Global and Regional Climate Changes due to Black Carbon

Transcription

1 Global and Regional Climate Changes due to Black Carbon V. Ramanathan and G. Carmichael Submitted to Nature-Geoscience as a Review Article November 16, 2007

2 Next to Carbon Dioxide, black carbon (BC) in soot, is most likely the strongest anthropogenic source for global and regional climate changes. Increased absorption of solar radiation by BC is shown to be just as important as CO 2 in the early melting and retreat of the arctic sea ice and the Himalayan glaciers. During transport, BC is mixed with other aerosols to form Atmospheric Brown Clouds (ABCs). Global dimming due to ABCs can help explain the decrease in South Asian and Sahelian rainfall. Large reductions in BC emissions can significantly reduce the retreat of sea ice and glaciers. 2

3 Origin and sources of atmospheric black carbon Soot starts off as smoke. Indoor smoke is due largely to cooking with bio fuels (wood, dung and crop residue) and outdoor smoke/haze is due to fossil fuel combustion (diesel & coal), open biomass burning (associated with deforestation and crop residue burning), and outdoor cooking with biofuels 1. Soot aerosols absorb and scatter solar radiation. The absorbing species (elemental carbon and some condensed organics) are popularly referred to as black carbon (BC) 2. This definition is some what arbitrary since recent findings suggest other secondary organics contribute to strong absorption in the UV, components which were presumably ignored in the original definition of BC 3. Globally the annual emissions of BC are (for the year 1996) ~8Tg/yr 4, with about 21% from biofuels, 38% from fossil fuels, and 41% from open biomass burning. The uncertainty in the published estimates for BC emissions is a factor of 2 to 5 on regional scales and at least ± 50% on global scales. High BC emissions (Fig. 1) are found in both the northern (associated largely with fuel combustion) and the southern (due predominately to open burning) hemispheres. BC can be transported thousands of kilometers and become widespread as trans-oceanic or trans-continental plumes of ABCs. These ABCs have been extensively documented by surface observatories, field observations and satellite data As revealed by single particle mass spectrometer data, ABCs consist of BC internally mixed with numerous other sub-micron aerosols such as sulfates, nitrates, organics and super-micron aerosols such as dust and sea salt 16. Although BC starts off as hydrophobic particles close to the source, during its transport it is mixed with soluble aerosols (e.g. sulfates) to become hydrophilic and subsequently removed as rain drops 2. This wet removal as well as direct deposition to the surface limits the atmospheric life time of BC to a few weeks or less 17 Regional Hotspots Until about the 1950s, North America and Western Europe were the major sources of soot emissions, but now developing nations in the tropics and East Asia are the major source regions 18, 19 (Fig. 1). Historical BC emissions are only available for fossil fuel combustion and bio fuel cooking 18, 19. Past emissions of BC from bio mass 3

4 burning are very uncertain 20, although, reports of biomass burning and visual sightings of extensive brown clouds date back to the 1880s 21. Integration of field observations 7, 14 and new satellite aerosol sensors 15 have revealed the current global distribution of ABCs and their radiative forcing Their concentrations peak close to major source regions and give rise to regional hotspots of BC-induced atmospheric solar heating (Fig. 1b) and surface dimming (Fig. 1c). Such hotspots include: i) Indo-Gangetic plains in South Asia; ii) eastern China; iii) most of Southeast Asia including Indonesia; iv) Regions of Africa between sub-sahara and south Africa; v) Mexico and central America; and vi) most of Brazil and Peru in South America. Populations of about 3 billion are living under the influence of these regional ABC hot spots. Radiative Forcing of the Climate system Among anthropogenic sources of atmospheric pollutants BC is the dominant absorber of visible solar radiation, and its absorption increases inversely with wavelengths from near IR (1 micron) to UV wavelengths with a power law of 1 to 3 depending on the source 3, 25, thus giving the brownish color to the sky. Unlike the straightforward nature of the greenhouse effect of CO 2,which leads to a positive radiative forcing of the atmosphere and at the surface 26 with moderate latitudinal gradients 27, 28, black carbon has opposing effects of adding energy to the atmosphere and reducing it at the surface. It alters the regional gradients of radiative forcing through a complex web of processes 7 that include: I) Increase in top-of-the atmosphere (TOA) radiative forcing. This occurs via several pathways. A) By absorbing the solar radiation reflected by the surfaceatmosphere-cloud system, BC reduces the albedo of the planet. B) Soot deposited over snow and sea ice can decrease the surface albedo C) Soot inside cloud drops and ice crystals can decrease the albedo of clouds by enhancing absorption by droplets and ice crystals All three of the above processes increase TOA forcing and contribute to global mean surface warming. 4

