Black carbon aerosol plays a unique and important role in Earth's climate system. Black carbon is a type of carbonaceous material with a unique combination of physical properties. This assessment provides an evaluation of black-carbon climate forcing that is comprehensive in its inclusion of all known and relevant processes and that is quantitative in providing best estimates and uncertainties of the main forcing terms: direct solar absorption; influence on liquid, mixed phase, and ice clouds; and deposition on snow and ice. These effects are calculated with climate models, but when possible, they are evaluated with both microphysical measurements and field observations. Predominant sources are combustion related, namely, fossil fuels for transportation, solid fuels for industrial and residential uses, and open burning of biomass. Total global emissions of black carbon using bottom-up inventory methods are 7500 Gg yr−1 in the year 2000 with an uncertainty range of 2000 to 29000. However, global atmospheric absorption attributable to black carbon is too low in many models and should be increased by a factor of almost 3. After this scaling, the best estimate for the industrial-era (1750 to 2005) direct radiative forcing of atmospheric black carbon is +0.71 W m−2 with 90% uncertainty bounds of (+0.08, +1.27) W m−2. Total direct forcing by all black carbon sources, without subtracting the preindustrial background, is estimated as +0.88 (+0.17, +1.48) W m−2. Direct radiative forcing alone does not capture important rapid adjustment mechanisms. A framework is described and used for quantifying climate forcings, including rapid adjustments. The best estimate of industrial-era climate forcing of black carbon through all forcing mechanisms, including clouds and cryosphere forcing, is +1.1 W m−2 with 90% uncertainty bounds of +0.17 to +2.1 W m−2. Thus, there is a very high probability that black carbon emissions, independent of co-emitted species, have a positive forcing and warm the climate. We estimate that black carbon, with a total climate forcing of +1.1 W m−2, is the second most important human emission in terms of its climate forcing in the present-day atmosphere; only carbon dioxide is estimated to have a greater forcing. Sources that emit black carbon also emit other short-lived species that may either cool or warm climate. Climate forcings from co-emitted species are estimated and used in the framework described herein. When the principal effects of short-lived co-emissions, including cooling agents such as sulfur dioxide, are included in net forcing, energy-related sources (fossil fuel and biofuel) have an industrial-era climate forcing of +0.22 (−0.50 to +1.08) W m−2 during the first year after emission. For a few of these sources, such as diesel engines and possibly residential biofuels, warming is strong enough that eliminating all short-lived emissions from these sources would reduce net climate forcing (i.e., produce cooling). When open burning emissions, which emit high levels of organic matter, are included in the total, the best estimate of net industrial-era climate forcing by all short-lived species from black-carbon-rich sources becomes slightly negative (−0.06 W m−2 with 90% uncertainty bounds of −1.45 to +1.29 W m−2). The uncertainties in net climate forcing from black-carbon-rich sources are substantial, largely due to lack of knowledge about cloud interactions with both black carbon and co-emitted organic carbon. In prioritizing potential black-carbon mitigation actions, non-science factors, such as technical feasibility, costs, policy design, and implementation feasibility play important roles. The major sources of black carbon are presently in different stages with regard to the feasibility for near-term mitigation. This assessment, by evaluating the large number and complexity of the associated physical and radiative processes in black-carbon climate forcing, sets a baseline from which to improve future climate forcing estimates.

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Bounding the role of black carbon in the climate system: A scientific assessment

There is growing evidence that the rate of warming is amplified with elevation, such that high-mountain environments experience more rapid changes in temperature than environments at lower elevations. Elevation-dependent warming (EDW) can accelerate the rate of change in mountain ecosystems, cryospheric systems, hydrological regimes and biodiversity. Here we review important mechanisms that contribute towards EDW: snow albedo and surface-based feedbacks; water vapour changes and latent heat release; surface water vapour and radiative flux changes; surface heat loss and temperature change; and aerosols. All lead to enhanced warming with elevation (or at a critical elevation), and it is believed that combinations of these mechanisms may account for contrasting regional patterns of EDW. We discuss future needs to increase knowledge of mountain temperature trends and their controlling mechanisms through improved observations, satellite-based remote sensing and model simulations.

