The following article has been taken from "Environment" magazine - a publication of the Environment Centre of Western Australia. "Environment" is a quarterly publication that acts as a forum for the discussion of environmental issues in WA and its region.
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The climatic changes associated with global warming are likely to have a dramatic influence on farming. How farmers are to enable their farms to cope given the inherent unpredictability of those changes must be considered very carefully and as soon as possible, particularly if long-term sustainable farming practises are to be adopted.
Miko Kirschbuam has worked as a research officer for the Global Change Program of the Australian Conservation Foundation. He has also written on this same subject for the South Australian agricultural journal, Acres.
Theo is the rural liaison officer for the WA branch of the Australian Conservation Foundation.
In recent years, public awareness has grown of the potential of significant global warming as a result of a build-up of trace gases such as carbon dioxide in the atmosphere. This global warming is commonly referred to as the Greenhouse Effect. Concern about the Greenhouse Effect is not new and has been discussed in the scientific community for over hundred years.
Agriculture is most vulnerable
In the possible effects of the Greenhouse Effect, agriculture is particularly vulnerable. There are directly damaging effects, such as heat waves, cyclone damage, increased erosion potential due to heavier rainfalls and others, as well as the special problems associated with having to deal with change and uncertainty itself.
temperature
The most certain change is an increase in temperature. It will affect crops that have specific vernalisation requirements such as stone and pome fruits and might necessitate a shift to species or cultivars with different temperature requirements. This is a major problem for fruit trees due to their long life spans, and fruit growing areas that are only just cool enough to allow flowering to be initiated will be likely to be facing difficulties and only be able to survive as fruit growing regions if they can switch to fruits with lower chill requirements. A 3oC mean temperature rise would see Manjimup with temperatures similar to those Geraldton currently has, and be clearly above the chilling requirements of its deciduous fruit industry.
Increasing temperature will also mean a greater incidence of heat waves. Perennial plants and summer crops will be most at risk. On the positive side, higher winter temperatures may increase productivity in regions that are currently of low productivity due to cold winter temperatures. Slower maturing varieties of plants may be more suitable under warmer temperatures since they are less likely to "bolt" to maturity, thereby reducing potential yields. Higher temperatures in winter could also reduce post-natal lambing losses in sheep.
Disease problems are likely to become more prevalent, as warmer regions generally are more prone to diseases. Similarly, insect pests may become more of a problem, as the higher metabolic rate of many insects and diseases will allow faster growth and multiplication. Many pest species are also killed by frost. The disappearance of frost would, therefore, also allow many pests to extend into regions from which they are currently excluded by frost.
rainfall
Warmer air can hold more water, and in a warmer world, surface water is likely to evaporate faster. This means that the hydrological cycle is likely to intensify with more rainfall and faster evaporation rates, at least in globally averaged terms. Most Greenhouse models suggest that, in general terms, summer rainfall will increase so that water availability to crops in regions reliant on summer rainfall may not change very much; winter rainfall, however, is likely to decrease sharply. However, our current models are not reliable enough to predict with any certainty which regions may experience what changes, but it seems certain that there will be both winners and losers in the distribution of changes in temperature and rainfall. Since rainfall intensity is almost certain to increase, problems such as erosion and flooding are very likely to increase.
Reduced rainfall will affect the recharge of aquifers, groundwater level and possible salinity problems in some areas. It will also have important implications for farmer reliance on farm dams, and mean that farmers may need to invest in expanding their farm management practises to more variable rainfall regimes.
Extreme events
Cyclones are generally expected to increase in both their frequency and intensity and are likely to occur in regions further south. There are, however, still major uncertainties about the physics of cyclones so that this change cannot yet be predicted with certainty.
Other extreme events, such as prolonged dry spells, long spells at temperatures above or below their averages or increased non-cyclonic wind speeds may or may not change in their frequency. Current models cannot yet make any reliable predictions on any of these questions, and there is no simple reason to assume that their incidence should change. On the other hand, if it gets warmer and the variability about the new average remains the same, it would, nonetheless, mean that the incidence of damaging high temperatures would increase.
the carbon dioxide fertilisation Effect
Growth of plants is dependant on the uptake of carbon dioxide from the atmosphere. The pre-industrial concentration of carbon dioxide was about 280 ppmv; it has increased to over 350 ppmv by now, and is currently increasing further at a rate of about 1.5 ppmv per year. This should increase the ease with which plants can take up carbon dioxide in the process of photosynthesis. Plant physiological studies support that contention, but it is not clear yet to what extent growth enhancements will be expressed in plants growing under natural conditions when other factors, especially phosphorus nutrition, may be limiting growth.
