2010 March 16: U.S. House of Reps.: Dr. Drew T. Shintell testimony on Black Carbon impacts


2010 March 16: U.S. House of Reps.: Dr. Drew T. Shintell testimony on Black Carbon impacts





March 16, 2010

Statement of

Dr. Drew T. Shindell

Senior Scientist

NASA Goddard Institute for Space Studies

before the

Select Committee on Energy Independence and Global Warming

United States House of Representatives

I thank the committee for the opportunity to testify on the impacts of black carbon. I have been a

researcher at NASA’s Goddard Institute for Space Studies since 1995, and have taught the

graduate level course on atmospheric chemistry and pollution at Columbia University since 1997.

Black carbon is one of many products of incomplete burning (combustion). It is not produced in

large amounts from very high temperature combustion such as that which takes place in power

plants, but in numerous types of much less efficient burning such as in diesel engines, agricultural

and forest fires, and residential cooking stoves. Most of these emission sources are a direct result

of human activities, while emissions from fires can be thought of as natural activities that are

influenced by humans. The largest sources of black carbon emissions from human activities in

the US (and Europe) are diesel engines while residential stoves (use for cooking and heating, and

fueled by agricultural waste, wood, coal, dung, etc) and industrial processes are typically most

important in developing countries and for the global total 1, 2.

Black carbon influences climate in multiple ways. It absorbs sunlight, leading to large-scale

surface warming (though locally there may be cooling as less sunlight reaches the surface). It can

also influence clouds in numerous, complex ways that are not fully understood at present. Hence

the overall impact of those effects is not known. When black carbon falls on snow and ice

surfaces it darkens them, reducing their ability to reflect sunlight away from the Earth’s surface,

and thus causing warming 3, 4. Furthermore, the absorption of sunlight by black carbon particles

on or in snow and ice leads to melting, creating a positive feedback that enhances the original

warming effect substantially. A broad assessment of current scientific knowledge leads to a best

estimate that black carbon causes substantial global mean warming, but with a very large

uncertainty 5-12. Near snow or ice covered regions, emissions of black carbon are almost certain

to have an overall warming impact

3, 6.

Direct observations of the climate seldom reveal cause and effect, so that the influence of black

carbon on surface temperature must be estimated by models. The models are continually tested

against observations, however. NASA provides many useful measurements of atmospheric

particulate (aerosols), including satellite observations, surface-based radiation detection networks,

and airborne field campaigns in collaboration with other agencies such as the Department of

Energy, the National Oceanic and Atmospheric Administration (NOAA), and the National

Science Foundation, as well as carrying out research and analysis, and modeling and data

assimilation. Earth observing satellites with a direct role in observing aerosols include NASA’s

Terra, Aura, Aqua and Calipso satellites (which include instruments from other US agencies and

foreign partners) and NOAA’s polar orbiting and geostationary environmental satellites. New

missions in development are expected to make a direct contribution to the investigation of

atmospheric aerosols, including the Glory satellite that should provide much more detail on

aerosol properties than previously available, and longer range planning includes important

follow-on capabilities. NASA ground based networks include the Aerosol Robotic Network

(Aeronet) and the Micro-Pulse Lidar Network with sites located around the world. Developed

primarily for satellite calibration and validation, these networks have proven to be useful and

productive data sources for aerosol research as well. Annual NASA investment in aerosol

missions and science has historically been approximately $130 million per year. As is described

below, black carbon’s role in climate cannot be understood in isolation, making it fitting that this

research is embedded in a broader, multi-agency effort to understand the Earth’s climate.

Multiple techniques have been used to investigate the effect of black carbon on surface

temperature. In one type of study, emissions are put into a model of atmospheric chemistry and

climate, and the results analyzed to isolate the effect of black carbon 6, 8, 9, 11. The model is

evaluated against observations both of particulate in the atmosphere and of climate. How well the

model is able to reproduce measured particulate amounts and locations and observed

temperatures gives us a sense of its accuracy and the credibility of its future projections. In

addition to this type of study, changes in atmospheric energy fluxes that are due to particulate

have been measured by aircraft and satellite and then put into climate models, and the response

evaluated 10, 13. Still another line of enquiry has used statistical comparisons between timevariations

in climate model results and in observations to isolate the influence of black carbon in

the surface temperature measurements

14. A fourth, related technique has used regional

temperature changes derived from the NASA Goddard Institute climate model and the observed

regional temperature trends to calculate the influence of particulate on climate during the 20th

century in comparison with other agents driving climate change

7. Encouragingly, all these

studies find results that are generally fairly similar, with an overall global mean warming due to

