part of an annually reversing wind system, characterised by the contrast between the Asian sub-continent and the Indian Ocean.

 As the temperature gradient increases in the summer (June-August), variation in land and sea heat capacities cause the atmospheric circulation to reverse advecting moisture-rich air masses northward from the Indian Ocean over India to the Himalayas (Figure 1). This causes south Asia’s wet season that provides most of the precipitation for snow accumulation to the Himalayan Mountains.

 Figure 1: Video describing seasonal changes to south Asia's monsoon.

SPATIAL AND TEMPORAL VARIABILITY

The Asian monsoon varies spatially and temporally on inter-annual, decadal and centennial timescales (Mayewski et al., 1980; Duan et al., 2006) Spatial variability occurs within a region - with a sharp decrease from south to north across the Himalayan region- and between monsoons depending on the strength of the mid-latitude westerlies and oscillations in the sub-tropical jet stream (SJS) which control the upper-most extent of the Asian monsoon (Benn and Owen, 1998).

Temporal variability is highly complex. It changes depending on the dynamic feedbacks that influence the land-ocean thermal gradient including insolation, snow albedo and atmospheric and surface temperature. Generally, the greater the temperature difference between the land and the ocean, the more intense the Asian monsoon, thus advecting more precipitation to higher latitudes which falls as snow in high Himalayan regions (Benn and Owen, 1998).

COMPLEXITIES

Kripalani et al., (2003) investigated the correlation between winter/spring snow estimates over the western Himalayas using satellite data between 1986-2000. The satellite images indicated a decrease in spring snow area and an increase snow melt rate from winter to spring (Feb-May) after 1993. More importantly, Kripalani et al., (2003) proposed that there were correlations between the snow cover, area and spring snow melt and the subsequent Indian Monsoon Rainfall (IMR) for that year. May snow cover was negatively related with subsequent summer monsoon rainfall, while February to May snow melt was found to have a positive correlation. This suggests a smaller snow area and faster snow melt is conducive for a strong monsoon across India (Kripalani et al., 2003). These findings were also supported by the conclusions of Prasad et al., (2009) from their general circulation model (GCM). The GCM indicated an inverse snow-monsoon relationship observed between monsoon rainfall and pre-monsoon (April) snowfall over the Tibetan Plateau (TP).  The weakening of the monsoon suggested by these studies is due to high snow-albedo, with a greater proportion of the incoming insolation being reflected back out into the atmosphere and hence less is absorbed to heat the land surface. Thus there is less of a temperature gradient between the land and the ocean inducing a weaker monsoon.

However, this contrasts with several studies that have observed a decrease in the strength of the monsoon in the latter part of the 20^th century coinciding with a decrease in snow extent in the Himalayas (Duan et al., 2006; Li et al., 2011). Duan et al., (2006) study of ice cores on the Dasuopi glacier in the Central Himalayas highlighted an inverse relationship between Northern Hemisphere temperature and the strength of the Asian monsoon (Table 1). Overall, the reconstructed correlation from the Dasoupi ice cores indicated that an average 0.1°C change in Northern Hemisphere temperature is associated with about 100± 10mm change in net snow balance. Thus, the de-linking of the historic snow-monsoon relationship may be attributed to rising global and regional temperatures due to global warming.

Table 1: Relative changes in Northern Hemisphere temperature and the Asian monsoon summer precipitation between 1770-1995 from a reconstructed ice core.

Year	Change in relative Northern Hemisphere temperature	Relative changes to Asian monsoon precipitation
1700-1770	Slight increase	Slight decrease
1770-1850	Decrease -0.3°C	Increase +300mm
1850-1920	Lowest in the past 300 years	Generally highest
1920-1995	Rapid rise +0.5°C	Gradual decline from 900mm in the 1920s to 400mm in the 1990s.

 (Source: Duan et al., (2006)

The IPCC (2007) support this claim, reporting that global warming will cause an increase in Asian summer monsoon variability and strength. Kripalini et al., (2003) also noted a reversal of the negative relationship between winter snow and summer rainfall in the most recent satellite data. This may be attributed to changes in winter snow cover extent and/or depth due to global warming. However the negative spring Himalayan summer-snow monsoon relationship was still maintained highlighting the complexity of the feedback forcing between the land and atmosphere.