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Elevation-dependent warming in mountain regions of the world

Himalayan glaciers are a focus of public and scientific debate. Prevailing uncertainties are of major concern because some projections of their future have serious implications for water resources. Most Himalayan glaciers are losing mass at rates similar to glaciers elsewhere, except for emerging indications of stability or mass gain in the Karakoram. A poor understanding of the processes affecting them, combined with the diversity of climatic conditions and the extremes of topographical relief within the region, makes projections speculative. Nevertheless, it is unlikely that dramatic changes in total runoff will occur soon, although continuing shrinkage outside the Karakoram will increase the seasonality of runoff, affect irrigation and hydropower, and alter hazards.

https://www.zora.uzh.ch/id/eprint/72075/1/2012_BolchT_BolchAl12_science_GlacHimal_supplm_subm_Kopie_.pdf

The state and fate of Himalayan glaciers

Glaciers are among the best terrestrial climate indicators, an important water resource in mountains1,2 and a major contributor to global sea level rise3,4. In the Hindu Kush - Karakoram - Himalaya region (HKKH), a paucity of appropriate glacier data has prevented a comprehensive assessment of current regional mass balance5. However, there are indirect evidences of a complex pattern of glacial responses5-8 in reaction to heterogeneous climate change signals9. Here, we provide the first coherent data set of detailed glacier thickness changes over the HKKH during 2003-2009 by combining satellite laser altimetry and a global elevation model. In the eastern, central and south-western parts of the HKKH, glacier wastage is widespread with regional thinning rates up to 0.66 ± 0.09 m a-1 in the Jammu-Kashmir region. Conversely, in the Karakoram, glaciers are close to balance with only a slight thinning of 0.07 ± 0.04 m a-1. Regionally averaged thinning rates under debris-mantled ice are similar to those of clean ice despite insulation by debris covers. The 2003-2008 specific mass balance for our HKKH study region is -0.21 ± 0.05 m a-1 water equivalent (WE), significantly less negative than the global average of ~ -0.7 m a-1 WE for glaciers and ice caps4,10. This difference is mainly an effect of the balanced glacier mass budget in the Karakoram. The corresponding HKKH sea level contribution is +0.035 ± 0.009 mm a-1 amounting to 1% of the present-day sea level rise11. Our 2003-2008 mass budget of -12.8 ± 3.5 Gt a-1 is more negative than recent satellite gravimetry based estimates of -5 ± 6 Gt a-1 over 2003-2010 (ref. 12). For the mountain catchments of the Indus and Ganges basins13, the glacier imbalance contributes ~3.5% and ~2.0%, respectively, to the annual average river discharge13, and up to ~10% for the Upper Indus basin14.

Contrasting patterns of early twenty-first-century glacier mass change in the Himalayas

The present state and future evolution of Himalayan glaciers has been controversial. An analysis of remotely sensed frontal changes and surface velocities from glaciers in the greater Himalaya between 2000 and 2008 shows large regional variability in the responses of Himalayan glaciers to climate change.

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Spatially variable response of Himalayan glaciers to climate change affected by debris cover

We find evidence that black soot aerosols deposited on Tibetan glaciers have been a significant contributing factor to observed rapid glacier retreat. Reduced black soot emissions, in addition to reduced greenhouse gases, may be required to avoid demise of Himalayan glaciers and retain the benefits of glaciers for seasonal fresh water supplies.

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Black soot and the survival of Tibetan glaciers

The post-depositional enrichment of black soot in snow-pack was investigated by measuring the redistribution of black soot along monthly snow-pits on a Tien Shan glacier. The one-year experiment revealed that black soot was greatly enriched, defined as the ratio of concentration to original snow concentration, in the unmelted snow-pack by at least an order of magnitude. Greatest soot enrichment was observed in the surface snow and the lower firn-pack within the melt season percolation zone. Black carbon (BC) concentrations as high as 400 ng g−1 in the summer surface snow indicate that soot can significantly contribute to glacier melt. BC concentrations reaching 3000 ng g−1 in the bottom portion of the firn pit are especially concerning given the expected equilibrium-line altitude (ELA) rise associated with future climatic warming, which would expose the dirty underlying firn and ice. Since most of the accumulation area on Tibetan glaciers is within the percolation zone where snow densification is characterized by melting and refreezing, the enrichment of black soot in the snow-pack is of foremost importance. Results suggest the effect of black soot on glacier melting may currently be underestimated.

