Part 1 of 2: 2010 March 16: U.S. House of Representatives Hearing: Ramanathan testimony on Black Carbon

2010 March 16: U.S. House of Representatives Hearing: Ramanathan testimony on Black Carbon
 
II. Black Carbon, Atmospheric Brown Clouds and Greenhouse Effect: Background
.
1. Origin of Black Carbon: Black carbon (BC), a major component of soot, is emitted through cooking with solid fuels (coal, wood, cow dung and crop residues), by biomass burning and fossil fuel combustion (solid coal or combustion of diesel fuel). These sources also emit organic aerosols and the mix of BC and organics is popularly referred to as soot. In the atmosphere, BC is mixed (Moffet and Prather, 2009) with other particles such as sulfates, nitrates, dust and other pollutants. A single particle can contain a mixture of BC and one or more of these other chemical species, in which case, the particle is referred to as internally mixed. BC can also exist as a separate particle, coexisting with other aerosols side by side, and in this instance BC is referred to as externally mixed. BC and these other particles remain in the atmosphere for several days to few weeks, during which they can be transported thousands of kilometers away from their source.
2. Atmospheric Brown Clouds and BC Hotspots: Such vast plumes of pollution aerosols containing BC are sometimes referred to as Atmospheric Brown Clouds (ABCs). Hot spots of ABCs with large concentrations of BC as well as other man-made aerosols such as sulfates, organics, nitrates and others have been identified by synthesizing satellite observations with ground base and aircraft observations: The following regions fall under the hot spot category: (1) east Asia (eastern China, Thailand, Vietnam and Cambodia), (2) Indo-Gangetic Plains in south Asia (the northwest to northeast region extending from eastern Pakistan, across India to Bangladesh and Myanmar), (3) Indonesian region, (4) southern Africa extending southward from sub-Saharan Africa into Angola and Zambia and Zimbabwe, and (5) the Amazon basin in South America. However, ABCs are a world wide problem, including developed nations. For example, per capita emissions of black carbon in US is comparable to that in E. Asia.
3. Policy Implications of the Regional Nature of BC Effects: The regionally concentrated nature of BC concentrations is a potential advantage for policy makers. While one of BC’s major effect is on global and regional climate change, the immediate effect of BC reductions will be felt locally (wherever mitigation actions are taken) as an improvement in air quality and visibility accompanied by mitigation of the impacts of BC on human health, agriculture, and local precipitation.
Images of Internally Mixed BC
Atmospheric Brown Cloud: Hot Spots
Monthly mean aerosol optical depths derived from MODIS aerosol instrument on NASA’s TERRA satellite. The optical depth is a good index for the product of the aerosol number concentration and their surface area from the surface through the depth of the atmosphere. The color shading is dark blue for AODs smaller than 0.05 (clean marine background); green for 0.2 (visible brown clouds) , yellow for 0.4 to 0.5 (very hazy)and red for AODs>0.6 (heavily polluted).
Ramanathan et al, 2007c
Testimony of V. Ramanathan March 16, 2010 5
4. Black and Brown Carbon Terminology: Black carbon is not a ‘greenhouse gas’. It is a particle and it is the strongest absorber of solar radiation in the atmosphere. It also absorbs and emits infra red or heat radiation and contributes to the greenhouse effect. However, the latter effect is much smaller than the solar warming effect. The term "black carbon" is not rigorously defined. Climate models have largely assumed black carbon is the same as elemental carbon. All man-made sources of black carbon also emit hundreds of organic aerosols and gases (which later become aerosols). Most climate models treat these organic aerosols as purely reflecting (and not absorbing) aerosols, i.e., they have a cooling effect. Recent experimental work has given compelling evidence that some of these organic aerosols also absorb sunlight in UV and visible wavelengths and thus enhance the warming effects of BC (Magi, 2009). These absorbing organic aerosols are popularly referred to as ‘Brown Carbon’ (Andreae and Gelencser, 2007). For the purpose of this report, absorption of solar radiation by BC and brown carbon are treated together, since they occur in the same wavelength region.
