GM Opel to Introduce Turbocharged CNG Van
Nanoscale Chemical Imaging of Catalysts at Work

Widespread and Complex Climatic Changes Outlined in New UNEP Project Atmospheric Brown Cloud Report

Anthropogenic aerosol optical depth (AOD) in the period 2001-2003 for four seasons. AOD is an index for the fraction of sunlight intercepted by particles and total aerosol concentration in the vertical column. Click to enlarge. Source: UNEP

Cities from Beijing to New Delhi are getting darker, glaciers in ranges like the Himalayas are melting faster and weather systems becoming more extreme due in part to the combined effects of man-made Atmospheric Brown Clouds (ABCs) and greenhouse gases in the atmosphere.

These are among the conclusions of scientists studying a more than three km-thick layer of soot and other manmade particles that stretches from the Arabian Peninsula to China and the western Pacific Ocean. The team, drawn from research centers in Asia including China and India, Europe and the United States, has just published their latest and most detailed assessment of the phenomenon. Their preliminary assessment was published in 2002.

ABCs start as indoor and outdoor air pollution consisting of particles and pollutant gases, such as nitrogen oxides (NOx), carbon monoxide (CO), sulfur dioxide (SO2), ammonia (NH3), and hundreds of organic gases and acids. Widespread ABC plumes resulting from the combustion of biofuels from indoors; biomass burning outdoors; and fossil fuels are found in all densely inhabited regions and oceanic regions downwind of populated continents.

There are five identified regional ABC hotspots around the world:

  • East Asia
  • Indo-Gangetic Plain in South Asia
  • Southeast Asia
  • Southern Africa
  • Amazon Basin

ABC hotspots are defined as regions where the annual mean anthropogenic aerosol optical depth (AOD) exceeds 0.3 and the percentage of contribution by absorbing aerosols exceeds 10% (absorbing AOD > 0.03).

Although substantial loadings of ABCs over Eastern USA and Europe have also been observed, in these extra-tropical regions the atmospheric concentrations of ABCs are large mainly during the summer season since precipitation removes the aerosols efficiently during other seasons. Furthermore, the soot concentrations are lower and hence these extra tropical regions are not included in the hotspots category.

In Asia, new aircraft and satellite data have revealed that ABC plumes, measuring 1-3km thick, surround the Hindu Kush-Himalayan-Tibetan glaciers, both from the South Asian and the East Asian sides. Between 1950 and 2002, soot emissions increased three-fold in India and five-fold in China, while sulfur emissions have increased ten-fold in China and seven-fold in India.

The brown clouds are in some cases and regions aggravating the impacts of greenhouse gas-induced climate change, says the report. Particles like black carbon and soot in ABCs absorb sunlight and heat the air and gases such as ozone enhance the greenhouse effect of CO2.

Globally, however, brown clouds may be masking the warming impacts of climate change by between 20 and up to 80%, the researchers suggest, due to other particles in the ABCs such as sulfates and some organics which reflect sunlight and cool the surface.

Another effect of the aerosols in the brown clouds is to make hot spots dimmer. Dimming of between 10-25% is occurring over cities such as Karachi, Beijing, Shanghai and New Delhi. Guangzhou is among several cities that have recorded a more than 20% reduction in sunlight since the 1970s. For India as a whole, the dimming trend has been running at about 2% per decade between 1960 and 2000—more than doubling between 1980 and 2004. In China, the observed dimming trend from the 1950s to the 1990s was about 3-4% per decade, with the larger trends after the 1970s.

Regions with large concentrations of ABCs may be getting cloudier which can also contribute to dimming but data are not sufficient to quantify this effect. Particles and aerosols in the ABCs may act to inhibit the formation of rain drops and rainfall.

The cloud is also having impacts on air quality and agriculture in Asia increasing risks to human health and food production for three billion people.

ABCs and Global Warming. The increase in global temperature linked with greenhouse gas emissions may currently be between 20-80% less as a result of brown clouds around the world, says the report.

As a result, if brown clouds were eliminated overnight, this could trigger a rapid global temperature rise of as much as 2° C. Added to the 0.75° C rise of the 20th century, this could push global temperatures well above the +2° C threshold considered by many scientists to the point for dangerous climate change. Thus, simply tackling the pollution linked with brown cloud formation without simultaneously delivering big cuts in greenhouse gases could have a potentially disastrous effect.