ATMOSPHERIC BROWN CLOUDS

Another cause for the weakening of the monsoon may be attributed to anthropogenic climate change due to atmospheric brown clouds (ABC). Ramanathan et al., (2008) argue that regional dimming is a major cause for the weakening of the Indian summer monsoon. The formation of ABCs over northern India is suggested to introduce north-south asymmetries that disrupt the temperature gradient. This statement is supported by the findings of Duan et al., (2006: 1) who observed a spatial decrease in monsoon precipitation in Nepal, Bangladesh and NE India by around 19%, 9% and 6% respectively.  However, the feedbacks that occur between the land and the atmosphere are highly complex with direct and indirect feedbacks. As Kripalini et al., (2003) paper demonstrated, global warming has resulted in changes to some relationships whilst others are currently unaffected. Further work is needed to evaluate the affect global warming and other anthropogenic-induced climate change may have on monsoon systems, which is an important factor determining glacier mass balance.

CONCLUSIONS

The strength of the Asian monsoon varies temporally responding to differences in the land-ocean temperature gradient between the Asian subcontinent and the Indian Ocean. Research has shown that intense monsoons coincide with greater temperature contrasts between the land and ocean, and in the past was inversely related to winter snow cover in the Himalayas. However, although the association between monsoons and land-sea temperature contrasts has remained constant, there has been a weakening in the monsoon system during the mid-20^th century coinciding with a decline in snow cover. Historically, the decline in snow cover would have caused a strengthening of the monsoon due to changes in surface albedo. Deviation from this trend suggests a de-linking of the snow-monsoon relationship, with another factor becoming dominant in influencing the temperature gradient in the region. Several studies have suggested global warming may be this forcing factor with additional secondary forcing due to ABCs.

Precipitation is widely accepted as one of the three most important factors influencing glacial mass balance. Thus the weakening of the monsoon will undoubtedly affect the distribution and coverage of Himalayan glaciers. Tele-connections to the El-Nino Southern Oscillation (ENSO) and other atmospheric circulations across the globe mean substantially more research is needed to understand the dynamics influencing the monsoon system. However, it is apparent that at a regional scale, a dynamic snow-monsoon feedback exists with changes in Himalayan snow cover affecting the monsoon, which, in turn, causes changes to snow extent. This is a fundamental point to note in my subsequent post as it suggests that changes to the glacial extent may also affect the future strength of the monsoon. Overall, observations indicate the Asian monsoon has weakened during the 20^th century leading to a decline in precipitation over northern India and the Himalayas. Previous posts have highlighted the importance of Himalayan glaciers as a source for freshwater for Asian communities, and it seems, if this decrease continues, this dependence and hence the importance of the Himalayan glaciers is likely to increase in the future.

Reference:

Benn, D. I. and L. A. Owen (1998) ‘The role of the Indian summer monsoon and the mid-latitude westerlies in Himalayan glaciation: review and speculative discussion’, Journal of the Geological Society’ 155: 353-363.

Duan, K., T. Yao and L.G. Thompson (2006) ‘Response of monsoon precipitation in the Himalayas to global warming’, Journal of Geophysical Research, 111, D19110: 1-8.

Goswani, B.N. (2005) ‘South Asian Monsoon’,  in W.K.M. Lau and D.E. Waliser (eds) Intraseasonal Variability in the Atmosphere Ocean Climate System, Springer: London, 19-61.

IPCC (2007) Climate Change 2007: The Physical Science Basis, in S. Solomon, D. Quin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor and H. L. Millers (eds) Contributions of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press: Cambridge.

Kripalani, R.H., A. Kulkami and S.S. Sabada (2003) ‘Western Himalayan snow cover and Indian monsoon rainfall: a re-examination with INSAT and NCEP/NCAR data’, Theoretical and Applied Climatology, 74, 1: 1-18.

Oerlemans, J. and J.P.F Fortuin (1992) 'Sensitivity of glaciers and small ice caps to greenhouse warming', Science, 258, 5079: 115-117.

Prasad, A.K., K.-H.S. Yang, H.M. El-Askary and M. Kafatos (2009) 'Melting of major glaciers in the western Himalayas: evidence of climatic changes from long ter, MSU derived tropospheric temperature trend (1978-2008)', Annuals of Geophysics, 27: 4505-4519.

Mayewski, P.A., G.P. Pregent, P. A. Jeschke and N. Ahmad (1980) ‘Himalayan and Trans-Himalayan glacier fluctuations and the South Asian monsoon record’, Arctic and Alpine Research, 12, 2: 171-182.

