Black carbon

Tackling crop-residue burning in South Asia

Every year a brownish haze envelops South Asia and its fringing seas during the dry season. Extending up to three kilometres high this atmospheric brown cloud is a cocktail of pollutant gases and particles. An important ingredient is black carbon, or soot, which is produced in the region mostly from human activities and is implicated in warming the lower atmosphere. It is the second most important contributor to global warming, next only to carbon dioxide, according to a comprehensive assessment published last year by Tami Bond and colleagues (1).

Black carbon has emerged as an attractive candidate for mitigating climate change in the short term. Diesel engines, which are the dominant source of soot in Europe and the Americas, provide the most promising avenue in terms of technological and institutional feasibility1. In South Asia, however, biomass burning has been shown to be just as important a source2. Biofuels are widely used for cooking in rural parts of the region, and it is also common to observe farmers burning rice stalks and residue after harvest. The Bond et al. synthesis suggests that the burning of crop residues contributes just over 10% of South Asia’s black carbon. But it also releases significant quantities of organic compounds that tend to have a cooling effect: as a result this process contributes little to the global warming signal in the near term (1).

Relatively little is known about this source, though, and we cannot rule out significant impacts at the regional scale. Indeed, the unusually sooty brown cloud over South Asia and its environs has been linked to changes in rainfall patterns and the melting of Himalayan glaciers3. According to one estimate, burning residues is responsible for about a quarter of India’s emissions of carbon-rich particles4; it is thus a potential target of mitigation measures. The mixture of black carbon and other fine particles – often arising from the same sources – has been linked to adverse health effects such as respiratory problems and heart disease (5).

We need answers to several questions so as to test the feasibility of targeting crop burning to mitigate climate and improve respiratory health in the region. Why do farmers burn residues? What might discourage them from doing so? Would the same incentives work across the region characterised by diverse cultures, institutions and practices? We now have answers to some of these questions thanks to a series of surveys undertaken by researchers from the South Asian Network for Development and Environmental Economics (SANDEE) in Nepal. The surveys were undertaken in parts of Pakistan, India, Nepal and Bangladesh (6-9).

Testing financial incentives
Once rice has been harvested, farmers across South Asia burn tens of millions of metric tonnes of leftover stalks each year to make way, quickly and cheaply, for wheat. The volumes of smoke have worsened in recent years with the advent of the combine harvester, a machine that leaves the stalks scattered in the field at a time when the race is on to plant wheat. “Once the combine harvester became popular, residue burning increased,” says Priya Shyamsundar, Executive Director of SANDEE and a member of IGBP’s Scientific Committee.

The farmers burn for compelling reasons and there is no easy, cost-effective way to tackle the problem. Burning leaves a soil that is easy to plough in readiness for the next crop and rids farmers of a material that has little current use. It might have been used for animal fodder but the need for this has diminished with the advent of the motorised tractor. Many farmers believe burning rids the land of weeds, pests and diseases and that the ash is good for the soil, which does seem to be the case in the short term. In the longer term, studies show it is far better to plough the residue back into the soil or compost it: but this is labour intensive and hence expensive.

Would financial incentives encourage farmers not to burn their fields? How much do farmers save when they burn? To find out, Krishna Prasad Pant, of Kathmandu University in Nepal, recruited farmers from the country’s southern lowland Terai region (Figure 1). He invited them to submit sealed bids for compensation for refraining from burning for a season. He found that the median value of the bids was around 78 US dollars (USD) per hectare; farmers were willing to accept this amount to stop burning rice straw in their fields. Some 170 farmers bid for this amount or less and were selected for a follow-up experiment in which a majority complied with the agreement reached to not burn and accepted payment.

Pant’s team trained the farmers either to plough the stalks back into the soil (which necessitated hiring a bigger, more costly tractor) or to cut them by hand and compost them in a corner of the field (for which they needed to hire more labour). About 85% stuck to their agreement and did not burn that season: many, in a follow-up survey, requested a repeat of the intervention. Based on these results Pant thinks the government should pay farmers not to burn, alongside educating them on soil fertility and composting of residue. “Once we know the cost of non-burning, the government can do something for an alternative,” he says.  He is not convinced that other options, such as an outright ban on burning, are feasible. “Putting a legal ban is very difficult to do because it is their traditional right.”

Shyamsundar isn’t convinced, though: “I don’t think we want to take away from this that we want to pay farmers not to burn. This strategy may be un-implementable. But these numbers give us an idea of how big the problem is, and what needs to be done.”