5 Figure 2 compares the BC forcing (Fig. 2c) with forcing due to all greenhouse gases (GHGs; Fig. 2a), only CO 2 forcing (Fig. 2b) and forcing of all aerosols other than BC (Fig. 2d). At the TOA, the ABC (i.,e, BC+Non_BC) forcing of -1.4 Wm -2 may have masked as much as 50% (± 25%) of the global forcing due to GHGs. Similar conclusions on the role of ABCs have been inferred by other studies 35, 36 and can also be inferred from IPCC 37. The BC forcing of 0.9 Wm -2 (with a range of 0.4 to 1.2 Wm -2 ) (Fig. 2b) is as much as 55% of the CO 2 forcing and larger than the forcing due to the individual effect of other greenhouse gases such as CH 4, CFCs and N 2 O and tropospheric ozone 37. Similar conclusions regarding the large magnitude of the BC forcing have been inferred by others and these estimates range from 0.4 Wm -2 to 1.2 Wm -2. The estimate shown in Fig. 2c is obtained from the observationally constrained study of 24. Values generated by many general circulation climate models (GCMs) are generally in the lower range of 0.2 Wm -2 to 0.4 Wm -2 37, 42, 43. There are several reasons for the underestimation by GCMs. Many ignore the internally mixed state of BC with other aerosols. Such mixing enhances absorption by a factor of 2 or more 39. Field observations have consistently shown that BC is well mixed with sulfates, organics and others 16, 44. Another factor contributing to lower BC forcing in GCMs is that observed BC concentrations peak at about 2 km above the surface 7, 14 whereas, in most models they are concentrated close to the surface 45. BC at elevated levels enhance solar absorption significantly because they can absorb the solar radiation reflected by the highly reflective low clouds 38, 40, 46. Column integrated aerosol absorption has been retrieved from a world-wide surface network of solar spectral radiometers, referred to as AERONET 47. The retrieved aerosol absorption 11, 48 is a factor of 2 or more larger than the GCM simulated values 41, 49. The exceptions to the low forcing bias of GCMs, are the models 50 that constrain aerosol solar absorption with AERONET values and models 39, 40 that account for the mixing state of BC with other aerosols. The BC forcing estimated by these models are in the range of 0.6 to 0.8 Wm -2 and 0.8 Wm -2 to 1.2 Wm -2 41, , 40 5

