rice paddies and methane emissions

Another way domestication has had a significant impact on concentrations of greenhouse gases, is the effect of rice paddies on methane concentrations.  It is thought that rice paddies could contribute to 20% of current methane emissions.  

Rice paddies date back to the beginning of agriculture and archeological evidence shows the first paddy to be in Korea.  Since this time rice cultivation using paddy fields has developed all over the world, in Europe, the USA and across much of Southeast Asia. China is now the greatest producer, accounting for 36% of the worlds rice.  Methane is produced by anaerobic bacteria in the flooded paddy fields. (see picture below)

From 1940-1980 methane emissions have increased by 49% and during this time, the global rice harvest has increased by 41%.  This is as a result of higher yielding crops, expansion of crop areas and increasing use of fertilizers, but this increase in production of rice has meant increase in methane concentrations.  

According to Aselmann and Crutzen (1989) rice paddies cover 1.3x106 km2  of the Earth's surface.  They undertook a study determining methane emissions from both rice paddies and wetlands.  They estimated rice paddy methane emissions to be between 60-140 Teragrams per year.  They found emissions to be highly seasonal, with greatest emission levels to be in the summer in both Hemispheres.           

A paper by Cao et al. (1996), used modelling to predict methane contributions from rice paddies across the world.  They found this difficult as not only does it vary greatly between regions, but they also found great seasonal variation between emissions as well.  They estimated emissions to be 50-60Tg yr.^-1.  Another study by Liu and Wu (2004) uses models again to estimate methane emissions from Taiwanese paddy fields. Here they found temperature to be the most important factor affecting the amount of methane emitted, so the higher the temperature the higher the concentration of methane.  They also found there were seasonal variations in methane emissions.  

Bachelet and Neue (1993), estimated methane emissions from Asia, as this is where 90% of rice is produced.  Here they evaluated different approaches of estimating methane emissions, again they found a major weakness to be that some methods used a constant rate of emissions for all months, which affects the results, as there are seasonal variations.  From their comparisons they concluded that in the past it seemed emissions for this area had been over estimated.  They gave an estimate of methane emissions from Asian rice fields to be 63 Tg yr.^-1, which is different from Cao et al.  

Consequently, this shows that current methane emissions from rice paddies are uncertain, as suggested by Aselmann and Crutzen, due to the complexity of measuring/modelling as a result of spatial and temporal differences.  However, it is important that reliable predictions of methane emissions are calculated as methane has a a warming effect  21 times greater than carbon dioxide.  

Methane emissions are set to increase as rice production increases to supply a growing population.  However Nueu (2007) suggests that we do have sufficient understanding of rice production and its effect on methane emissions to put in place policies to reduce impacts. These are important and I will discuss mitigation strategies surrounding rice cultivation in a later post.  As Nueu says, many people rely on rice as a staple in their diet and at the rate that population is set to increase, at least 880 million tonnes of rice will need to be produced by 2025 to keep up with growth.  Unless changes are made this will lead to huge increase in atmospheric methane concentrations.