Warmed By What We Eat; Meat, Rice and Climate Impact

cattleWhile food supplies are in crisis as nations experiment with switching from edible crops to biofuels in order to limit fossil fuel use and its harmful effects, it may come as a surprise to many to learn that the meat and rice in our traditional diets are almost as much to blame for man's impact on the upper atmosphere and consequent climate change.

Somewhere around half the total greenhouse gas emissions since pre-industrial times have come from agriculture, and while fossil fuel use now contributes a larger share, with more and more people needing more and more food there seems little prospect of rolling back agriculture's impact.

However the race is on to find solutions to limit emissions from the two largest causes: livestock farming and rice growing.Fortunately, some good progress is being made; recent research in New Zealand (livestock) and China (rice) demonstrates there is potential to significantly reduce the impacts from both types of farming - as much by careful management as by genetic and technological enhancement.

But agricultural emissions remain a thorny problem for nations trying to get to grips with any form of holistic carbon trading or reduction scheme, primarily because food is one commodity humans cannot do without!

The size of the problem can best be appreciated by understanding how it fits into the scope of global emissions and, in particular, relative impacts. (And, further down, I'll provide news of some recent advances in tackling this issue.)

There are six main so-called "greenhouse gases" (GHG's) - carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbon-23 (CHF3), sulphur hexafluoride (SF6), and PFC-14 (CF4). Of these, the first three account for all but 0.02% of total emissions; which is just as well, as the other "industrial" gases are extremely potent and long-lasting. But thanks to strong international action on curbing their release, they no longer appear a significant problem.  

Carbon dioxide generally gets the most attention, because the majority of CO2 emissions are obvious: car exhausts, industrial smokestacks, power stations - anything that burns fossil fuels. And certainly it is both the most abundant greenhouse gas emitted (99.4% of total GHG's) and the one most easily addressed (though the task is by no means easy!).

However methane is about 23 times by weight more harmful to the upper atmosphere in terms of promoting climate change than CO2, while nitrous oxide is some 310 times more potent. The vast majority of emissions of both these gases come from agriculture, and together contribute around one quarter of the total GHG warming effect.

Fortunately nitrous oxide - released via bacterial activity both from animal dung and urine and as a side-effect of fertiliser application - is emitted in relatively minute quantities: only 0.08% of total GHG's. Nevertheless it contributes 6.1% of total effects.

Methane meanwhile makes up only 0.5% of total GHG emissions yet contributes a more significant 18.2% of warming effects. More than half - perhaps as much as 80% - of this comes from agricultural production.

Estimates vary, but the amount of methane produced by ruminant animals (sheep, cattle, goats, pigs, camels, buffalo, deer, etc) is thought to be around 18% of the total emitted, and that from rice paddies around 21%. (Other significant methane sources include coal-mining (6%) and biomass burning (10%).)

A ruminant animal breaks down food by enteric fermentation, naturally producing methane as a result. Indeed, one cow produces on average an equivalent of around 40 gallons of methane each day!

But regardless of escalating stock numbers, the animal itself is not entirely to blame for the rapid increase in this source of GHGs since pre-industrial times. Diet has a lot to do with it. Monocultured pastures and feedlot grains both lack essential variety, causing animals to spend longer digesting and causing their stomachs to become acidic. Both factors produce more methane per beast.

Studies in numerous countries are examining how diet can be altered or supplemented to reduce these emissions. For example, a recent study in Wales found that garlic may cut methane production, and some types of yeast may also help. But while such supplements may be practicable in feedlot situations, they are not feasible for grass-fed animals.

Changing pastures by introducing more variety, particularly plants low in fibre but high in sugars, is one suggested step. Plants high in tannin have been shown to help limit methane emissions, but come with a relatively high cost. Genetic selection is another partial answer: identifying and breeding stock that naturally emit less methane, and culling the rest to leave a low-emission herd.

methanogensOne major breakthrough has been achieved by researchers so far. A team led by Dr Graeme Attwood at New Zealand's Pastoral Greenhouse Gas Research Consortium has mapped the genome of a microbe - known as a methanogen - which produces methane from the rumen of cattle and sheep.  The picture shows the microbe in situ.

Having spent five years making the genetic map, the task now is to identify new genes and proteins that will target and inhibit methanogens without decreasing animal productivity or affecting other beneficial microbes. Dr Attwood estimates they are another five years away from providing practical solutions from this research.

Manure is the other main source of methane (and nitrous oxide) from stock. There's not much prospect of effectively dealing with this problem for pasture-grazed animals, but there is in feedlot set-ups where manure is easily and routinely collected.