6 II) Atmospheric solar heating. In addition to absorbing the reflected solar radiation, BC absorbs the direct solar radiation and together the two processes contribute to a significant enhancement of lower atmosphere solar heating, by as much as 50% in the hot spots (see Fig 1b; 14 ). Numerous studies have speculated on the large magnitude of the atmospheric solar heating. Direct measurements of this solar heating has evaded us until now, for it requires multiple aircraft flying over the same domain at different altitudes to measure flux divergences (i.e. heating rates) for an extensive period of time. These challenges were recently overcome by deploying 3 light-weight UAVs with well calibrated and miniaturized instruments to measure simultaneously aerosols, black carbon and spectral as well as broad band radiation fluxes 14, 51, 52. The study 14 demonstrated that ABCs with a visible absorption optical depth as low as 0.02, is sufficient to enhance solar heating of the lower atmosphere by as much as 50%. Absorption in the UV, visible and IR wavelengths contributed to the observed heating rates. Such large heating rates, if it is solely due to BC, require BC to be mixed or coated with other aerosols 14. Global average BC solar heating, as per the present estimate, is 2.6 Wm -2 (Fig. 2c) with a factor of 5 to 10 larger heating (Fig. 1b) over the regional hotspots. III) Surface Dimming. The BC absorption of direct solar reduces the solar radiation reaching the surface and leads to dimming (Fig. 2c). The BC dimming is further enhanced by the reflection of solar radiation by aerosols other than BC (Fig. 2d) and by the enhancement of cloud albedo by aerosol nucleation of cloud drops (indirect effect). The cumulative dimming effect is -4.2 Wm -2 (sum of Fig. 2c and 2d), about -3 Wm -2 from direct effect of ABCs (BC and non-bc aerosols) and the rest from the indirect effects. Regionally the dimming can be as large as 5% to 10% reduction over the regional hotspots (Fig. 1). It is important to note that the surface dimming and absorption of direct solar do not contribute much to TOA forcing since it is simply a redistribution of the direct solar between the surface and the atmosphere. However, globally, this redistribution can weaken the radiative-convective coupling of the atmosphere and decrease global mean evaporation and rainfall 26. 6

7 Is the planet dimmer now than it was during the early twentieth Century? Solar radiometers around the world are indicating that surface solar radiation in the extra tropics was less by as much as 5% to 10% during the mid twentieth century 53, 54, while in the tropics such dimming trends have been reported to extend into the twenty first century. But many of these radiometers are close to urban areas and it is unclear if the published trends are representative of true regional to global averages. The Indian Ocean Experiment used a variety of chemical, physical and optical measurements to convincingly demonstrate 7 that ABCs can lead to dimming as large as 5% to 10% (Fig. 1c), over widespread regions in the North Indian Ocean and South Asia. In order to get a handle on the global average dimming 24 integrated such field observations with satellite data and aerosol transport models to retrieve an observationally constrained estimate. As seen from Figure 1c, over large regions the reduction of solar absorption at the surface exceeds 10 Wm -2 (>5%), which is consistent with the dimming reported from surface observations. The global-annual average dimming (for ), however, is -4.2 W.m -2, as opposed to the -10 Wm -2 estimated by surface radiometers over land areas. Thus great care should be exercised to extrapolate surface measurements over land areas to global averages. The global dimming of -4.2 Wm -2 has been compared with the GHGs forcing of 3 Wm -2 from 1850 to present 54. Such comparisons, without a proper context could be misleading since, as shown in Fig. 2, for BC, even the sign of the surface forcing (negative) is different from that at TOA (Fig. 2c). Global Climate Effects Surface Temperature: The TOA BC forcing implies that BC has committed the planet to a warming of about 0.5 to 1 C, where we have assumed a climate sensitivity of 2 to 4 ºC for a doubling of CO 2. On the other hand, ABCs (BC + Non_BC) forcing has committed the planet to a cooling of about C to -2.5 ºC 35. Since BC forcing results in a vertical redistribution of the solar forcing, a simple scaling of the forcing with the CO 2 doubling climate sensitivity parameter may not be appropriate 40, 55, 56. For example, GCMs suggest that the reduction of sea ice and snow albedo by BC is three times as effective as CO 2 forcing for global average surface warming 56. 7

