Asian cities are getting dimmer

Posted: 13 November 2008

Cities from Beijing to New Delhi are getting darker, glaciers in ranges like the Himalayas are melting faster and weather systems becoming more extreme, in part, due to the combined effects of human-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.

Today the team, drawn from research centres in Asia including China and India, Europe and the United States, announced their latest and most detailed assessment of the phenomenon.

The brown clouds, the result of burning of fossil fuels and biomass, are in some cases and regions aggravating the impacts of greenhouse gas-induced climate change, says the report.

Black carbon emissions 1850-2000
Black carbon emissions 1850-2000
Black carbon emissions 1850-2000. The estimate includes only fossil fuel and biofuel combustion sources.
This is because ABCs lead to the formation of particles like black carbon and soot that absorb sunlight and heat the air; and gases such as ozone which enhance the greenhouse effect of CO2.

Globally however brown clouds may be countering or 'masking' the warming impacts of climate change by between 20 and up to 80 per cent, the researchers suggest. This is because of particles such as sulfates and some organics which reflect sunlight and cool the surface.

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.

Regional hotspots

Five regional hotspots for ABCs have been indentified: East Asia, covering eastern China; the Indo-Gangetic plains in South Asia from the northwest and northeast regions of eastern Pakistan across India to Bangladesh and Myanmar; Southeast Asia, covering Cambodia, Indonesia, Thailand, and Vietnam; Southern Africa extending southwards from sub-Saharan Africa into Angola, Zambia and Zimbabwe; and the Amazon basin in South America.

anthropogenic aerosol optical depth
anthropogenic aerosol optical depth
The integrated satellite data shows 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. The ABCs over South Asia peaked during the months of November-March. For July-August ABCs and dust reached peak values over Africa and Middle East. During the boreal spring, the ABCs and dust extended from East Asia across the North Pacific and further into Atlantic. The Amazonian Plume peaked during September to October.
There are hotspots too in North America over the eastern seaboard and in Europe - but winter rain tends to remove them and reduce their impact.

Around 13 megacities have so far been identified as ABC hotpots:Bangkok, Beijing, Cairo, Dhaka, Karachi, Kolkata, Lagos, Mumbai, New Delhi, Seoul, Shanghai, Shenzhen and Tehran where soot levels are 10 per cent of the total mass of all human-made particles.

ABCs can reduce sunlight hitting the Earth's surface in two ways.Some of the particles such as sulphates, linked with burning coal and other fossil fuels, reflect and scatter rays back into space.

Others, also linked with fossil fuel and biomass burning, in particular black carbon in soot, absorb sunlight before it reaches the ground. The overall effect is to make 'hot spot' cities darker or dimmer.

Cities and 'Dimming'

'Dimming' of between 10-25 per cent is occurring over cities such as Karachi, Beijing, Shanghai and New Delhi. Guangzhou is among several cities that have recorded a more than 20 per cent reduction in sunlight since the 1970s.

Monsoon cloud
Monsoon cloud
ABC-induced dimming is considered as the major causal factor for the rainfall decrease in India and for the north to south shift of the summer monsoon in Eastern China.
For India as a whole, the dimming trend has been running at about two per cent 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 cent per decade, with the larger trends after the 1970s," says the report.

Overall, global temperature rises -linked with greenhouse gas emissions - may currently be between 20 per cent and 80 per cent less as a result of brown clouds around the world says the report.

If brown clouds were eliminated overnight, this could trigger a rapid lobal temperature rise of as much as to 2 degrees C.

Added to the 0.75 degrees C rise of the 20th century, this could push global temperatures well above 2 degrees C - considered by many scientists to be a crucial and dangerous threshold.

Thus simply tackling the pollution linked with brown cloud formation without simultaneously delivering big cuts in greenhouse gases could have a potentially disastrous effect.

Variable climate

Achim Steiner, Director of the UN Environment Programme (UNEP) said: "I expect the Atmospheric Brown Cloud to be now firmly on the international community's radar as a result of today's report".

The phenomenon has been most intensively studied over Asia. This is in part because of the region's already highly variable climate, including the formation of the annual monsoon, and the fact that the region is home to around half the world's population and is undergoing massive growth.

But the scientists today made clear that there are also brown clouds elsewhere, including over parts of North America, Europe, southern Africa and the Amazon Basin which also require urgent and detailed research.

"Combating rising CO2 levels and climate change is the challenge of this generation, but it is also the best bet the world has for Green Growth,including new jobs and new enterprises from a booming solar and wind industry to more fuel efficient vehicles, homes and workplaces. Developed countries must not only act first but also assist developing economies with the finance and clean technology needed to green energy generation and economic growth," said Mr Steiner.

"In doing so, they can not only lift the threat of climate change but also turn off the soot-stream that is feeding the formation of atmospheric brown clouds in many of the world's regions. This is because the source of greenhouse gases and soot are often one and the same - unsustainable burning of fossil fuels, inefficient combustion of biomass and deforestation."

Glacier retreat

Professor Veerabhadran Ramanathan from the Scripps Institute in La Jolla, California, and head of the UNEP scientific panel which is carrying out the research said: "Our preliminary assessment, published in 2002, triggered a great deal of awareness but also scepticism...

"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 Hindu Kush-Himalayan-Tibetan glaciers, which provide the headwaters for most Asian rivers, and thus have serious implications for the water and food security of Asia," he said.

"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," added Professor Ramanathan.

The report, Atmospheric Brown Clouds: Regional Assessment Report with Focus on Asia can be found at www.unep.org