Black carbon – which gives soot its black colouring – is the result of incomplete fossil fuel and wood combustion and is emitted as particles into the atmosphere. The main sources being diesel vehicles and in less industrialised countries from the burning of biomass.
Up in the atmosphere it absorbs sunlight and increases air temperature, while on the ground it adds to air pollution, particularly in urban areas. It can also darken snow and ice, increasing absorption of sunlight and has been linked to accelerating melting of the Arctic.
The contribution of black carbon to warming is thought to be 100-2000 times greater than CO2. However, it has a much shorter lifetime, staying in the atmosphere for only a number of days or weeks compared to 100 years for carbon dioxide (CO2), meaning benefits from reducing emissions will be rapidly noticeable.
In a study this week, the UN Environment Programme (UNEP) estimate we could quickly reduce ‘near-term’ global warming by 0.5 degrees Celsius by tackling black carbon emissions. Reduction measures would have an even greater benefit in the Arctic where it could reduce warming by 0.7 degrees.
This represents as much as two-thirds of change anticipated in sensitive Arctic regions and could help to stem irreversible changes including melting of ice and release of methane from permafrost.
A small number of measures to reduce emissions, including particle filters on diesel vehicles and methane capture from waste management, were identified that would produce immediate benefits.
The report was released at the latest UN climate change meeting in Bonn, Germany, where delegates met to discuss strategies for tackling climate change ahead of the next major UN climate conference (COP 17) in Durban, South Africa, this November.
A reduction of 0.5 degrees could prove vital for nations who agreed to limit global warming to two degrees at last years UN climate summit in Cancun, Mexico, and may lead nations to agree to a more ambitious 1.5 degree limit in the future.
Health benefits of cutting black carbon
Reducing emissions of black carbon and other short-term pollutants like methane and ground-level ozone will have several co-benefits too, says the UNEP report, including improvements in air quality, which will benefit human health and agricultural production.
Cuts in air pollution levels through tackling black carbon or soot as it is sometimes referred to, would prevent between 700,000 and 4.6 million premature deaths each year. The biggest benefit would be felt in Asian countries where frequent smogs, of the type seen in Beijing before the 2008 Olympics, are a hazard to public health.
Air pollution is not only a problem that plagues Asian cities, with London facing legal action from the European Commission for failing to improve its air quality levels. The Mayor of London’s office estimated long-term exposure to air pollution in the city was responsible for 4,267 premature deaths a year.
Farming would also benefit - the UNEP report estimates significant reductions in black carbon and ground-level ozone, which is toxic to plants, could help increase crop yields by more than 50 million tonnes each year. Key food crops like maize, rice and wheat in Asia are believed to be most affected by the pollutants.
In an interview with the Ecologist, tropical forest expert Dr Simon Lewis says that unlike CO2, which plants need to grow, black carbon and other short-term pollutants offer no benefit.
‘Carbon dioxide gives some protection to plants from the impacts of drought, but other pollutants do not provide this protective effect, which suggests that we should make stronger efforts to quickly and substantially reduce non-CO2 emissions, such as black carbon and methane,’ he says.
No reason for delay
Importantly, the technologies required to control these emissions have already been developed and, unlike some strategies for controlling CO2, could be applied globally.
Dr Johan Kuylenstierna, scientific coordinator of the UNEP report, believes many of these measures will actually achieve cost savings over time. Methane recovery in particular is expected to be financially profitable and the value of the gas obtained can be worth more than the cost of recovering it.
Several measures which could make significant reductions to black carbon emissions were identified by the UNEP:
• Install particle filters on all particle filters diesel vehicles
• Ban vehicles that emit high levels of black carbon
• Replace Coal with coal briquettes in cooking and heating
• Replace wood-burning boilers with clean-burning ones in industrialised countries
• Introduce clean burning biomass stoves in developing countries
• Replace traditional brick kilns with low-emission kilns
• Replace traditional coke ovens with modern recovery ovens in developing countries
• Ban the burning of agricultural waste
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