Black carbon's impact on ecosystems

It is known to alter melt cycle of glaciers and interfere with rainfall patterns

Published: Wednesday 11 March 2015

Black carbon can accelerate snow melt (Photo: Swapnil Awaghade, May 2011)

Black carbon and snowmelt: Black carbon can also accelerate ice-melt when they settle on snow. The bright snow surfaces reflect a high amount of solar energy back into space. But black carbon absorbs substantial fraction of this energy and re-emit it as heat. The Arctic and the Himalayas are therefore vulnerable. Black carbon on glacial snow is a concern as it alters the melt cycle of glaciers in regions that rely on glacial melt to balance water supply through seasons. These impacts are highly regional depending on the local profile and trend of pollution and transport of pollution. There is now considerable focus on the Arctic and alpine glacier regions and the Himalayan glaciers.

Black carbon and rains: Black carbon is also known to interfere with cloud formation and the rainfall pattern. It also reduces sunlight that reaches the surface and that is reflected back to the space. Black carbon may change precipitation and surface visibility. Scientists say that plumes of emissions can suppress convection and stabilize the atmosphere in ways that obstruct normal precipitation patterns. It is described as dimming of the earth’s surface that reduces patterns of evaporation that make clouds. If black carbon heats up the layer of the atmosphere where clouds are forming, for example, they will evaporate. They can no longer reflect sunlight back into space, and so the soot-laced clouds end up warming the atmosphere. But black carbon that hangs above low-lying clouds has a different effect. It stabilizes the layer of air on top of the clouds, promoting their growth. These clouds are like shields, blocking incoming sunlight. As a result, black carbon also ends up cooling the planet. There are now several studies and evidences that will have to pieced together to assess the varied impacts of black carbon.

Some warm and some cool: What then adds up? According to the climate science there are good and bad particles. All particles do not warm. Some, especially organic carbon, have cooling effect as well. Amongst the various fractions of particulate matter the organic carbon and sulphate have cooling effect as they are lightreflecting. But black carbon is light absorbing. If the ratio of cooling particles is higher, sources may have more cooling effect. Science is still trying to figure out this threshold for different sources. The exact threshold from negative to positive forcing for the major sources is still an area of uncertainty and is the focus of ongoing research. Share of cooling and warming particle determines the net positive or net negative impact of different pollution sources.

Moreover, if the global radiative forcing of particles since the industrial revolution is tracked it would show that while CO2, methane, N2O, black carbon have strong influence on warming, the sulphates and organic compounds have had global cooling effect. So scientists conclude that tropospheric ozone, black carbon, methane, F-Gases are “bad” short-lived forcers. But sulfate aerosols, organic carbon, clouds, are “Good” short-lived forcers from climate perspective.

This means all sources cannot be blamed equally for warming the climate as the mix of cooling and warming particulate matter varies across pollution sources. For instance, open burning and residential biomass or biomass based cookstoves have much higher proportion of organic carbon that scatter sunlight and are cooling. Biomass burning is expected to be dominated by organic compound and therefore as a net cooling effect. So does sulfate from power plants. Ironically, when CO2 emissions are hastening the tipping point it is the biomass energy of the poor that is masking and cooling.

But black carbon emissions from transport that largely uses diesel or brick kilns with inefficient combustion technologies have higher share of light absorbing black carbon, which has definite warming impact. The combustion of fossil fuels that are low in sulfur is net positive radiative forcing. Evidences are stronger on the net warming impacts of diesel vehicles and brick kilns. Diesel engines account for nearly all of the black carbon from transport and diesel engines produce net warming – a case for win-win to reduce both health and climate risks. Thus, the sources like diesel and brick kilns that are rich in black carbon emissions need priority action.

Yet warm or cool – all particles must go if we want to save the lives of the poor. Cooling particles from the biomass based cookstoves of the poor is not an opportunity to save the climate. These particles – be it warming or cooling – harm the lungs and kill the poor. The rich will have to frame the strategies and the requisite funding to quickly give to the poor access to affordable and clean fuels.

Thus, there is actually no scope of shifting burden to the poor and underplay the role of CO2. Veerabhadran Ramanathan of the Scripps Institution of Oceanography, University of California, San Diego, has concluded that nearly 40 per cent of the warming that CO2 has already committed is not showing up as that is masked by the cooling particles like sulphates. Aggressive air pollution control will remove both cooling and warming particles and the committed warming of CO2 will show up more aggressively. This requires equally aggressive action on CO2.

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