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Post-depositional enrichment of black soot in snow-pack and accelerated melting of Tibetan glaciers

In this study, an optimized two-step heating-gas chromatography system is used to measure elemental carbon (EC) and organic carbon (OC) content in snow and ice, with the ability to quantify the elemental and organic carbon species in a snow or ice sample of 60-80 g. In this system, OC and EC are transformed into CO2 in a stream of oxygen at 3408C and 6508C, respectively. The resulting CO2 is accumulated in two molecular-sieve traps, and then put into a gas chromatograph equipped with a flame ionization detector by heating the traps to 2008C in a helium stream. Background contamination (mainly caused by impurities in the oxygen stream) and accuracy are dominated by the variability of the blank loads on the pre-cleaned filters, which are 0.50 � 0.04 (1) mgC for OC, and 0.38 � 0.04 (1) mgC for EC. The system is suitable for snow and ice sample measurements, with the same precision as shown for the blank tests. EC and OC concentrations have been measured in snow samples collected from different glaciers on the Tibetan Plateau. The results allow quantification for the first time of the different carbonaceous particle contents on the Tibetan Plateau and other regions. The concentrations of EC and OC particles in snow show a clearly decreasing trend from east to west and from north to south on the plateau, excluding the Pamirs region. The highest mean EC content, 79.2 ng g -1 , was found in the northeast region, and the lowest, 4.3 ng g -1 , was found in the western Himalaya. We note that even slight surface melting results in fresh snow getting dirtier, especially in regions with higher pollution such as seen on a glacier in the Qilian Shan. Here, the EC and OC concentrations in the fresh snow average 6.6 and 87.5 ng g -1 , but after 2 days of surface melting they increased to 52.6 and 195.5 ng g -1 . This suggests that surface snow melting can reduce snow albedo due to the accumulation of carbonaceous particles.

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Elemental and organic carbon measurements with a two-step heating-gas chromatography system in snow samples from the Tibetan Plateau

Ecosystems in the Central Asian Plateau, which includes the Mongolian Plateau, are becoming increasingly sensitive to human interventions, leading to deterioration of already fragile ecosystems. The goal of this paper is to illustrate human dependence on an ecosystem by identifying patterns of resource consumption in this region and investigating the knowledge and perceptions of herders living in these ecosystems. Data on consumption in the two regions were collected using structured questionnaires delivered to a total of 252 herders from Mongolia and China’s Inner Mongolia. Meat and other animal products remain the dominant food items for most households, accompanied by various vegetables and cereals. This unbalanced diet leads to excessive consumption of protein and fat from animal sources. The major energy sources used by herders are fuelwood, animal dung, crop residues, and dry grass, but consumption patterns differed between the two areas. Mongolian herders rely more heavily on livestock for meeting their consumption needs than herders in Inner Mongolia. Herder knowledge and perceptions of ecosystem conditions and consumption of resources differed between Mongolia and Inner Mongolia, reflecting the influence of different state policies. The data reported and the conclusions drawn are relevant for developing resource management policies for the Mongolian Plateau, but also provide useful insights for any region where livestock production dominates the use of rangeland resources.

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Comparing patterns of ecosystem service consumption and perceptions of range management between ethnic herders in Inner Mongolia and Mongolia

Carbonaceous particles concentrations of OC and EC are determined using a two-step gas chromatography system in Muztagh Ata ice core covering the time period of 1955–2000. Over the period represented by the core, OC and EC concentrations appear to have changed significantly, varied in the range of 17.7–216.7 and 6.5–124.6, and averaged 61.8, 32.9 ng·g−1, respectively. The average concentration of EC in Muztagh Ata ice core is much lower than that in an Alpine ice core record (100–300 ng·g−1) during the same period, but it is a factor of 14 in Greenland ice core (2.3 ng·g−1), this may induce a strong impact on the snow albedo in the last 46 years in our study area. Observations indicate two periods with obviously high deposition concentrations (1955–1965 and 1974–1989) and two periods with low concentrations (1966–1973 and 1990–1995), as well as a recent increasing trend. By comparing EC and SO42− concentration variations and deciphering OC/EC ratios recorded in the same ice core, we can judge roughly that the carbonaceous particles deposited in Muztagh Ata ice core were attributed to fossil fuel combustion sources.

Xianqin Liu

Papers

Black soot and the survival of Tibetan glaciers

Post-depositional enrichment of black soot in snow-pack and accelerated melting of Tibetan glaciers

Elemental and organic carbon measurements with a two-step heating-gas chromatography system in snow samples from the Tibetan Plateau

Comparing patterns of ecosystem service consumption and perceptions of range management between ethnic herders in Inner Mongolia and Mongolia

Carbonaceous particles in Muztagh Ata ice core, West Kunlun Mountains, China