doi:10.1038/nindia.2007.55 Published online 16 January 2008

Carbon fields mess up emission figures

As figures of a key greenhouse gas continue to be incorrect in national projections, Indian scientists are questioning the veracity of the region's climate change policies.

Subhra Priyadarshini

Methane from high carbon fields is overlooked

The data on India's greenhouse emissions need an immediate relook as more and more studies across Asia, many of them in India, indicate that methane emissions from the country's paddy fields have been grossly underestimated through the years.

A key question debated by scientists at the Bali conference of the Intergovernmental Panel for Climate Change (IPCC) late last year was what constitutes a 'dangerous level' of concentration of greenhouse gases. But with incorrect country figures (the research has implications in many countries), fixing responsibility is not only difficult but also erroneous.

A benchmark study1 in a high carbon content paddy field in West Bengal has questioned India’s projected methane emission levels, concluding that if the eastern and north-eastern parts of the country with their high carbon agricultural land were to be taken into account, India's cumulative emission figures might actually go up a few notches.

Not Just India

This holds true for countries like China, Indonesia, Philippines, Vietnam and Thailand where high carbon paddy fields are continuously flooded. These countries along with India account for 70 per cent of the world’s paddy harvest area.

India’s paddy field methane emission data is primarily derived from low-carbon lands spread across the country. However, University of Calcutta's S. K. Mitra Centre for Research in Space Environment has shown that during the summer of 2003-04, peak methane emission touched 16.5 mg per square metre per hour and the same winter it was 12.5 mg/sq m/hr.

The high methane emission in eastern India is due to the presence of higher organic carbon in the soil

This data presents a stark contrast with studies conducted by New Delhi’s National Physical Laboratory (NPL) on low carbon fields like Pant Nagar in Uttar Pradesh, where 7070 mg per square metre of methane was emitted in the entire summer harvest season (see ). There is no hourly data measured from these regions but roughly converted this would mean 2.46 mg/sq m/hr.

N. N. Purkait of the University and D. K. Chakraborty, a scientist who has now retired from the NPL, studied emissions in the field during 1998-99 and again in 2003-04. The soil carbon content here was between 0.89 and 1.08 per cent as compared to low-carbon fields (less than 0.7 per cent). The comparison was based on the assumption that no organic fertiliser was used in the fields.

Methane contributes a greenhouse effect of around 1.7 watt per square metre. Its concentration has been increasing at the rate of 1 per cent per year due to human activities. IPCC estimates that at current rates it could contribute an additional 0.5 watt per metre square in radiative heating in the next 50 years. About 20 per cent of total methane emission in the atmosphere comes from paddy fields.

In India, measurements of methane emission from paddy fields were initiated in the late eighties. The eastern region, with a major coal field belt, has high carbon content. In West Bengal, it is between 0.74 and 1.36 per cent. The total rice harvested area of West Bengal is nearly 6 million hectares and the total methane emission is nearly 1 terra gram per year. It represents a fair share of India's total harvested rice area of 42.23 million hectares and total methane emission of around 5 terra gram per year.

Measuring methane in a high carbon field

"This high value in the eastern region is due to the presence of higher percentage of organic carbon in the soil as compared to other regions," Purkait says.

The team chose two plots from this area, one for Kharif (monsoon harvest) and the other for Rabi (winter harvest). Observations were taken from August to January for Kharif crop and from January to May for Rabi crop. They found that emission was maximum after noon and that it had a direct link with temperature. The team found two peak emission periods – one after panicle emergence and the other after flowering -- in both the crops; for Kharif, the first peak was larger than the second one and for Rabi, the second.

Prabhat Gupta of NPL’s analytical chemistry division agrees with the findings. Gupta, who compiled a classic database of methane emissions from paddy fields across the country as part of India's methane campaign in 1991, says data on the influence of soil type on such emissions in India was scarce. "Limited studies indicate high emission values in some soils at Jorhat (Assam), Kuttanad (Kerala) and Gaberia (West Bengal). We have observed from the decadal data that emissions changed appreciably as the soil organic carbon varied," he says.