8 Hydrological Cycle: The surface and atmospheric warming due to GHGs would lead to an increase in atmospheric humidity (due to increase in saturation vapor pressure) and rainfall (due to increase in the radiative heating at the surface) 26, 57. With respect ABCs (BC +Non-BC), the over all negative forcing at TOA, as well as the surface dimming, should lead to a decrease in evaporation and rainfall 7, 37. It is difficult to predict the net effect of GHGs and ABCs on global rainfall, given the large positive forcing at TOA and the larger negative forcing at the surface. We can not resort to observed rainfall trends to infer the net anthropogenic effect on global rainfall since long term rainfall measurements are available only for land regions. Since GCMs generally underestimate the ABC forcing at the surface and on the atmosphere, we can not yet rely on GCMs for an answer to this fundamental issue. Regional Climate Effects Simultaneously masking and intensifying greenhouse warming. We have just begun to 12, 14, comprehend the chain of response and feedbacks on the regional climate due to BC 24, In regions where radiative-convective coupling of the surface and the atmosphere is strong (e.g, equatorial oceans; tropical land during wet seasons), the surfaceatmosphere response will be determined by the TOA forcing, and as a result BC by itself will lead to a warming of both the surface (in spite of the surface dimming) and the atmosphere (in spite of the atmospheric solar heating); while ABCs (BC + non-bcs) will lead to a cooling of both the surface and the atmosphere. In regions where such coupling is weak (e.g, dry seasons in the tropics), the surface dimming due to ABCs can lead to a surface cooling and thus mask the greenhouse warming 65, whereas the atmospheric solar heating by BCs can lead to a warming of the atmosphere and intensify the greenhouse warming of the troposphere. GCMs that include just the BC forcing 14, 63, 66 show that BC leads to a warming from the surface to about 12 km altitude, by as much 0.6 K over most of the northern hemisphere including the Arctic region( e.g, see Fig. 11 in Chung and Seinfeld 40 ). The magnitude of the BC atmospheric warming is comparable to the simulated warming due to GHGs forcing 67. Regionally, the combined effect of ABCs 8

9 (BC plus non-bc) is to cause a surface cooling 65 over South Asia, while warming the atmosphere by as much as 0.6 ºC during winter and spring 14, 59. Such differential warming of the atmosphere (with respect to the surface) over the South Asian region have also been observed with microwave satellite sensor observations of the trends from 1979 to , 59. The atmospheric warming by BC is a major contributor to the observed retreat of Himalayan glaciers 14 and the arctic sea ice 56. Retreat of Himalayan Glaciers: Analysis of temperature trends on the Tibetan side of the Himalayas reveal warming trends in excess of 1 0 C since the 1950s and this large warming trend at the elevated levels is sufficient to account for the retreat of glaciers through melting 68, 69. The model simulations suggest that advection of the warmer air (heated by BC and GHGs) from South and East Asia over the Himalayas contributes to a warming trend of about 0.6 K (annual mean) in the lower and mid troposphere (see Fig. 3) of the Himalayan region 14, 63. This is as large as that due to the GHGs (Fig. 3), leading to the inference 14 that BC forcing is as important as GHGs in the observed retreat of over 2/3 of the Himalayan glaciers 70. The BC induced atmospheric warming is further amplified significantly by the reduction of snow and sea ice albedo discussed next. Arctic warming and retreat of sea ice: The major source of removal of black carbon from the atmosphere is through wash out in rain and snow. When soot is deposited over snow and sea ice, it can significantly enhance solar absorption by snow and sea ice and accelerate melting of snow and glacier ice 31. Recent studies suggest this as one of the major factors for the retreat of the arctic sea ice and glaciers (see summary of earlier studies in 56 ). Simulations in 56 showed that deposition of BC from sources in North America and Europe over the Arctic sea ice is a major contributor to the warming trend (about 0.5 to 1 K) and the observed retreat 71. In addition, this study 56 estimated that black carbon induced reduction of snow albedo is a major forcing term in the Tibetan side of the Himalayas. BC and GHGs are working together to accelerate the retreat of sea ice and Himalayan glaciers. Ice core record of BC deposition over Greenland from the early nineteenth century onwards has now provided historical record for estimating BC 9