5. How Does BC contribute to Global Climate Change?
BC warms the climate in at least 5 different ways (Jacobson, 2010): i) It traps (absorbs) solar radiation in the atmosphere, directly heats the air and thus contributes to climate warming. There is now strong experimental evidence that internally mixed BC absorbs significantly more solar radiation than externally mixed BC (Moffet and Prather, 2009). ii) When BC is deposited on sea ice, snow packs and glaciers, it darkens the snow and ice surfaces, enhances absorption of sunlight and contributes to melting of snow and ice. iii) BC also absorbs and emits heat radiation (Infra red radiation) and adds to the atmospheric greenhouse effect. This effect, although much less than the warming from the solar heating effect, can be important in the arctic and during nights. iv) BC gets into cloud droplets (by nucleation or scavenging) and enhances absorption of solar radiation by drops. v) The day time warming of the lower layers of the atmosphere, first few kilometers, by BC can suppress the relative humidity and evaporate low level clouds, which will allow more solar radiation to reach the ground and amplify the warming.
BC has also a potential cooling effect. When aged and
mixed with other aerosols such as sulfates and oxidized organics, BC can also be efficient cloud nuclei. Formation of new cloud drops through BC nucleation, in low level stratus and cumulus clouds, can make the clouds brighter and shield the surface from solar radiation and cause surface cooling.
Testimony of V. Ramanathan March 16, 2010 6
6. Recent Estimates of BC Forcing: Compared with the climate forcing due to carbon dioxide which has been studied intensely for several decades, the science of BC and its climate effects is relatively new. Our understanding of the impact of black carbon (BC) aerosols has undergone major improvements and revisions during the last 10 years. The major contributing factors are listed below: i) Experimental findings from field and aircraft observations (e.g. INDOEX, ACE-Asia and others) in Asia, Africa, Arctic, Europe, and N America. ii) new satellite observations [e.g., MODIS, CALIPSO]. iii) surface observatories such as the IMPROVE network in USA; worldwide AERONET network by NASA and Atmospheric Brown Cloud observatories for the Indo-Asian-Pacific region by UNEP, NOAA and others; iv) Scripps’ Unmanned Aircraft Observing systems funded by NSF and NOAA; v) UCSD’s Time of flight mass spectrometer single particle measurements; vi) observationally constrained emission inventories; vii) aerosol chemical-transport models developed at Stanford, Caltech, NASA, NOAA and NCAR laboratories. We now have direct UAV measurements for the large enhancement of atmospheric solar heating by BC (Ramanathan et al, 2007b).
Global averaged estimates for the radiative heating (or radiative forcing) of the surface-atmosphere system by BC as of now (year 2005) is in the range of 0.3 Wm
-2 to 0.9 Wm-2 for the direct solar absorption by atmospheric BC; 0.05(± 50%) Wm-2 for the BC solar heating of ice and snow; 0.03 (±50%) Wm-2 for the greenhouse effect of BC. The combination of these three warming effects of BC is referred to as direct radiative forcing. The direct radiative forcing of BC (0.4 to 1 Wm-2) is about 20% to 60% of the pre-industrial to year 2005, CO2 greenhouse forcing. We have adopted Forster et al’s (2007) IPCC estimates of 1.66 Wm-2 for the pre-industrial to year 2005, CO2 radiative forcing. The heating due to BC in clouds (items iv and v above) and the cooling effect due to BC nucleation of cloud drops are not yet firmly established.
