2010 March 16: U.S. House of Reps.: Dr. Drew T. Shintell testimony on Black Carbon impacts
HOLD FOR RELEASE
March 16, 2010
Dr. Drew T. Shindell
NASA Goddard Institute for Space Studies
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
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
provide clean air and limit climate change.
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