Methane has attracted particular attention as a greenhouse gas, as once it is formed, mitigation strategies are not easy to devise. The US Environmental Protection Agency (EPA) estimated in 1990 that global methane emission is about 550 Tg (550 million tonnes) per annum, with about 110 Tg of this released from paddy cultivation. This estimate turned out to be wrong, but it could have had devastating consequences for the economy of India.
Nearly 90% of the world's rice production comes from Asia, and rice is the staple food for 2 billion people. India produces about 80 million tonnes from about 42 million hectares. The EPA estimated methane emission from India as 37.8 Tg per annum in 19901. The implications of this estimate were so serious that India's National Physical Laboratory led an international collaboration of 15 laboratories to measure methane emissions directly2.
Methane released during paddy cultivation was collected and analysed, with careful controls. Methane fluxes were typically between −10 and +80 mg per hour per m2. Samples were taken from 34 sites in India of varying agricultural conditions, and measurements were done at different cultivation stages, from seedlings to harvesting, and integrated over a full year to cover single and multiple crops. Methane emission depends on the paddy-water ecosystem, and this was also measured in various ways over time.
All this direct agricultural information was used to calculate the methane emission budget2, which was 4.07±1.25 Tg per year, only one-tenth of the 1990 EPA estimate. A repetition of the measurements in 1996 gave essentially the same result.
The main reason for the difference between the EPA estimates and the actual measurements is that long incubation periods lead to anaerobic conditions, favouring high methane genesis. The EPA extrapolated the 1990 data without realizing that the high methane flux conditions had resulted in an annual methane emission from India that was an order of magnitude larger than the experimentally measured values.
This error has now been corrected and the estimate of global methane emission from paddy fields has been scaled down to 60 million tonnes. Methane emission from rice cultivation in India and other similar Asian countries is no longer considered a major factor in global warming.
If this error had gone uncorrected, international protocols would have required Indian methane emissions to be brought down to global average levels, reducing paddy cultivation by at least 47% in the short term. The economic cost of this would have been about 135,000 million rupees (US$3.1 billion) per annum, nearly double the annual budget of all India's science and technology ministries combined3.
So the investments that enabled the National Physical Laboratory to perform and coordinate precise measurements saved the country from an economic crisis. This story illustrates that, even in developing countries, it is wise for governments to invest in research and development.
Ahuja, D. R. Estimating Regional Anthropogenic Emissions of Greenhouse Gases (US EPA Technical Series, Washington DC, 1990).
Parashar, D. C. et al. Chemosphere 33, 737–757 (1996).
Gopal, E. S. R. et al. in Proc. 2nd Int. Conf. Metrology, Quality and Global Trade (eds Banerjee, P. et al.) 432–439 (Alpha, New Delhi, 1999).
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Gopal, E. EPA error risked halving India's rice harvest. Nature 403, 130 (2000). https://doi.org/10.1038/35003040