black carbon that is about 15-55 percent of the warming due to carbon dioxide. These studies

clearly still present a substantial range of values, and are further limited by our incomplete

knowledge of interactions between black carbon and clouds but nevertheless all suggest a

substantial warming impact from black carbon. It is important to keep in mind that while carbon

dioxide increases have contributed more than any other single factor to warming, emissions of

long-lived greenhouse gases other than carbon dioxide have in total contributed nearly as much to

warming as has carbon dioxide itself

15. At the same time, reflective aerosol particles such as

organics, sulfates, and nitrates have offset a substantial portion of the warming from greenhouse

gases 15. This means that the percentage contribution of any individual factor to the total forcing

of climate change depends upon how the comparison is made (against net forcing, or total

positive forcing, for example). In terms of percent contribution to net warming since the mid-18


Century, the above estimate implies 15-55 percent of that warming may have been driven by

increased black carbon (the contribution from carbon dioxide alone and the net warming from all

climate change drivers has been approximately the same, so that comparison with either of these

leads to comparable values). If the comparison is against the impact of all the greenhouse gases

contributing to warming, then black carbon has added 10-35 percent to the greenhouse-gas

induced warming, some of which has been offset by reflective aerosols and aerosol-induced cloud


Black carbon has likely had even larger regional effects, especially due to its strong impact on

snow and ice. In the Arctic, so called ‘Arctic Haze’ has been observed by pilots for decades, and

results largely from transport of pollution from lower latitude industrialized areas. Though it is

difficult to separate the effects of black carbon on Arctic temperatures from the effects of other

factors, several results suggest that black carbon has contributed a larger share of warming in that

region than it has globally 3, 6-8, 11. In other words, more than 15-55 percent of Arctic warming

since the mid 18th century might be attributable to black carbon. It may have had an especially

large effect in the early 20th Century, when coal burning was commonly used in the Northeast US

for residential heating, and along with reductions in sulfur emissions, may have also helped drive

the very rapid Arctic warming of the past several decades

7. In the Himalayan region, located

very close to the world’s largest emissions of black carbon, detailed observations of glaciers

covering large areas and long periods of time are unfortunately quite sparse. While it seems that

glaciers in this region are retreating overall

16, the role of black carbon in that retreat remains

difficult to quantify, though it is likely to have played some role, especially in glaciers on the

southern flank of the Tibetan plateau


Since black carbon absorbs sunlight in the atmosphere much as it does in snow and ice, it can also

affect other aspects of climate in addition to surface temperature. When sunlight is absorbed by

the dark particles, the air a few kilometers above the surface containing the particles warms. This

alters the temperature differences that create winds, which affects both regional temperatures and

precipitation. Several studies have indicated that the large amounts of smoke and haze (so-called

atmospheric brown clouds) observed near Asia can cause shifts in the timing and intensity of the

monsoon, with large impacts for rainfall in India and China 18-20. As with most aspects of climate

change, it is difficult to verify this link exclusively with observations as many other factors also

influence the monsoon, and other types of particles such as windblown desert dust contribute to

the brown clouds. However, the physical mechanism linking black carbon to changes in

precipitation is clear and operates worldwide. Unlike temperature changes, shifts in precipitation

nearly always have negative net economic impacts as long-term infrastructure has quite sensibly

been designed for norms over past decades.

Emissions of black carbon may affect the quality of the air we breathe as well as our climate.

Policies are typically designed with the goal of limiting damage to one or the other, but largely

treat the air quality and climate effects separately. For example, US regulations on the emissions

of air pollutants including particulate matter (of which black carbon is a component) primarily

consider their adverse effects on air quality public health, and the environment

21. In most of the

world, air quality regulations are created at local, state, or national levels, and do not consider

climate impacts, while international climate change mitigation efforts (e.g. the Kyoto Protocol,

the Copenhagen Accord) generally address greenhouse gases such as carbon dioxide but do not

include shorter-lived pollutants such as lower atmospheric ozone or black carbon and does not

consider the effects of the greenhouse gases on air quality. This separation is driven by policy

rather than science, however, as the emissions of many pollutants affect both aspects of our

environment. Encouragingly, research has shown that the optimal strategies to reduce black

carbon and some ozone precursors are similar whether the goal is improving air quality or

limiting global warming 22. This is not the case for all pollutants that influence air quality, such

as sulfur dioxide. However, for black carbon, carbon monoxide, volatile organic compounds and

methane in particular, these two goals align. Even in the absence of a broad strategy

encompassing both goals this argues for a stronger emphasis on reductions in emissions of these

pollutants in air quality policies, for which there would be a climate co-benefit, and in climate

policies, for which there would be an air quality co-benefit.