Ramanathan, V. M. Agrawal, H. Akimoto, M. Aufhammer, S. Devotta, L. Emberson, S. I. Hasnain, M. Iyngarasan, A. Jayaraman, M. Lawrence, T. Nakajima, T. Oki, H. Rodhue, M. Muchirawat, S.K. Tan, J. Vincent, J.Y. Wang, D. Yang, Y.H. Yang, H. Autrup, L. Barregard, P. Bonasoni, M. Brauer, B. Brunekreef, G. Carmichael, C.E. Chung, J. Dahe, Y. Feng, S. Fuzzi, T. Gordon, A. K. Gosain, N. Htun, J. Kim, S. Mourato, L. Naeher, P. Navasumrit, B. Ostro, T. Panwar, M. R. Rahman, M. V. Ramana, M. Rupakheti, D. Settachan, A. K. Signh, G. Sat. Helen, P. V. Tun, P.H. Viet, J. Yinlong, S.C. Yoon, W. C. Chang, X. Chang, J. Zelikoff and A. Zhu (2008) Atmospheric Brown Cloud: Regional Assessment Report with Focus on Asia, UNEP: Kenya.

In the news this week...

Well 11/11/11 passed with no major mishaps but on this momentous day the Seventeeth Conference of the South Asian Association for Regional Cooperation (SAARC) drew to a close in the Maldives. Founded in 1985, this conference includes representatives from Bangladesh, Bhutan, India, the Maldives, Nepal, Pakistan and Sri Lanka, with the admission of Afghanistan in 2005.

The aim of the conference was to try and improve trade within the region, good governance, and most relevant to this blog, to discuss and suggest solutions for reducing water security threats to the region due to climate change.

Encapsulated under the conference theme ‘building bridges’, the conference highlighted that basin-wide, trans-boundary cooperation will be required to reduce the impacts of a decrease in the meltwater discharge to the major rivers within the region. This includes rivers such as  the Indus, Brahmaputra, Ganges that support most of the agricultural production in the basin as well as being the main source of freshwater for the growing south Asian population.

WATER WARS: FIGHTING THE FOOD CRISIS

If you would like to see some of the geo-engineering solutions that may have been discussed at the conference, then there is currently an exhibition entitled ‘Water Wars’ being held at the Science Museum in London that is definitely worth a visit. Although most of the exhibition shows geo-engineering methods that are still in the early stages of development, it also shows how the effects of water security in regions such as Asia may be felt in the United Kingdom. After all, if the countries cannot produce enough food to support their own population, why would they export it to us? This point is supported in the video available on the website and is really worth a look if you have a few moments (even if only for the animation and the girls voice). There is also an interactive game where you can see how you can cope against providing for a growing population in the context of reduced water availability. For a chance to put yourself in the policy makers shoes and to experience the difficulties that some countries in south Asia may have to solve in the future, click here.

The impact of glacial retreat on countries down valley/ downstream of the Himalayas demonstrates the importance of understanding how glaciers respond to climate change. The effects of glacial melt on surrounding communities are growing on the political agenda, and I will discuss a few of these such as water security and glacial lake outbursts in the oncoming weeks.

YET ANOTHER CONFERENCE

Figure 1: Map showing the region studied by the ICMOD.

The International Centre for Integrated Mountain Development (ICMOD) has revealed a new project this week to be held in Thimpu, Bhutan between the 14^th and 19^th of November 2011. Entitled ‘Connecting from Space to Village’, a series of events shall be conducted to coincide with the Bhutan’s ‘Climate Summit for a Living Himalayas’. The aim is to draw attention to policymakers, development agencies, local communities and other stakeholders, the potential of using remote sensing and Earth observations to better understanding and address issues of climate change.

The project seeks to act as a platform for sharing experiences of Earth observation applications that can be incorporated into the Summit’s discussions. These include themes based around ensuring food security, securing natural freshwater and also securing and maintaining biodiversity for long-term, sustainable use.

The events will be conducted over the next few weeks, and it will be interesting to follow how the events inform and direct the discussions at the summit.

In the news this week...

THE BIRTH OF A NEW ICEBERG.

Putting the halloween and fireworks news to one side, a recent flight by NASA's operation 'IceBridge' has discovered a major rift in the Pine Island Glacier located in western Antartica. Extending over eighteen miles and over 50 metres deep in places, this crack could result in the formation of an iceberg over 800 square kilometres in area. Though some scientists had speculated that the Pine Island Glacier was due to calve again, until the flight in October there had been no indication that the glacier was beginning to break apart. Now that the rift has been identified, this could provide a chance for scientists to observe the break up of the glacier and may help to improve understandings of the mechanisms and feedbacks that drive it.

For more information on the IceBridge mission in Antartica and other work in glacial regions click here.

Black carbon: The Dark Side of the Force.



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).



Figure 1: The movement of black carbon from the atmosphere to the surface by wet deposition.

Nucleation scavenging occurs when cloud liquid and ice grow directly onto the aerosol particles. Coagulation of particles causes some of the particles to become large enough to overcome air viscosity, and fall out below the cloud where the particles begin to evaporate or sublime (Jacobson, 2004). If it reaches as precipitation over glacial environments, black carbon is added to a thin layer on top of snow or sea ice. 