Complex Regional Impacts on Temperature. The science of ABCs, woven with the science of greenhouse gases, is not simple and may be behind some highly complex warming and cooling patterns witnessed on continents and in different regions of specific countries.

  • The masking of greenhouse warming by ABCs may in part be the explanation for the lack of a strong warming trend over India since the 1950s during the dry season which runs from January to May.

  • ABCs may explain in part why the warming trend in India’s nighttime temperatures is much larger than the trend in daytime temperatures.

  • Annual mean temperatures in mainland China have risen by more than one degree C in the past half century. However the trends have not been uniform with the Tibetan Plateau and the North, Northeast and Northwest of China experiencing the highest temperature rises. Conversely Southwest and central Eastern China has experienced a strong cooling trend of between 0.1 to 0.3 degrees C per decade.

The authors note that the combined effects of greenhouse gases, ABCs and rapid urbanization are required to explain the complex pattern of warming and cooling trends in China.

Impacts on Weather Patterns Including the East Asian Monsoon. The large heating and cooling effects of ABCs respectively in the atmosphere and at the surface, combined with the impacts of greenhouse gases, may be also triggering sharp shifts in weather patterns.

This is being aggravated by dimming over the Northern Indian Ocean versus the relatively clean Southern Indian Ocean setting up new gradients in surface sea temperatures and surface sea evaporation rates.

ABCs, along with the global warming, may thus be acting to trigger significant drying in northern China and increased risk of flooding in southern China while in part also triggering other environmental and economic effects.

  • Overall decrease in monsoon precipitation over India and Southeast Asia by between 5-7% since the 1950s.

  • Since the 1950s the Indian summer monsoon is not only weakening but shrinking with a decrease in early and late season rainfall and a decline in the number of rainy days.

  • In both China and India extreme rain events of more than 100 mm a day have increased.

  • In both India and China very heavy rainfall of more than 150 mm a day have nearly doubled.

Impact on Glaciers. The Hindu Kush-Himalaya-Tibetan glaciers provide the head-waters for the major river systems including the Ganges, Brahmaputra, Mekong and Yangtze rivers. The Ganges basin is home to more than 400 million people and holds 40% of India’s irrigated croplands.

The Chinese Academy of Sciences estimates that the glaciers have shrunk 5% since the 1950s and the volume of China’s nearly 47,000 glaciers has fallen by 3,000 square km over the past quarter century.

Glaciers in India such as the Siachen, Gangotri and Chhota Shigiri glaciers are retreating at rates of between 10 and 25 meters a year. The retreat has accelerated in the past three and-a-half decades. The Gangotri glacier alone provides up to 70% of the water in the Ganges.

  • The report suggests that ABC solar heating of the atmosphere, due to the absorption of soot and black carbon pollution, is as important as greenhouse gas warming in accounting for the anomalously large warming trend observed in the elevated regions such as the Himalayan-Tibetan region.

  • Decreased reflection of solar radiation by snow and ice due to increasing deposits of black carbon is emerging as another major contributor to the melting of ice and snow.

  • Elevated regions of the Himalayas within 100 km of Mount Everest experience large black carbon concentrations ranging from a few hundred to a few thousand nano grams per cubic metre.

Impacts on Agriculture. A great deal more research is required, the authors say, to determine crops at risk and the precise role various ABC-linked effects—separately or in combination with those of greenhouse gases—may or may not be having. However, possible effects may include:

  • Damage to crops as a result of increased ground level ozone. In Europe a threshold concentration at which damage can occur is deemed to be 40 parts per billion The report says that in parts of Asia ground level ozone can reach 50 parts per billion during February to June and peaking again between September and November at 40 parts per billion.

  • The studies suggest that growing season mean ozone concentrations in the range 30-45 parts per billion could see crop yield losses in the region of 10-40% for sensitive cultivars of important Asian crops such as wheat rice and legumes.

  • A recent study translated such impacts on yield into annual economic losses estimating that for four key crops—wheat, rice, corn and soybean—these may amount to around $5 billion a year across China, the Republic of Korea and Japan.

  • Other effects may include damage linked with the various acidic and toxic particles from brown clouds depositing on plants from the atmosphere.

  • Reduced levels of photosynthesis and thus crop production due to dimming.

Health Impacts of ABCs. Brown clouds contain a variety of toxic aerosols, carcinogens and particles including particulate matter (PM) of less than 2.5 microns in width. These have been linked with a variety of health effects from respiratory disease and cardio-vascular problems.

