in ranges like the Himalayas are melting faster and weather systems becoming more extreme, in part, due to the combined effects of man-made ‘Atmospheric Brown Clouds’ (ABCs) and greenhouse gases in the atmosphere, said the UN Environment Programme (UNEP).

In a report, the agency said 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 centres in Asia including China and India, Europe and the US, recently released 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, said the report.

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 carbon dioxide.

The report said that globally the brown clouds may be countering or ‘masking’ the warming impacts of climate change by between 20% and up to 80%. This is because of particles such as sulphates 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.

Achim Steiner, UN Under-Secretary General and Executive Director, UN Environment Programme (UNEP) said: “One of UNEP’s central mandates is science-based early warning of serious and significant environmental challenges. I expect the Atmospheric Brown Cloud to be now firmly on the international community’s radar as a result of this 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, the fact that the region is undergoing massive growth and is home to around half the world’s population.

But the scientists have 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.”

Veerabhadran Ramanathan, 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 skepticism. That has often been the initial reaction to new, novel and far reaching, counter-intuitive scientific research.

“We believe the 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.”

Professor Ramanathan, who is based at the Scripps Institution of Oceanography, in La Jolla, California, said: “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.”

Highlights of the report on the atmospheric brown clouds

The cloud covers five regional hotspots: East Asia especially 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.

There are hotspots too in North America over the eastern seaboard and in Europe – but winter precipitation tends to remove them and reduce their impact.

Cities and regions are “dimming”: Around 13 large cities have so far been identified as ABC hotpots. They are Bangkok, Beijing, Cairo, Dhaka, Karachi, Kolkata, Lagos, Mumbai, New Delhi, Seoul, Shanghai, Shenzhen and Tehran where soot levels are 10% 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 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.

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.

Impacts on agriculture: 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 said 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% to 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 soya bean – these may amount to around $5 billion a year across China, the Republic of Korea and Japan.

Reduced levels of photosynthesis and thus crop production due to ‘dimming’

Health impacts: 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.

Outdoor exposure – Increases in concentrations of PM 2.5 of 20 microgrammes per cubic metre could lead to about 340,000 excess deaths per year in China and India.

Indoor exposure – the World Health Organisation estimates that over 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 PM2.5 has been crudely estimated at 3.6% of GDP in China and 2.2% of GDP in India.