10 forcing over the Arctic and examine its role in the retreat of sea ice during the twentieth century 72. The Asian monsoon. Precipitation trends over many regions of the tropics during the last 50 years have been negative, particularly over Africa, South Asia and northern China (Fig. 4) 67. These drying patterns are not explainable solely from global warming A combination of natural variability and anthropogenic aerosol forcing are emerging as major players in the observed trends 59, 73, 74, 76. The impacts of ABCs and BC on the S. Asian monsoon have received attention recently 40, 58, 59, 61-64, 66, 73. Precipitation over land is driven by evaporation from the land surface and long range transport of moisture from the surrounding Indian Ocean (IO). These model studies reveal that ABCs have three competing effects on the long range transport of moisture and its convergence over South Asia. i. Decrease in IO evaporation due to dimming: Emissions of BC and other aerosol precursors from South Asia have increased significantly since 1950s 18, 19. This has resulted in a dimming trend of about 7% as detected by surface radiometers in India 59. Similar dimming has also occurred over the Indian ocean 7 (See Fig. 1c). Since about 75% or more of the surface radiative heating is balanced by evaporation 26, the dimming trend leads to a decrease in evaporation from the North Indian Ocean 59 feeding less moisture to the monsoonal inflow into South Asia. ii. Decrease in meridional sea surface temperature (SST) gradient: Since ABCs are concentrated over the North Indian Ocean (NIO) (Fig 1), the dimming is suppressing the greenhouse warming over the NIO while the GHGs warming is proceeding unabated over the southern IO. As a result, the summer time north to south SST gradient (with warmer waters over the NIO) has decreased sine the 1950s, as indeed seen from observations 59, 73.. The weakening of SST gradient weakens the monsoonal circulation as shown by numerous studies 59, 73, 74 and in turn weakens the monsoonal rainfall during summer time. It is important to note that, although the ABC dimming peaks in winter and spring, the SST response is delayed until summer time due to the slower response time of the ocean 59, 63,

11 iii. Increase in atmospheric meridional heating gradient: The stronger BC solar heating of the atmosphere over South Asia (Fig. 1b) strengthens the monsoonal outflow flow with stronger rising motions over the sub-continent accompanied by stronger moisture flux into South Asia 59-63, 73. This effect, which increases rainfall, peaks during spring when the BC heating is at its peak values 63. The atmospheric heating is solely due to BC, whereas the dimming is due to both the BCs and Non-BC aerosols in ABCs (Figs 1c and 2d). In order to account for the delayed oceanic response to the dimming, fully coupled ocean-atmosphere models are required. To-date three such studies have been published 59, 61, 63 and all of them estimate an increase in pre-monsoon rainfall during spring followed by a decrease in summer monsoon rainfall, in agreement with observed trends (Fig. 4; 59 ). The link between dimming, north-south SST gradient and decrease in land rainfall has also been invoked to explain the Sahel drought 74. Climate system response and feedbacks Soot-dust-ice cloud interactions Spring season dust storms from Inner Mongolia and Taklimakan transport large quantities of dust across the Pacific Ocean 77, 78. Likewise, Saharan dust storms load the tropical Atlantic with dust 79. But during long range transport, the dust is mixed with industrial soot from East Asia 78 and biomass burning related soot from Africa. Such dust-soot mixtures, increase the atmospheric solar heating and surface dimming significantly 77, 78. In addition, the dustsoot mixtures can serve as nuclei for ice clouds and feedback on precipitation 80, and climate. For the first time, such dust-soot mixtures were tracked in an aircraft all the way across the Pacific Ocean from near the surface to the top of the troposphere 81. Layers of these dust-soot mixtures between 1 km and 14 km were observed from the western Pacific to North America. 11