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.



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	Warren and Wiscombe (1980)
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.

 

Something to think about...

The articles discussed in my blog about ABCs state that ‘global’ dimming has masked some of the impacts of global warming during the 20^th century. With stricter regulations being put in place to reduce aerosol emissions how could this affect the world’s climate and glacial retreat in the future? Could it:

1)      Cause rises in global mean temperatures as the opposing dimming force is reduced. As a result feedbacks cause a further increase in the rate of glacial retreat?

Or,

2). Cause rises in global mean temperatures, but because atmospheric solar heating is reduced, glacial retreat decreases slightly as local surface heating at higher altitudes is lower?

And if, reducing local aerosol pollution to improve air quality results in enhanced global warming, how are policy makers going to weigh these decisions up against each other?

Just something to think about until my next post.

What's that? Is it a bird? Is it a plane? No it's an atmospheric brown cloud!

So far my blog has focused on the impact of global warming on glacial retreat in the Himalayas. However as Lei et al., (2011) stated in their article (see October 31^st entry), although global warming is the main forcing driving glacial retreat, there are also regional factors that account for the accelerated decline observed in these mountain regions. One of these driving forcings is the influence of atmospheric brown clouds (ABCs) and will be focus of this blog.

ATMOSPHERIC BROWN CLOUDS (ABCs)

Atmospheric brown clouds (ABCs) consist of a mixture of light-absorbing (mainly black carbon) and light-scattering (nitrous oxide, sulphate) aerosols (Ramanathan et al., 2007). These plumes of internally mixed aerosols result from anthropogenic combustion of biomass and fossil fuels and are characterised by polluted tropospheric layers with an aerosol optical depth (AOD) greater than 0.3 absorbing AOD>0.03 (Bonasoni et al., 2010).

IMPACTS OF ABCs.

The different aerosols in ABCs act in two main ways to influence the climate. Firstly, light absorbing particles such as black carbon (BC) contribute to global warming by increasing atmospheric solar heating through greater absorption of incoming insolation and outgoing radiation reflected from the Earth’s surface (Ramanathan et al., 2007). Secondly, light-scattering particles reduce the proportion of incoming insolation that reaches the surface, scattering more radiation back out into space cooling the Earth’s surface. This process acts in opposition to global warming and is referred to as ‘global dimming’ (Magnus et al., 2011).

The term ‘global’ dimming itself is somewhat misleading as ABC plumes tend to occur in five main regional hotspots: East Asia, the Indo-Gangetic Plain (IGP), Southeast Asia, South Africa and the Amazon Basin (Ramanathan et al., 2008). The first three regions, particularly the IGP, are located within relatively close proximity to the Himalayas and influence regional climate change within the region.

ABCs AND THE HIMALAYAS.

ABCs have a significant impact on global climate and some studies suggest that the cooling effect due to global dimming may have masked as much as 50% (Ramanathan et al., 2007) to 20-80% (Ramanathan et al., 2008) of global warming during the 20^th century.

Though ABCs reduce solar radiation reaching the Earth’s surface, their distribution at 1-3km (around the Himalayas) means the formation of these plumes in contribute to an enhanced warming effect at higher altitudes (Ramanathan et al., 2008). Lau et al., (2010) express this through the concept of the ‘elevated heat pump (EHP)’ effect. The paper noted a 1-2ºC cooling anomaly over land regions of India due to a combination of high AOD and increased cloud shielding with a similar anomaly observed in East Asia. However, at higher altitudes where the surface was amongst or above the ABC layer, atmospheric heating by BC (and dust brought in by spring dust storms) enhanced surface heating in these regions accelerating snow melt in the Himalayas and in the Tibetan Plateau. Ramanathan et al., (2008) extensive paper into the impacts of ABCs in Asia also proposes that ABCs enhance heating at higher altitudes, suggesting that BC increased solar heating at elevated levels (1-4km) over India and China by as much as 20-50% (6-20Wm^-2).

Relating these findings to the recent report published by Lei et al., (2011) may account for the statistically significant relative temperature increases observed at higher altitudes.

IMPACTS OF ABCs ON HIMALAYAN GLACIERS.

Increased emissions of ABC precursors sulphur dioxide and soot are shown in Table 1 and have resulted in an AOD and AOD_abs rise of 0.22 and 0.02 respectively in South China (Ramanathan et al., 2007).

Table 1: Rise in regional aerosol emissions in India and China between 1950 to 2002.