Actual policies will usually impact many species simultaneously, since, as discussed previously,

black carbon is a product of incomplete combustion and this also produces substantial amounts of

other particulates and gases. The amount of sunlight absorbed by black carbon can be

substantially altered by interactions with these other compounds, and they themselves also affect

climate. This means that it’s necessary to examine the net effect of all emissions from a

particular activity on climate rather than the effect of black carbon alone. Research suggests that

strategies to simultaneously improve air quality and mitigate global warming differ from region

to region. Preliminary results from ongoing work at my institute suggest that in the United States,

reductions in overall emissions from diesel vehicles appears to be a method to achieve both goals,

with a substantial part of the climate benefits coming from reduced black carbon. This could

result from a shift from trucks to rail for cargo transport, for example. Imposition of diesel

particulate filters on diesel vehicles, another method to reduce emissions, would in practice have

different effects on emissions of different compounds. For example, these filters reduce

particulate matter by about 90% but could result in a slight increase in carbon dioxide emissions

due to decreased engine efficiency. Proper vehicle operation and maintenance practices optimize

the air quality benefits of filters and other emission reduction technologies. The overall

conclusion that such emissions reductions represent a win-win for air quality and climate does not

change, however. More generally, increases in fuel efficiency coupled with reductions in

emissions (carbon monoxide and volatile organic compounds) from both gasoline and diesel

fueled vehicles show the most positive results for both climate and air quality 23, 24.

In contrast, many countries in the developing world use fuel with high sulfur content (as the US

did years ago). Reductions in across-the-board emissions from those areas would improve air

quality, but could actually increase near-term warming because these reductions would reduce

reflective particles in the atmosphere that produce a cooling effect and increase atmospheric

methane 24. However, most of the developing nations are expected to follow the pattern of the

developed nations and switch to low-sulfur fuels (to both directly reduce emissions that lead to

particulate formation and to enable advanced emission controls) as their populations become

more affluent and demand better air quality. While the transition to low-sulfur fuels may lead to

near-term warming, simultaneous use of particulate filters would reduce that warming and may

even lead to an overall cooling. In other words, policies that consider both air quality and

climate, and hence strongly reduce emissions of black carbon, carbon monoxide and volatile

organic compounds (as diesel filters do) as well as sulfur, are considerably more climate-friendly.

In developing Asia, where particulate emissions are larger than in any other part of the world,

reductions in emissions from both industrial processes and residential cooking stoves offer ways

to simultaneously improve air quality and mitigate warming 24, 25. Additional work is ongoing to

characterize the effects of emissions from other activities, including aviation and shipping which

may increase substantially and/or change location in the future. While there is more to learn,

several things are already clear. Reductions in emissions of products of incomplete combustion

will virtually always improve health. By targeting emissions rich in black carbon, carbon

monoxide and volatile organic compounds relative to sulfur dioxide and nitrogen oxides, many

options are available that will simultaneously benefit climate change.

It is worth noting that these options are by no means the default choices, and to date air quality

regulations made for the sake of public health in the US, Europe and Japan have often been much

more successful in reducing pollutants that cool climate (sulfur dioxide and nitrogen oxides) than

those which lead to warming (for example, methane and black carbon). Changes in emissions of

short-lived pollutants resulting from air quality policies along with the continued growth in

Northern Hemispheric emissions of some warming pollutants such as black carbon have been

linked to the accelerated warming of the Northern Hemisphere since the 1970s and the very rapid

heating of the Arctic during recent decades (they may account for more than half the 1970-2007

warming trends, which have been nearly two degrees F (1 C) for the Northern Hemisphere and 3

F (1.5 C) for the Arctic)

7. This highlights the substantial impact of these pollutants on climate

change, especially at regional scales. It also emphasizes the importance of coordinated air quality

and climate policies to achieve progress in both areas simultaneously rather than continuing our

record of improvement in one at the expense of the other.