  • Increases in concentrations of PM 2.5 of 20 micrograms per cubic meter in the air could lead to about 340,000 excess deaths per year in China and India.

  • The World Health Organization estimates that more than 780,000 deaths in the two countries can be linked to solid fuel use in the home.

  • Economic losses due to outdoor exposure to ABC-related PM 2.5 has been crudely estimated at 3.6% of GDP in China and 2.2% of GDP in India.

Project Atmospheric Brown Cloud was established by the United Nations Environment Programme (UNEP) in 2002 following the documentation of brown clouds and haze by the Indian Ocean Experiment (INDOEX). The science secretariat of ABC is located at the Center for Clouds, Chemistry and Climate, Scripps Institution of Oceanography, UCSD.

We believe today’s report brings ever more clarity to the ABC phenomena and in doing so must trigger an international response—one that tackles the twin threats of greenhouse gases and brown clouds and the unsustainable development that underpins both.

One of the most serious problems highlighted in the report is the documented retreat of the Hind Kush-Himalayan-Tibetan glaciers, which provide the head-waters for most Asian rivers, and thus have serious implications for the water and food security of Asia. The new research, by identifying some of the causal factors, offers hope for taking actions to slow down this disturbing phenomenon; it should be cautioned that significant uncertainty remains in our understanding of the complexity of the regional effects of ABCs and more surprises may await us.

—Professor Veerabhadran Ramanathan, head of the UNEP scientific panel carrying out the ABC research

The current project is funded by UNEP with support from the governments of Italy, Sweden and the United States.




An excellent impetus to have the Developing World contribute more financially, technically, and politically to climate change strategies.

It will be interesting to see whether these 2nd/3rd-world phenomena tend to spread, move, or affect other areas of the world that are not currently as directly affected, such as Europe and North America.


This situation is similar to the cooling seen in the 40's and 50's. During this period the CO2 warming (a smaller forcing at the time) was temporarily overwhelmed by an increase in human particulates and aerosol pollution. Pollution regulations and improved technology saw a decrease in this different kind of emissions over the 60's and 70's and as the air cleared, the CO2 signal again emerged and took over.

And if you look at the temperature record for the 1990's you'll notice a sharp drop in '92, '93 and '94. This is the effect of massive amounts of SO2 ejected into the stratosphere by Mount Pinatubo's eruption.


Is the ABC problem presently restricted to South East Asia areas?

How Mexico City, Los Angelos, Rio, Buenos Aires and other large cities in SA and NA compare?

Will it be possible to reduce or eliminate the major sources?

Mr. Environment

ALL HAIL THE GOD OF GLOBAL WARMING!!! Pay homage to my god or be put in jail. Pay homage to my god or be mocked as a "denier." Simply because Earth's temperatures have been FALLING over the past decade is no indication that the Earth is not warming. Global Cooling IS Global Warming. ANY temperature change (Climate Change) is the fault of man. Simply because these changes have been recorded since the beginning of recorded time doesn't matter.

Now, the solution: MASSIVE government power grabs. HUGE increases in TAXES. Elimination of private industry. Daily prayer to the god of Global Warming.

Ed Harley

I suspect that natural climate variations played a large part in the cooling that occurred from the mid 40's to the mid 1970's. Is it a coincidence that the Pacific Decadal Oscillation (PDO) was in a cool phase during that time period... or that it was in a warm phase during the "global warming" years of 1977-2007? By the way, the PDO is in a cool phase again, possibly for the next 15-30 years. If I had to rank climate factors in order of importance, atmospheric CO2 would come in a distant third behind natural cycles and particulate pollution.


Harvey, Los Angeles has actually cleared up quite a bit. When I was in Palm Springs several years ago, the topic of smog came up at a meeting and an LAPD officer laughed at our thoughts that it was bad. He said that dispatchers had gotten a ton of calls from people who were calling to ask what the hell those things to the east of them were. "Mountains, ma'am, mountains", was my favorite response.
Apocryphal or not, the air has definitely cleared a lot in LA (and in the Coachella valley) in the past 20-25 years.


So what to do ?
Fit scrubbers to power stations,
Switch to CNG for buses instead of diesel,

All the stuff we do in the west - it is just a problem of money and priorities. Stopping burning fossil fuel is a much harder problem, but is not this problem - this one is solvable.