12 Non-linear feedbacks between Greenhouse Warming and Soot Forcing. Increase in drought frequency and intensity due to global warming can intensify occurrence of forest fires as has been documented for California 82. Increase in forest fires, such as the boreal forest fires of 2001, can increase soot deposition in sea ice and enhance its melting 56 by a factor of two. Desertification associated with decrease in rainfall (Fig. 4) accompanied by an increase in evaporation from a warmer surface can lead to more aerosol emissions. Surface cooling due to dimming occurring simultaneously with lower atmosphere warming (due to BC heating) can stabilize the boundary layer during the dry season and increase the life times of aerosols in ABCs and increase persistence of smoke filled fog. Smoke can also influence precipitations formation mechanisms 83, 84. Two extreme scenarios have been proposed for such feedbacks. For South Asia, GCM simulations suggest that a 2 to 3 fold increase in soot loading (from present day levels) is sufficient to spin-down the monsoon circulation, decrease rainfall by more than 25% and increase drought frequency significantly 59. Since wash out by rain is a major sink for BC, large decreases in rainfall can feedback on BC concentrations positively. The other is the so-called nuclear winter scenario 85-87, in which large scale increase in BC from fires resulting from a global scale nuclear war, can nearly shut down sunlight at the ground (total dimming) accompanied by a large solar warming of the atmosphere which can collapse the troposphere and decrease rainfall drastically. Reducing Future BC Emissions Using BC reduction to delay dangerous climate change. Given its much shorter life time compared to CO 2 (with a lifetime of 100 years or more), and its significant contribution to global radiative forcing, a major focus on decreasing BC emissions offers an opportunity to reduce the effects of global warming trends in the short term (as also suggested by others, e.g ). Reductions in BC are also warranted from considerations of regional climate changes and human health 91, 92. It is clear from Figure 2 that air pollution mitigation steps can have significant impacts on future climate changes. The net forcing from ABCs is -1.4 Wm -2, with a contribution 12

13 from non-bc of -2.3 Wm -2 and from BC of +0.9 Wm -2 (Fig. 2). The logical deduction from Figs 2a, 2c and 2d is that the elimination of present day ABCs through emission reduction strategies would intensify surface warming by about 0.4 to 2.4 ºC, where the range is due to a two-fold range in the assumed climate sensitivity (2 to 4 ºC for a doubling of CO 2 ) and a ± 50% uncertainty in the ABC forcing (of -1.4 Wm -2 ) (also see 35 ). If only the non-bc aerosols (sulfates, nitrates, organics) were controlled, it could potentially add 2.3 Wm -2 (± 50%) to the TOA forcing and push the system closer to the 3 K cumulative warming (since 1850s), considered by many to be the threshold for dangerous climate change. If on the other hand, the immediate target for control shifts entirely to BC (due to its health impacts) without a reduction in non-bc aerosols, the elimination of the positive forcing by BC will substantially decrease both the global warming and the retreat of sea ice and glaciers. This BC reduction can delay the onset of dangerous climate change by a decade to three (given that CO 2 increase is expected to add 0.3 Wm -2 /decade 67 ). It is important to emphasize that BC reduction can only help delay and not prevent dangerous climate change. It is merely an interim step for buying time to implement effective steps for reducing CO 2 emissions. Asian emissions and future trends Given the fact that technology exists for large reductions of soot emissions, we explore the impact of a major focus on soot reductions. We focus on Asia, where emissions from China and India alone account for ~25 to 35 % of global BC emissions and the regional climate responses to BC are (expected to be) large. In addition, with the economies of China and India expanding with double digit growth rates, Asia can become a much larger source of BC and Non-BC aerosols, depending on the energy-path taken to sustain this growth rate. In fact new estimates indicate that BC emissions for China in 2006 have doubled since 2000, while SO 2 emissions have grown during this period by more than 50% 93. East Asia and South Asia also represent a different mix of emissions, and therefore can illustrate potentials for various control options, that are also representative of global choices. The majority of soot emission in South Asia is due to bio fuel cooking, while in East Asia, coal combustion for residential and industrial uses plays a larger role. 13

14 The large BC emissions are reflected in the geographical extent of the large absorbing component of AOD, simulated with a regional aerosol-chemistry-transport model 94 (see areas with BC-AOD >0.01 in Fig. 5a). What are the opportunities to reduce the radiative effects of BC? Providing alternate energy efficient and smoke free cookers and introducing (transferring) technology for reducing soot emissions from coal combustion in small industries will have major impacts on the radiative forcing due to soot. Figure 4b shows the impact of replacing bio-fuel cooking with BC-free cookers (solar; bio and natural gas) in South and East Asia. The impacts are dramatic; over South Asia a 70% to 80% reduction in BC heating, and a 20% to 40% reduction in East Asia. The impact on human health will potentially be even more dramatic since over 400,000 annual fatalities among women and children are attributed to smoke inhalation during indoor cooking 91,