	Rises in aerosol emissions between 1950-2002 (-fold)
	Soot	Sulphur
India	3	7
China	5	10

As well as enhancing surface warming at higher altitudes, the increased AOD of the aerosol plumes have also affect the strength and distribution of the Asian Monsoon. Coinciding with the increase in aerosol emissions, since the 1980s in particular, the Asian monsoon has decreased in strength becoming less reliable and stable (Prasad et al., 2009). In addition to a 20% fall in rainfall over the IGP since the 1980s, the number of rainy days has also decreased across all of India (Ramanathan et al., 2008). There has also been a north-south shift in the monsoon in East China which has been attributed to a reduction in convection from the earth’s surface due as a consequence of global dimming.



A reduction in precipitation results in lower glacial accumulation rates as less rainfall falls as snow. This act to enhance glacial retreat as the glacier cannot replace the mass that is lost due to the accelerating snowmelt rates.

CONCLUSIONS

Overall, ABCs are major driving forcings influencing regional climate change. Though the plumes may act in opposition to global warming at the Earth’s surface, at higher latitudes enhanced solar heating contributes to the greenhouse effect causing greater surface temperatures and accelerating glacial retreat. Furthermore, a weakening of the Asian monsoon system and a decrease in annual precipitation has reduced the ability of glaciers to respond to these regional temperature rises.

Currently, China and India continue to emit aerosols at an ever-increasing rate. Therefore it seems likely that the regional temperature rise at higher altitude will continue into the future, showing another way humans have managed to affect even the most remote environments on earth.

Reference:

Bonsoni, P., A. Marinoni, M. Sprenger, F. Angelini, J. Arduini, U. Bonafe, F. Calzolani, T. Colombo, S. Decessari, C. Di Biagio, A. G. di sarra, F. Evangelisti, R. Duchi, M. C. Facchini, S. Fuzzi, G.P. Gobbi, M. Maione, A. Panday, F. Roccato, K. Sellegri, H. Venzac, G. P. Verza, P Villani, E. Vuillermoz and P. Cristofanelli (2010) 'Atmospheric brown clouds in the himalayas: first two years of continuous observations at the Nepal Climate Observatory-Pyramid (5079m)', Atmospheric Chemistry and Physics, 10: 7515-7531.

Lau, W.K., M.-K. Kim and W.-S. Lee (2010) 'Enhanced surface warming and accelerated snow melt in the Himalayas and Tibetan Plateau induced by absorbing aerosols', Environmental Research Letters, 5:1-11.

Li, Z., Y. He, W. An, L.Song, W. Zhang, N. Catto, Y. Wang, S. Wang, H. Liu, W. Cao, W. H. Theakstone, S. Wang and H. Du (2011) 'Climate and glacier change in southwestern China during the past several decades', Environmental Research Letters, 6, 4: 1-25.

Magnus, J.R., B. Melenberg and C. Muir (2011) 'Global warming and local dimming: the statistical evidence', Journal of the American Statistical Association', 106, 494: 452-464.

Prasad, A. K., K.-H.S. Yang, H. M. El-Askary and M. Kafatos (2009) 'Melting of major glaciers in the western Himalayas: evidence of climatic changes from long term MSU derived tropospheric temperature trend (1978-2008)', Annuals of Geophysics, 27: 4505-4519.

Ramanathan, V. M. Agrawal, H. Akimoto, M. Aufhammer, S. Devotta, L. Emberson, S. I. Hasnain, M. Iyngarasan, A. Jayaraman, M. Lawrence, T. Nakajima, T. Oki, H. Rodhue, M. Muchirawat, S.K. Tan, J. Vincent, J.Y. Wang, D. Yang, Y.H. Yang, H. Autrup, L. Barregard, P. Bonasoni, M. Brauer, B. Brunekreef, G. Carmichael, C.E. Chung, J. Dahe, Y. Feng, S. Fuzzi, T. Gordon, A. K. Gosain, N. Htun, J. Kim, S. Mourato, L. Naeher, P. Navasumrit, B. Ostro, T. Panwar, M. R. Rahman, M. V. Ramana, M. Rupakheti, D. Settachan, A. K. Signh, G. Sat. Helen, P. V. Tun, P.H. Viet, J. Yinlong, S.C. Yoon, W. C. Chang, X. Chang, J. Zelikoff and A. Zhu (2008) Atmospheric Brown Cloud: Regional Assessment Report with Focus on Asia, UNEP: Kenya.

Ramananthan, V., M. V. Ramana, G. Roberts, D. Kim, C. corrigan, C. Chung and D. Winker (2007) 'Warming trends in Asia amplified by brown cloud solar absorption', Nature, 448: 575-578.