Uncertainty can be reduced

Gupta confirms that a good deal of uncertainty in calculating India’s methane budgets can be reduced if data is categorized according to soil carbon levels and use of organic fertilizers. "Some ground truths in data quality certainly need a relook. We don't have information on the exact area under high carbon paddy cultivation," he says. Figures with the Soil Survey of India are too sketchy to base scientific estimates on.

There is a need to revise emission factors laid down by IPCC in the light of these studies

Eminent atmosphere scientist A. P. Mitra, who headed the Asian Development Bank sponsored Methane Asia Campaign in 1998 showed in a classic study that soil carbon played a significant role in determining methane emissions. He compiled data from India, China, Indonesia, Philippines, Vietnam and Thailand to conclude that average methane emissions from continuously flooded paddy fields with high carbon content was 36 gm per square metre as opposed to 12 gm per square metre in low carbon fields. The average IPCC emission factor is 20 gram per square metre. These six countries account for 70 per cent of the world’s paddy harvest area.

"There is need to revise emission factors laid down by IPCC in the light of these studies for different paddy growing ecosystems," he told Nature India days before his demise in September 2007. Mitra's dream of projecting a just methane emission scenario is being carried forward by ISRO's geosphere and biosphere programme, which is launching a country-wide mapping of agricultural land based on many parameters including carbon content.

Gupta says there have been vocal demands by members to reduce uncertainties in data collection as it did not reflect the exact state of greenhouse emissions by the country. "Everyone realises the need for fair calculations. And India should certainly not lag behind," he says.

Methane blues

Alongside carbondioxide from fossil fuels, that has traditionally been identified as the major culprit in national reports worldwide, methane was also picked up as one of the key greenhouse gases contributing massively to warming in early estimates. In 1990, the US Environmental Protection Agency (USEPA) came out with a report that identified India as contributing to the global methane burden by producing over 37 metric tonne of the greenhouse gas from paddy fields every year.

India in turn had accused the report of being biased against developing countries since China's methane emission figures from paddy fields were also quite high in the report. Indian officials contended that the calculation of methane emission from paddy fields was based on non-specific data, meaning it was roughly estimated using a standard global formula into which the total area under agriculture in India was simply fed. India's Council for Scientific and Industrial Research (CSIR) made its own calculations an year later and said the actual amount of methane emitted from paddy fields was less than a tenth of what USEPA had projected.

However, neither of these calculations took into account methane emissions from high carbon fields as a separate variable. Later studies across the country confirmed the need for segregating carbon and non-carbon fields while formulating country reports.

The way forward

Chairman of the Inter-governmental Panel for Climate Change Rajendra Pachauri feels determining what constitutes "dangerous anthropogenic interference with the climate system" in relation to Article 2 of the UNFCCC (United Nations Framework Convention on Climate Change) involves value judgements. "Science can support informed decisions on this issue, including by providing criteria for judging which vulnerabilities might be labelled 'key'," he says.

Science can support informed decisions by providing criteria for judging which vulnerabilities might be labelled 'key'

Environmentalists and policy makers have debated these issues threadbare during the recent IPCC Bali convention. The agriculture and forestry sectors are being considered as key areas with economic mitigation potential by the year 2030, he adds.

According to him IPCC has projected certain abrupt irreversible impacts of anthropogenic (man-made) warming which include partial loss of ice sheets on ice polar land. It could mean metres of sea level rise, major changes in coastlines and inundation of low-lying areas. "There could be great effects in river deltas and low-lying islands. Approximately 20-30% of species assessed so far are likely to be at increased risk of extinction," Pachauri says.

Large scale and persistent changes in Meridional Overturning Circulation (MOC) could have impacts on marine ecosystem productively, fisheries, ocean carbon dioxide uptake and terrestrial vegetation, he says.


  1. Purkait, N. N. et al, Indian Journal of Radio & Space Physics, 36, 52-58, 2007
  2. Gupta, P. K. et al, Nutrient Cycling in Agrosystems, 64, 19-31, 2002 | Article |
  3. Mitra, A.P. et al, Nutrient Cycling in Agroecosystems, 64, 147-155, 2002  | Article |