With its belching cows and giant diesel-powered tractors, the farm sector is widely known to be an important contributor to Australia's impact on climate change. Just how important, however, is not often recognised.
In its latest National Greenhouse Gas Inventory, for the year 2006, the federal Department of Climate Change calculates the share of national greenhouse gas emissions coming from agriculture and stock-raising at 15.6% (expressed as carbon dioxide equivalent).
Though substantial, this figure is much less than the 49.9% attributed to "stationary energy", which consists mainly of power station emissions.
But scientists are increasingly recognising the figure of 15.6% for the farm sector is misleading — and only partly because the official greenhouse data counts the tractors and other farm machinery under the headings of stationary energy.
A more appropriate calculation puts agricultural emissions at about 30% of the total — similar to the burning of coal. In the struggle to lower Australia's emissions — the world's worst per person — the key targets must include not only coal trains, but cows and sheep.
At the same time, farmlands hold an important part of the solution to global warming. Properly used, farm soils can take carbon back from the atmosphere, locking it away for the long term.
The confusion over the emissions figures for agriculture arises from a perverse interpretation by the Intergovernmental Panel on Climate Change (IPCC), the United Nations body which reports on global warming, of the way the greenhouse gas methane (CH4) behaves in the atmosphere.
Methane is produced, among other ways, by microorganisms in the intestinal tracts of cattle and sheep. For the most part exhaled or burped rather than farted, these emissions of methane account for more than 80% of the impact of the Australian farm sector on climate change.
Methane molecules have a half-life in air of about seven years before they are oxidised to carbon dioxide and water. After 12 years they are almost completely gone.
In its 1995 report, which underlay the Kyoto Protocol, the IPCC assigned methane a Global Warming Potential (GWP) 21 times that of carbon dioxide. This was calculated on the basis of the warming effects of methane — and those of the carbon dioxide it turns into — over 100 years.
This (GWP) figure of 21 for methane is still used by the Australian Greenhouse Office when it calculates emissions totals.
The fate of many of the Earth's ecosystems, however, will not be decided over the next 100 years but in the next few decades, as climate "tipping points" are approached and quite likely, exceeded.
The really meaningful GWP figure for methane, therefore, is the one that describes the effect the gas will have during this critical shorter period.
Calculated over 20 years, methane has a GWP of 72. On this basis, the Zero Emissions Network explains, the contribution of livestock to Australian greenhouse gas emissions is not the 11% reported by the Department of Climate Change, but 25%.
Adding to the impacts of Australian agriculture on global warming are emissions of the gas nitrous oxide (N2O), resulting mainly from the breakdown of nitrogen fertiliser used in soils.
In government figures, nitrous oxide is reckoned to account for 3% of national emissions.
Cutting farm sector emissions is clearly indispensable, but the task will not be straightforward. There are about 130,000 farms in Australia and they have diverse combinations of climate and soil type. There can be no question of simply drafting regulations, tweaking a few incentives, and expecting results.
Finding answers will depend on investing serious sums in agricultural research, consulting at length with farmers to determine which methods work in specific settings, and fine-tuning policies to make sure environmental imperatives are met while farms stay viable.
How, for example, to achieve the central goal of cutting methane emissions from cows and sheep? Here, fortunately, the demands of greenhouse abatement and the need of farmers to make a living tend to back one another up.
Fodder that creates methane in the gut of animals does not produce meat or milk. Figures cited by the Conservation Council of Western Australia say the losses of ingested energy are between 10% and 15%.
There are substantial gains for pastoralists if these losses can be cut.
One partial solution could be selective breeding, because individual animals vary significantly in their methane output.
Another approach is through careful management of nutrition, since in general, sheep and cattle produce less methane when fed on good-quality forage.
Particular plant chemicals also appear to suppress methane production. Research with sheep in New Zealand shows that pasture mixes that include species rich in tannins have major benefits.