15 Figure Captions Figure 1: Global distribution of black carbon emission (a); atmospheric solar heating due to ABCs (b); and reduction of solar radiation absorbed by the surface due to ABCs, i.e, Dimming. BC emissions data are taken from 4 and includes emissions from fuel combustion (fossil fuels and bio fuels) and open bio mass burning (forest fires, savanna burning and outdoor cooking) for the year The data for Figures 1 b and 1c are taken from 24 and is applicable for the 2001 to 2003 period. This study integrates satellite aerosol optical data with surface network of aerosol remote sensing instruments and field observations to derive aerosol absorption, scattering optical depth and vertical distribution. It then uses an aerosol-transport-chemical model to partition the aerosol optical depths into natural and anthropogenic. Figure 1b shows that solar radiation absorbed n the atmosphere due to anthropogenic aerosols which is attributed to BC. Figure 1c shows the reduction in absorbed solar radiation at the surface by all anthropogenic aerosols (BC and non-bc) in ABCs and this reduction : i) Absorption of direct (downwards) solar radiation by BC and ii) scattering back to space by aerosols such as sulfates, nitrates, organics, fly ash among others. Figure 2: Globally averaged radiative forcing at the top-of-the atmosphere (top set of numbers); for the atmosphere (numbers within the blue box) and at the surface (numbers within the brown box at the bottom). For each panel, the sum of the surface and the atmosphere forcing equals the TOA forcing. Figure 1a is the forcing for all greenhouse gases (CO 2 ; CH 4 ; N 2 O; Halons; Ozone) and Figure 1b is just for CO 2. The forcing values represent the change in radiative forcing due to increase in gases from pre-industrial to year The TOA numbers are taken from 67 and the atmospheric and surface forcing is derived from an atmospheric radiative transfer model and the numbers at the surface and the atmosphere are slightly adjusted to agree with the TOA IPCC forcing. The uncertainty in the forcing values are ± 20%. The sum of the BC and Non-BC forcing is taken from the Chung et al study and this equals to sum of the numbers shown in Fig 2c and 2d. The BC forcing values were obtained by running the Chung et al analysis with and without BC. The published values for TOA forcing that allows for internal mixing of BC has a range of 0.4 to 1.2 Wm -2 and that for non-bc has an uncertainty of ± 40%. The surface and atmospheric forcing values for both BC and non-bc has uncertainties of ± 50%. All of he uncertainties and ranges have 95% confidence interval. Figure 3: Average temperature change over the northern S Asian region from 20N to 40N and from 70E to 100E. The blue line is the change due to the increase in all greenhouse gases and non-bc aerosols and the results are taken from 59. The red line is the estimated temperature change due to BC taken from the GCM study of 66. Figure 4: Precipitation trend from 1950 to 2002 (units: mm/day change from 1950 to 2002). The plot is adopted from 64. Figure 5: Simulated annual mean optical depth of black carbon aerosols for 2004/2005 using the regional aerosol/chemical/transport model described in

16 a) With BC emissions from bio fuel cooking (indoor cooking with wood/dung/crop residues), fossil fuels and biomass burning. b) Same as (a), but without biofuel cooking. 16

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23 a) BC emissions (Tons/yr) b) BC Atmos Heating (W/m 2 ) c) Dimming due to ABCs (W/m 2 )

24 3 1.6 TOA Atmosphere Surface a) All GHGs b) CO 2 c) BC d) Non-BC (Direct ) (Direct + indirect)

25 annual mean temperature change internal mixed BC CO2+SO2 Pressure [mb] annual mean temperature change [C]

26 Observed Trends in Summer Rainfall: 1950 to 2002 N-S Shift in Asian rainfall The Weakening Indian Monsoon The Sahelian Drought

27 a) Baseline BC AOD for 2004/05 b) BC AOD without biofuels