7. BC forcing in the context of the Greenhouse Blanket: The greenhouse gases (water vapor, carbon dioxide, ozone and others) surround the planet like a blanket. A blanket keeps us warm on a cold winter night by retaining (or trapping) our body heat. Similarly, the GHGs retain much of the infra red radiation (or heat radiation) given off by the surface and the atmosphere (including clouds) within the planet. The energy retained (thickness of the blanket) by the atmosphere has been determined from satellite radiation budget data and other correlative surface temperature data, to be about 163 Wm-2 (5%) for the 1985 to 1989 period. H2O in the form of SOLAREFFECTDirect Atmospheric Heating Ice/Snow HeatingGREENHOUSE EFFECTExternallyMixedInternally MixedObservationallyConstrainedSTANFORD STUDY(Jacobson, 2001) CAL TECH STUDY(Chung & Seinfeld, 2002) NOAA_GFDL STUDY(in Forster et al, 2007)IPCC AR4(Forster et al, 2007)NASA STUDY(Sato, 2003)SCRIPPS STUDY(Ramanathan & Carmichael)0.310.50.50.340.55to0.62 0.810.9 (±50%)0.050.10.03 to0.04
Black Carbon Global Radiative ForcingC: Smoke in the Greenhouse BlanketThe Greenhouse gases (water vapor, carbon dioxide, ozone and others) surround the planet like a blanket. A blanket keeps us warm on a cold winter night by retaining (or trapping) our body heat. Similarly, the GHGs retain much of the infra-red radiation (or heat radiation) given off by the surface and the atmosphere (including clouds) within the planet. The energy retained (thickness of the blanket) by the atmosphere has been determined from satellite radiation budget data and other cor-relative surface temperature data, to be about 163 Wm-2 ( 5%) for the 1985 to 1989 period. H2O in the form of vapor, cloud drops and ice crystals provide about 2/3 of the total effect, CO2 about 20% and the balance is due to other GHGs including ozone, methane, nitrous oxide. !Since pre-industrial times, the increase in GHGs (CO2; meth-ane; CFCs; nitrous oxide; and others) by human activities have thickened the blanket and retained more energy. CO2 alone has increased from 280 ppm to 387 ppm. This increase as well as increase in the other anthropogenic GHGs has retained addi-tional energy of about 3 ( 15%) Wm-2 or thickened the blanket !!!!
BC forcing in the context of the Greenhouse Blanket
Testimony of V. Ramanathan March 16, 2010 7
vapor, cloud drops and ice crystals provide about 2/3 of the total effect, CO2 about 20% and the balance is due to other GHGs including ozone, methane, nitrous oxide.
Since pre-industrial times, the increase in GHGs (CO2; methane; CFCs; nitrous oxide; and others) by human activities have thickened the blanket and retained more energy. CO2 alone has increased from 280 ppm to 387 ppm. This increase as well as increase in the other anthropogenic GHGs has retained additional energy of about 3 (15%) Wm-2 or thickened the blanket by about 1.8% (3 Wm-2 out of 163 Wm-2). The 1.8% may seem small, but it should be noted that, the increase in global mean surface temperature from glacial to the current interglacial period is about 5° K (5°C) and in the absolute Kelvin temperature scale, 5°C is only 1.7 % of the global mean temperature of 289° K (15.5°C). Reverting back to the effect of BC, we can think of BC as smoke in the blanket that traps sunlight (0.3 to 0.9 Wm-2) directly into the blanket and into snow and sea ice (0.05 Wm-2) as well as retaining infra-red radiation (0.03 Wm-2) in the blanket.