As the amount of methane emitted is higher when collected waste is stored (as it traditionally is) in uncovered tanks or lagoons, a solution is to instead store manure in closed vessels and encourage anaerobic digestion, then capture the methane emitted and use it to generate electricity. Anaerobic digestion is a biological treatment process in which microscopic bacteria break down organic materials into gases and liquids. (Anaerobic means without oxygen, as opposed to aerobic - with oxygen.)

For example it's estimated that the 7,000+ dairy and swine operations in the U.S. could generate enough electricity to power nearly 600,000 homes annually, preventing the release of 1.3 million tons of methane into the atmosphere. There are already significant numbers of farms utilising such systems, and given that with sufficient scale you can make a profit from selling excess power, the market is growing fairly rapidly.

And the digested manure can be used as a fertilizer, further reducing GHGs caused by artificial fertiliser production. Of course, you have to have enough land to make use of the compost - or have a market for it.

The argument over which stock method - grassfed or feedlot - is better environmentally and more easily managed is complex, but it's worth noting that grassfed beef is not only lower in overall fat and in saturated fat, but provides more healthy omega-3 fats. Cattle taken off grass to be feedlot-fattened on grain immediately begin losing the omega-3s they have stored in their tissues; the meat from feedlot animals typically contains only 15- 50 percent as much omega-3s as that from grassfed livestock.

Meat from pastured cattle is also up to four times higher in vitamin E than meat from feedlot cattle, and much higher in conjugated linoleic acid (CLA), a nutrient associated with lower cancer risk.

Consumers used to one type often find the taste of the other off-putting, but at the end of the day this is habit; provided emissions from natural grazing can be limited sufficiently, then this is a healthier way to consume meat that should be encouraged.

An additional "tool" for pasture-based farming is to offset grazing by planting trees. New Zealand's Ministry of Agriculture has calculated that a hectare of pinus radiata - a mainly pulp and paper pine which grows to maturity in NZ in 25 years or less - reduces and absorbs 19 times more GHGs than a hectare of stock land would emit. Consequently they have recommended reducing the country's pasture by 1% and replanting this with pine in order to offset 19% of the nation's emissions.

Of course, any solution comes with a cost, and the farming lobby is a powerful one - arguably becoming more powerful by the day. New Zealand has already seen a proposed carbon tax shot down five years ago by farmers objecting to paying for their stock's flatulence, and NZ's current proposed emissions trading scheme that will bring agriculture into its ambit in 2013 is similarly under alarmist attack from farmers who argue that food sources should be either exempt or heavily discounted. This is a particularly fraught issue for countries like New Zealand, where more than half of total GHG emissions come from pastoral farming. 

ricefieldsBut while the cow has often been held up as a major cause of environmental havoc, another food generally seen in a very favourable light - rice - is actually just as big a problem. Because rice grows in water-logged paddies, the warm wet soil provides ideal conditions for methanogenesis, and most of the methane produced is released to the atmosphere.  

Differences in average temperature, water depth and the length of time the paddy soil is waterlogged can all result in large regional variations in emissions - indeed, from the same paddy at different times of the year. But on average a rice paddy is waterlogged for about 4 months a year, and this is the period of high emissions.

Solutions to cutting back rice emissions run a similar gamut to those for stock, and include introducing a more integrated approach to rice paddy irrigation and fertilizer application, and selecting varieties that require less water. There are many types that can be grown under much drier conditions than traditionally employed, while high-yield varieties can reduce paddy area without reducing overall production. However, with demand for rice expected to increase by 70 percent in the next 30 years, the chances of a reduction in paddy area are slim.   

Supplements can also play a part; compounds such as ammonium sulphate, which favour activity of other microbial groups over that of the methanogens, have proved successful under some conditions.

The good news is that big gains have already occurred through better crop management. In the 1980's, Chinese growers began draining paddies midway through the growing season in an effort to conserve water. To their surprise, they discovered this actually increased yield, because draining stimulates root development and accelerates decomposition of organic matter to produce more nitrogen (fertilizer).

An unintended side-effect was to reduce the amount of methane produced; so much so, that a study led by Prof. Changsheng Li of New Hampshire University has concluded that a drop in world methane emissions in recent years directly correlates to the estimated reduction in emissions from Chinese rice paddies due to this change in practice.

However so long as mankind needs to eat and prefers a meat diet in the West and a rice diet in the East, the conundrum of food-sourced methane will continue to confront us. It seems vegetarians who have always decried eating meat because it requires animals to be slaughtered now have an even better reason: vegetarianism may help save the planet. Provided, that is, they also avoid rice.

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  • Posted on July 22, 2008. Listed in:

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