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.