MANILA, Philippines – Good news: the total methane produced by all the rice fields in the world is actually much less than previously thought.
So said a team of Chinese and Japanese researchers in the April 3 issue of the journal Global Biogeochemical Cycles.
Even better news: the researchers said the methane emissions from all these rice-fields could be reduced by nearly eight million metric tons or eight Teragrams (Tg) each year if the two recommendations they made were applied.
Methane emissions estimates
Xiaoyuan Yan of the Nanjing-based Chinese Academy of Sciences teamed with Japanese researchers from the Tsukuba-based National Institute for Agro-Environmental Sciences and the Yokohama-based Japan Agency for Marine-Earth Science and Technology. They estimated the methane emissions using the 2006 guidelines set by the Intergovernmental Panel on Climate Change, which they said was the first time such estimates had been calculated using those methods.
“These results indicate that the emission of methane from rice paddies was overstated in most earlier atmospheric models, which allows for a new methane source or higher estimated methane emissions for other sources,” Yan and his colleagues said in their study. Some of the differences could be explained by criteria applied such as various farming practices or management techniques.
Assigning the correct emissions value matters since the amount of atmospheric methane, a greenhouse gas at least 20 times more potent than carbon dioxide, has nearly tripled since the industrial age began. Since being identified as a source of methane emissions in the 1960s, rice fields have been tagged with wildly fluctuating numbers.
Studies have indicated ricefields produce anywhere from 20 Tg to 100 Tg annually. Just for comparison, ruminants such as cows are thought to the source of roughly 80 Tg of methane emissions. The methane emissions figure Yan and his colleagues reported based on their own calculations is 25.6 Tg, which they note is on the low end of the range but is also close to the total emissions numbers reported by various nations included in the study.
Using the IPCC guidelines, Yan and his colleagues estimated that the methane emissions of Philippine rice fields is 744,000 metric tons or 744 Gigagrams (Gg), which, they said, agrees “reasonably well” with the value provided by the country itself. Not all of their estimated values were as close, though; their calculations for Indonesia and Thailand were a quarter to half of what those countries had reported, for example, while the estimated methane emissions from India were half again as high as the official number.
Comparison
The researchers compared their estimate to a range of methane emission values calculated for the country based on data collected in a number of studies. One report said, for example, that the annual methane emissions of rice fields in the Philippines was an estimated 566 Gg based on an average from data collected between 1983 and 1993. The methane emissions number officially provided by the Philippines in a later study, the researchers noted, was 636 Gg.
Yan and his colleagues concluded their report by suggesting two ways to reduce the methane contribution of rice-fields, breaking down the potential methane emissions reductions if their recommendations were applied by country. For example, they suggested methane emissions from the Philippines could be reduced by roughly 30 percent per year by applying rice straw off season and draining the ricefields at least once during the growing season.
The latter activity carries its own risks, likely increasing emissions of the greenhouse gas nitrous oxide. But researchers said that “the global warming potential resulting from this increase is negligible when compared to … the methane reduction.”
(E-mail the author at massie@massie.com.)
