Black soot on the Tibetan Plateau

A paper by Xu et al. (2009) examines the impact of black carbon (soot) on the Tibetan Plateau - which contains the largest ice mass outside of the polar regions – and how this affects their survival for the future.

As has often been examined in this blog, the human as well as physical impacts of any changes to the region can be plainly seen. Most importantly, the authors offer clear explanation of the regional significance of water originating from the glaciers of the Tibetan Plateau. Firstly, the meltwaters are key in supplying the Indus, Ganges and Brahmaputra river systems, amongst others. Secondly, this freshwater source sustains over 1 billion people. Thirdly, for 25 percent of the people in western China, the meltwater from the Tibetan Plateau provides the main dry season water source (December – March). It is hard to envisage any clearer evidence of the reliance that so many people have on mountain glaciers.

Indeed, with the loss of headwater glaciers (supplying the source of the rivers), there will inevitably be a rapid decline in the availability of meltwater during the dry season. The increased melting of these glaciers is instead likely to provide spring time flooding. This then brings us onto what is causing this melting. The answer remains mostly obvious, in that glacier retreat is driven primarily by increased air temperature, on which much research exists. However, as Xu et al. (2009) note; areas of over 4000m elevation have warmed by 0.3°C per decade over the last 30 years, which is twice the rate of observed global warming. This high rate of temperature increase and subsequent rapidity of glacial retreat suggests that “additional mechanisms may be involved”. This of course then brings us onto the influence of black carbon on rates of glacier melt.

Whilst black carbon has an atmospheric impact (of warming the troposphere), the focus of this paper remained on the incorporation of black carbon onto snow and ice and the effect that this has for surface melt and albedo. When black soot forms on snow and ice surfaces, it serves to darken them, which as a result increases solar absorption and increases surface melting. As for the input pathways of black carbon, two factors are seen to bring it to the glaciers of the Tibetan Plateau. The first of these is the positioning of the plateau close to SE Asian industry. As the authors state, the region is presently and predicted to continue as the largest source of black carbon globally. Secondly, this positioning allows for the black soot to be carried upwards by winds and attached to snowflakes, prior to their deposition on top of the glacier surfaces. Thus, the glacier surface is darkened.

The study was conducted by taking ice cores from five key locations across the plateau. The aim of this was to establish temporal changes in the amount of black carbon since the 1950s. As well as this, the sources of snowfall for each part of the Plateau were established. For the N and NW parts:

“... associated mainly with the westerly jet stream, which moves southward toward the Himalayas in winter. Thus, black soot deposited on Himalayan glaciers derives primarily from two directions: west and south... so its upwind sources are principally Europe and the Middle East.”

For the S parts of the plateau:

“... [they] receive deposits from the west in winter and from the south in summer.”

Finally, the largest source of snowfall for the entire plateau is shown to be the Indian monsoon, which can move as far as 30-32°N in the summer.

Evidence of these soot and snowfall sources can be seen in the ice core records, which saw decreasing carbon in the NW and central regions of the plateau during the 1970s and 1980s. Such a reduction wasn’t seen in other areas, and reflects the successful implementation of new environmental regulations in Europe at the time. As well as this, the ice core records showed that carbon concentrations have increased in the southern Tibetan Plateau since 1990. This north/south plateau contrast shows both a difference in global attitudes to the emission of black carbon as well as showing the effect of different transport lengths (flow from Europe more easily disrupted/dissipated).

Figure 1. Black and Organic Carbon levels of the 5 ice cores. With annual and 5-year running averages.

The paper showed that the observed amount of black carbon was enough to influence the surface reflectivity of the glaciers. This comes together with the increased industrial activity, rapidly increased soot deposition from the 1990s and the accelerating rate of glacier retreat in the last 30 years. The study also showed that black carbon concentrations of 10 ng g^-1 reduce the visual albedo of thick snow to 0.01 – 0.04. This means that absorption of visible radiation is increased by 10 – 100 percent (dependent on snowflake type and the extent of soot mixing with snow). As well as this, the arrival of soot in spring allows for the melt season to begin earlier. Indeed the deposition of black carbon peaks between November and March, when the snow is at its maximum extent. More worrying, is the presence of black carbon in the accumulation zone of glaciers (where natural snow melt isn’t seen to any great extent). As well as all of these factors, the study has found that the melting process in the ablation zone can serve to increase the concentration of black carbon (and therefore intensifying its effects).

The authors also reference several studies which attempt to model the impacts of black soot on the regional climate of glaciated areas. (Hess et al., 1998) (Hansen & Nazarenko, 2004) (Flanner et al., 2007) (Shindell & Faluvegi, 2009). As Xu et al. note, the findings of these initial studies are very significant:

“These studies suggest that black soot is responsible for a substantial fraction of the regional warming of the past century, comparable to the fraction attributable to carbon dioxide. Assessment of black soot’s impact on glaciers will need to include the contribution that black soot makes to regional climate change, as well as direct effects on the glacier.”

Evidently, the influence of black soot on glacier change has been shown to be quite significant. If loss of the Tibetan Plateau as a fresh water source is to be prevented, then policy aimed at reducing SE Asian black carbon emissions is a necessity. It has been shown not only to directly alter surface albedo, but also to significantly affect regional climate as well. The authors state finally that if black soot levels are reduced and CO₂ levels are stabilised (somewhat unknowingly), then there remains a possibility that Tibetan glaciers may remain beyond this century. It is perhaps better put by stating that the future of the glaciated Tibetan Plateau remains very uncertain indeed.

Further reading: Whilst not looked at directly in this blog, this paper by Kopacz et al. (2011) takes a more technical look at the sources to the Tibetan Plateau and Himalayas, by modelling the global transport and radiative forcing of black carbon.

In the news: This topic was also covered in an article from The Guardian (2009), which looks at black soot originating from developing countries, impacting across the Tibetan Plateau and Himalayas.