Supplementing high-energy finishing diets with vegetable fats such as canola oil has also been found to reduce methane emissions by as much as a third. Still under development are "germ warfare" methods that target methane-producing bacteria, infecting them, for example, with viruses.
Though useful, these methods are unlikely to go anywhere near eliminating the emissions from stock-raising. To get these emissions down, the number of sheep and, especially, of beef cattle will have to be cut. But this raises questions of food supply, and of providing new income sources to stock-raisers.
In Australia, cattle and sheep are very often a low-cost choice for land that cannot be used for crop-raising. These stock are run precisely because they need relatively little labour, and will survive and grow on whatever forage is available.
Amid the finely balanced economics of stock-raising, lowering the emissions of animals in this low-input, low-productivity setting would very often be too costly. Logically, this sector of Australian pastoralism needs to be shut down.
How might this be achieved, and the stock-raisers be compensated?
In the Rudd government's proposed Carbon Pollution Reduction Scheme (CPRS), the rural sector will be incorporated into a general Australian emissions market from 2015.
Pastoralists, no doubt, will gain saleable carbon credits for turning their land over to uses such as agro-forestry that lock carbon away in the form of trees.
In wetter areas, extra income from carbon credits could make turning pastures over to timber production an attractive option.
The real challenges, however, will come in the drier regions where most Australian stock are pastured. If graziers in these less-favoured areas are to stop running cattle and sheep, generous support and transition schemes will be needed, funded out of taxes on big-business carbon emitters.
Will the pastoralists raise kangaroos on regenerating woodland? Or will they be, in effect, rangers in new conservation parks?
Meanwhile, how are people to be fed? Ideally, beef and lamb would be relatively expensive specialty meats in a market dominated by pigs, poultry, fish and perhaps kangaroos, none of which emit methane in significant amounts.
Yet kangaroos aside, farming these animals would need very large amounts of grain-based feed.
The situation calls for the rapid development of an algal biomass industry, growing microalgae in ponds filled with brackish groundwater and fertilised — with due care to avoid contamination by heavy metals — using urban sewage wastes.
Microalgae grow very quickly, yielding protein-rich animal feeds that also contain large amounts of omega-3 oils, vital for fish nutrition. In New Zealand and the US, work to create such an industry has recorded some promising results.
Would Australians give up beefburgers for farmed barramundi? Global warming may force them to.
So long as cows and sheep remain in any numbers, methane emissions from livestock will be significant. It should, however, be possible to cancel out their greenhouse impacts — and conceivably, those from other "intractable" greenhouse gas sources — through capturing carbon in soils.
Here, we need to take a trip to the wheat-growing property of farmer Brian Krieg near Snowtown, north of Adelaide.
As related by the Stock Journal in April 2008, Krieg over three years raised the organic matter level of his land from 1.5-1.8% to 2.5-3% by using biological farming techniques. In terms of soil carbon, the increase was from about 1% to about 1.7%.
In its native state, Krieg's land probably had a soil carbon content at least twice the 1% level it now has after a century of conventional farming. Over decades, environmentally aware management can probably restore all or almost all the original soil carbon of most farm soils.
If the carbon content of the top 30 centimetres of a hectare of farmland can be raised by 1%, the amount of carbon stored is about 42 tonnes.
Australia has about 50 million hectares of periodically cultivated soil. A 1% increase in their carbon would lock away ABOUT 2.5 times the country's total annual greenhouse gas emissions (calculated using a GWP value of 72 for methane) of 833 million tonnes in 2005.
So long as sustainable practices were maintained, this carbon would remain in place.
In many soils, rises in soil carbon of far more than 1% are almost certainly attainable. The best-practice conservation farming practised by broad-acre farmers like Krieg involves combining no-till or minimum-till methods with rotation between grains and fodder crops, especially legumes.
This mode of farming is not, strictly speaking, "organic", since it relies on herbicides to suppress weeds and makes some use of artificial fertilisers.