Please read the recent study (above) on southern California crrent situtation. It doesn't look all that good.




Harvey, I read the post earlier, and reread it again, but I missed the part about Southern California not improving. I have looked at the pollution numbers for ozone and small particulate matter, i.e. PM10- PM2.5, and they are all down to half their levels 20 years ago. California is doing well, I thought! I have looked up numbers on California and their air is supposed to be much better. Almost all of the US is doing better these past few years, and we should be ready to say that this is what we expect in the future.

Andrey Levin

Some time ago I looked at couple of dozen of sc. articles on the subject. What a mess! Influence of aerosols and particles on local and global climate is not even not quantified, it is not even agreed it is generally cooling or warming. Only couple of effects are quite clear:

1.Emission of sulfur oxides cools the atmosphere. China could expect warming signal when SOx scrubbers will be installed on coal power plants.

2.Particles drastically increase snow cover and ice melting.

BTW, it is not glaciers who feed the rivers. It is yearly precipitation, in form of rain and snow. The role of glaciers is to supply some water in dry year (and shrink), and accumulate excess of snow in wet and cold year. Flow stabilization function of glaciers is much smaller than, say, groundwater, and could be enhanced by construction of hydro dams and reservoirs.

Henry Gibson

It is quite clear that there are too many creatures similar to humans on the planet. ..HG..


Andrey "BTW, it is not glaciers who feed the rivers. It is yearly precipitation"

The Colorado River is almost entirely glacier fed. Take a trip to Rocky Mountain Natl Park.


It is quite clear that there are too many humans on the planet.


Hey, wait a minute! Too many creatures? Too many humans? Remember Rodney King? "Can't we all just get along?"

The solution to these particulate reports is well within reach these days. Obviously pressure on all industrial nations to adopt non-fossil standards for power generation. If coal is to be used - weigh the losses to agriculture and health against building CCT or switching to NG in the near-term.

THEN, move on with the standard green portfolio (wind,solar,geotherm,wave), some new nukes (actinide burning) AND the offloading of residential grid use to Residential Power Units. This would cut the demand for electric energy by 35% in North America. The technology is here - someone's got to lead - so why not NA, and the Obama revolution?

BTW Mr. Environment, why does one want to pray to a god causing the problem?

Andrey Levin

“The Colorado River is almost entirely glacier fed”

No. Colorado River is fed by melting snow and rain precipitated yearly over its 243 000 square miles drainage basin. Glaciers provide only flow stabilization function during dry period, when most of winter snow already melted. Same stabilization functions are more successfully performed by Shadow Mountain and Lake Granby reservoirs.

Glacier’s fed rivers seized to exist about 8 thousand years ago, when glacier shields from last Ice Age stopped their recess and stabilized.



""An excellent impetus to have the Developing World contribute more financially, technically, and politically to climate change strategies. ""

Except, that they do. They still produce much less CO2 than us North Americans, they are taking more initiative to reduce fossil fuel usage than we are, and the vast majority of excess CO2 in the atmosphere has been caused and is continuing to be caused by OUR extravagant lifestyles. This problem is our fault, yet the developing world will be sharing the brunt of the impacts, and is doing as much if not more than we are to address the problem..

But beyond this, you are missing the point. The reason our skies are clearing is in large part because we aren't doing anything over here anymore. Our economies are becoming more and more service based, as eviddenced by the increasing trade deficit. Instead, all the factories making the products we like to buy have moved over to Asia. So in a sense, all this pollution in China is in large part our responsibility, because through the laws of economics wee all demand cheaper products, which means less environmental considerations. We've shifted our bad air over to China; yet we blame China for this, and at the same time demand cheap toys.

Andrey Levin

“The reason our skies are clearing is in large part because we aren't doing anything over here anymore.”

It is quite popular URBAND legend. However, it is not true. Just an example:

“… US September crude steel production fell by 1.3%, bringing the year to date total to 76 million tonnes, an increase of 4%. Canadian production was up 3.2% in September, while the year to date total increased by 7.9% to 12.6 million tones… US exports of steel rose by 28% in the first eight months of 2008 to 9 million tonnes. Canada and Mexico still accounted for about 75% of the total, followed by Brazil, China and Italy.”

Kit P

In the US we are demonstrating that coal can be used without causing significant environmental impact. Steel produced in the US has 75% of the carbon content of that in China.

The comments to this entry are closed.