8. Role of Non-CO2 Climate Warmers in Mitigation: BC is but one of several non-CO2 climate warmers. Human activities have added several other GHGs. These include, methane, nitrous oxide, halocarbons and tropospheric ozone. Compared with the centuries to millennial time scales of CO2, the life times of several non-CO2 warmers are much shorter. BCs life time is less than few weeks; Ozone molecule’s life time is few months, methane less than 15 years and halocarbons range from a year to a decade. Because of their shorter life times, reductions in the emissions of short lived warmers will lead to quicker reductions in the concentrations and their radiative warming of climate. In fact, such a mitigation action, will also help the science of climate change test its model predictions of cause and effect. Several studies have estimated that, with continuation of current trends in GHGs emissions, there is more than a 50% probability of surpassing the 2°C warming threshold during this century. A IIASA study estimates that with currently available technology and stringent implementation of current air pollution laws, it is possible to achieve about 30% reductions in methane, ozone, HFCs and more than 30% for BC. Such reductions in non-CO2 warmfor crossing the 2°C threshold by few decades or more. climate warmers Contribution to 2005 forcing relative to CO2(1.66 Wm-2) Greenhouse Gases Ozone (troposphere) : 20% Methane : 30% Halocarbons : 20% Particles (Aerosols) Black Carbon : 27% to 55%*(soot/smoke)_________________________________________Total Non-CO2: 97% to 125%_________________________________________All numbers except the red are IPCC values; Long lived N2O not included* Ramanathan & Carmichael; 2008
Non-CO2 Climate Warmers
Deposition of BC on Snow in California: Direct Measurements
Measured Black Carbon Deposition on the Sierra Nevada Snow PackO Hadley, C. Corrigan, T W Kirchstetter, C. S. Davis, V. Ramanathan California CEC Project
Testimony of V. Ramanathan March 16, 2010 8
9. Regional Climate Effects of BC: The regional effects of BC are estimated to be particularly large over Asia, Africa and the Arctic. In these regions its effects include alteration of surface and atmospheric temperatures, disrupting monsoon circulation and rainfall patterns (Menon et al, 2002; Ramanathan et al, 2005; Lau et al, 2008). The interaction of the regional climate effects of greenhouse gases and ABCs deserve more attention. For example, a recent study (Ramanathan et al, 2007b) employing unmanned aerial vehicles suggests that BC enhances atmospheric solar heating by as much as 50%. When these data are combined with CALIPSO and other satellite data over S, SE Asia and the Indian Ocean and employed in a climate model, the simulations suggest that the elevated atmospheric warming over the S and SE Asian region, (including the elevated Himalayan regions) by ABCs is as much as that due to the greenhouse warming. Thus the atmospheric solar heating by BC may be intensifying the effects of greenhouse gases on the Himalayan-Tibetan glacier region. Climate model studies also suggest that fossil fuel and biofuel BC emissions in Asia and Europe induce as much springtime snow cover loss over Eurasia as anthropogenic carbon dioxide, a consequence of the darkening of the snow by deposition of snow and strong snow-albedo feedback. (Flanner et al, 2009). We now have direct measurements of efficient removal of BC by snow over the Sierras in California.
10. Effects of BC and ABCs on Regional Water Budget: Reverting to the general effects of all aerosols (and not just BC), ABCs enhance scattering and absorption of solar radiation and also produce brighter clouds (IPCC, 2007) that are less efficient at releasing precipitation (Rosenfeld et al, 2000). These in turn lead to large reductions in the amount of solar radiation reaching the surface (also known as dimming), a corresponding increase in atmospheric solar heating, changes in atmospheric thermal structure, surface cooling, atmospheric warming, alterations of north-south and land-ocean contrast in surface temperatures, disruption of regional circulation systems such as the monsoons, suppression of rainfall, and less efficient removal of pollutants (Ramanathan et al, 2001b, 2005, 2007a; Menon et al, 2002). Together the aerosol radiation and microphysical effects can lead to a weaker hydrological cycle and drying of the planet.This connects aerosols directly to availability of fresh water, a major environmental issue of the 21st century (Ramanathan et al, 2001b). For example, the Sahelian drought during the last century is attributed by some models to the north-south asymmetry in aerosol forcing (Rotstayn and Lohman, 2002). In addition, new coupled-ocean atmosphere model studies suggest that aerosol radiative forcing may be Simulated effects of BC and CO2on atmospheric temperatures over India. (Ramanathan and Carmichael, 2008)
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