But by leaving the soil mostly undisturbed, with plant roots in place and stubble and crop litter on the surface, it allows soil micro-organisms to flourish. When these micro-organisms die, their decay creates a carbon-rich humus that improves soil structure and water retention.
There is also a big role for the composting of urban green wastes, and for the return of this carbon to soils. In addition, there is the challenge of using sewage sustainably for soil enhancement.
Carbon can also be added directly to soils in the form of biochar — that is, charcoal made from the heating of plant matter in an oxygen-poor environment.
Research continues on the best levels for particular soils and climates, but it is known that in Australian wheatbelt soils even a few tonnes of biochar per hectare can sharply increase crop yields.
Biochar is extremely stable in soils, lasting for hundreds or even thousands of years. By providing a haven for soil micro-organisms, it promotes the further accumulation of soil carbon through natural processes.
Through making soil nutrients more accessible to plants, higher levels of soil organic carbon — and also of biochar — allow applications of fertiliser to be cut. Nitrogen already in the soil is kept in place. As a result, nitrois oxide emissions fall.
Taking these potential cuts together, might Australian agriculture become emissions-neutral?
If we recall the figures cited by the Zero Emissions Network, livestock account for 25% of total Australian greenhouse gas emissions. If stock numbers were sharply cut, especially in the beef cattle sector, and if a broad range of other mitigation measures were applied, this 25% could probably be cut to 15%.
Extensive use of biochar and conservation farming methods could likely cut a further 1% from total emissions through reducing nitrous oxide emissions from soils.
That would still leave the entire farm sector responsible for 19% of total Australian carbon emissions, corresponding to 43 million tonnes of carbon. This would need to be offset through the draw-down of carbon from the atmosphere.
If fast-rotation tree crops were planted on tens of millions of already-cleared hectares, available data suggests that biochar use could eventually sequester 20 to 30 million tonnes of atmospheric carbon a year, and perhaps much more.
If soil organic carbon were increased at the rate of one tonne a hectare a year over 30 million hectares of cropland, an extra 30 million tonnes a year could be locked away.
Plant regrowth from ending grazing on semi-arid woodland would draw down an unknown amount, but probably at least 10 million tonnes a year.
These are imprecise figures, but a carbon-neutral Australian agriculture nevertheless emerges as a perfectly realistic proposition. Indeed, the sector might finish up strongly carbon-negative, able to offset emissions elsewhere.
The timelines would not be rapid, since trees would have to be planted and to grow to harvestable size, but the process could be well under way within 10 years and complete within 20.
Most farmers would probably do well from the shift to carbon-neutral agriculture, with biochar and higher soil organic carbon levels improving productivity. Nevertheless, economic incentives will be needed to set the process in motion.
The Rudd government proposes that agriculture be decarbonised through the mechanisms of the new emissions market. But in an October 2008 paper, Australia Institute researchers Hugh Saddler and Helen King argued powerfully that a carbon market would fit poorly with agriculture.
In particular, measuring emissions from farmland with the accuracy required would be prohibitively expensive.
A workable alternative would be for the government to sign contracts with farmers and graziers to pay them for using conservation farming methods, for maintaining low stocking rates, and for planting and maintaining trees.
Once again, however, it must be remembered that the agriculture sector is diverse, and that blanket solutions are few.
The great majority of Australian farms are of modest size, often family owned and run. To compensate these farmers for the costs of cutting stock levels and introducing progressive farming methods is reasonable.
But especially in the beef industry, and particularly in northern Australia, giant agribusinesses have made large inroads.
The stock-raising that occurs on these vast agribusiness properties is weighted toward the rough pasturing of beef cattle — precisely the activity that creates inordinate amounts of methane emissions.
A sharp distinction needs to be made between working farmers and the kind who visit their properties by corporate jet. The latter do not need or deserve subsidies.
In the case of large corporate agriculture and stockraising, the measures needed include nationalisation and the planned introduction of responsible land use.