The health benefits that could be gained from particulate and ozone precursor emissions

reductions are clear from epidemiological studies. These studies span both long periods of time

and wide areas and also short, local changes due to events such as temporary industrial strikes 26-

30. Both particulate matter and tropospheric (lower atmospheric) ozone, a gas produced from

carbon monoxide, volatile organic compounds and methane (in the presence of nitrogen oxides),

contribute to a variety of adverse health effects. Reductions of emissions directly into living

spaces are likely to yield substantial health benefits. One recent study estimated that roughly 2

million deaths could be prevented in India by bringing advanced biomass stoves to 15 million

homes per year over the next 10 years 31. While the health benefits of emissions reductions are

most strongly felt in the nearby population, long-range transport of air pollution can also be

substantial: one recent study found that ozone levels a few kilometers above the Western US can

be significantly influenced by emissions from East Asia 32. Another recent study estimates that

the difference between Chinese emissions of particulate following a ‘high-end’ or ‘low-end’

projected trend would be several hundred premature deaths annually in the US in 2030


Though small compared with the hundreds of thousands of additional premature deaths within

China itself, this nonetheless shows that the health impact of air pollution is not simply a local

issue. Climate impacts extend even more broadly, with most of the Northern Hemisphere north

of the tropics responding strongly to emissions from anywhere within that region, for example


In a study of the projected climate during the 21

st Century, substantial warming and drying of the

continental interior of the US was seen, and much of this was driven by changes in air quality

pollutant emissions from East Asia


There are other co-benefits from control of air pollution in addition to improved public health.

Particulate matter and tropospheric ozone precursors both impair visibility, with potential

detrimental economic impacts on tourism and recreation. Elevated levels of tropospheric ozone

also causes damage to plants, leading to economic losses from reduced agricultural and forestry


35. Air pollution can also degrade many types of materials used in buildings, such as

stonework and metalwork. In economic analyses developed by the EPA and others, the valuation

of human health impacts tend to dominate, however

35. Economic analyses including the benefits

of reduced pollution of course show vastly different net economic impacts of controlling

emissions from incomplete combustion than estimates based simply on the cost of implementing

the controls. This is true even without including any monetary value for reduced damages due to

climate change. A compelling example of the use of co-benefits to motivate a strategy to mitigate

emissions that lead to warming is the international ‘Methane to Markets’ program led by the

United States. This program has provided funding and expertise to advance projects that capture

methane from farms, landfills, pipelines and coal mines. The projects then use the captured

methane to produce energy at a net profit while also mitigating warming. When the economic

benefits from avoided health impacts are included, many projects to control black carbon and

carbon monoxide may have higher benefits than costs even without including the value of

reduced warming. For example, recently proposed emissions regulations for diesel vehicles in

California were estimated to lead to a reduction in human health damages of approximately five

times the cost of implementing the particulate reductions

36. Numerous federal diesel rules have

shown similar and even greater ratios of health benefits to costs. Policies that consider both

human health and climate change mitigation simultaneously are likely to provide substantial

health benefits in associated health care cost savings

37. In the US alone, air pollution has been

calculated to lead to 70-270 billion dollars in damages per year 35, so that there is a great deal of

potential for co-benefits that should be considered when evaluating the costs of emissions


Reducing emissions of the short-lived warming agents is unlikely to eliminate global warming

even in the near-term, and reductions in carbon dioxide emissions are clearly required to mitigate

long-term warming. However, the combined influence of all the short-lived warming agents,

black carbon, carbon monoxide, volatile organic compounds, methane and hydrofluorocarbons, is

quite large, so that reductions in all these together could achieve a substantial reduction in nearterm

warming. With the exception of the hydrofluorocarbons, all these reductions would lead to

significant improvements in air quality as well, making them attractive options from many

perspectives. And for all these short-lived forcing agents, technology to reduce emissions is

already readily available for deployment, with the primary barriers being structural rather than

technological (unlike, for example, carbon dioxide produced from burning fossil fuels).

Further research is needed to provide a clearer understanding of how much black carbon is

emitted by different types of burning, how it interacts with other types of particulate and with

clouds, and how to improve the ability of models to simulate black carbon in the atmosphere and

cryosphere (snow and ice). Such research would lead to more reliable estimates of black carbon’s

role in climate change. However, taking into account the current range of estimates for black

carbon’s global impact, along with its known ability to substantially influence snow and ice

covered regions and to shift precipitation, emissions reductions are likely to be a useful

component of strategies to mitigate climate change. Realistic emissions reductions would affect

several types of particles and gases, and thus require careful analysis of their net impact. This

type of research, that integrates knowledge of many different aspects of the climate system, is

needed to compliment federal programs that are typically focused on single components of

climate research. Ideally, future research should provide policy makers a menu of mitigation

options covering technological, structural and behavioral, and regulatory approaches for

individual emission sources in different regions of the world. As stated earlier, reductions in

emissions from incomplete burning are virtually always good for health. Reductions of emissions

rich in black carbon, carbon monoxide, volatile organic compounds and methane are typically

good for climate as well, allowing many ‘no regrets’ options to be identified already. Further

work can allow much better optimization of emission reduction strategies to simultaneously

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