There are likely to be differences between the ways in which different species respond to increases in carbon dioxide concentration. With ample water supply, it is likely that plants using the C3 photosythetic pathway such as wheat, legumes and most tree species may grow better, whereas growth in plants using the C4 pathway, such as maize and sorghum, may not be increased. Under water-limited conditions, both types of plants should be similarly stimulated. the CO2 fertilisation effect should be particularly important at higher temperature, and could thereby to some extent counteract the negative effects of the Greenhouse Effect.
However the absence of dramatic yield improvements up to now should make us cautious about expecting major improvements to eventuate in the near future. There is also the possiblity that plants which adapt gradually to warming temperatures may not respond favourably to the fertilisation effect. It is unlikely to be able to compensate for the negative aspects of the Greenhouse Effect.
Change and uncertainty
Potentially the most serious threat imposed by the Greenhouse Effect is change and uncertainty itself. Agriculture is heavily dependant on the expectation that next year's weather will be predictable. A particular crop species, for example, is chosen because it is expected to grow best under the environmental conditions expected for the coming season. This is particularly important for long-lived plants like trees. Tree varieties must be chosen that respond well not only in the next year, but over the next decades.
Uncertainty in future rainfall is bound to increase as we do not yet have any clear understanding of how changes in global temperature may affect local weather patterns and rainfall distributions. Changes in temperature may further compound the difficulties, as it is again not clear how global changes in temperature will be expressed at the local level. All models of climate change clearly predict there to be significant regional differences in the extent of global warming, with some regions experiencing less than the global average.
We cannot yet predict what regions will experience more and what regions less warming. Computer models are continuously being refined to develop more reliable regional predictions. However, the complexity of the global climate system may be such that we will never be able to predict these local effects. This leaves agriculture in the quandary of having to contend with a changing climate where the nature of change cannot yet be foreseen.
Some aspects of change are predictable, and that poses its own costs and challenges. As it gets warmer, regions that were once suitable for, say, winter wheat may become suitable for summer wheat, or a region may change from being suitable for wheat to maize. How do farmers respond? When is the appropriate time for switching from one production system to the other? Do individual farmers have the necessary skills to adapt; do they have the right tools and machinery? How easy is it for a farmer who has grown wheat for twenty years to switch to the production of maize?
Some regions may also become totally unproductive; current models predict that winter rainfall may decrease, which together with warmer temperatures, may make much of the areas in South and Western Australia that rely on winter rainfall less productive. If Australia is lucky this loss in production potential may be compensated for by increased productivity in the summer rainfall regions of NSW and Queensland. But much of the human and physical infrastructure in South and Western Australia may be lost to the extent that it is not relocatable.
In addition there is likely to be immense human hardship involved in the translocation of farmers once certain areas become unproductive, with the attendant break-down of social networks. To make this transition harder, there will be no clear point at which an area becomes unproductive and there would be likely to be a long period of increasing hardship with decreasing yields before it finally becomes unavoidable to give up unproductive enterprises.
In addition to the anticipated gradual change, there is the possibility of sudden shifts from one climate regime to another. Such quick changes have apparently occurred during past episodes of climate change that were associated with the ice ages.
influencing the Greenhouse Effect
The most important Greenhouse gases are carbon dioxide, methane, nitrous oxide and the chlorofluorocarbons. The relative contribution from each of these gases from Australian sources is shown in Figure 1. Of Australia's carbon dioxide emissions, 2.2% originate from agricultural activities. Agriculture plays a more significant role as a source of methane and nitrous oxide. The fourth major group of greenhouse gases, the CFCs, originate to no significant extent from agricultural activity.
It is worth describing what the major sources of Greenhouse gas emissions from agricultural activities are in some detail, and also what scope there is for limiting these emissions.
carbon dioxide
The emissions of CO2 originating from agricultural activities come predominantly from the operation of farm machinery and the production of fertilisers. In principle, farmers can reduce these emissions by 1. reducing the use of high energy options for farming operations (eg. mustering with horses rather than helicopters; or limiting the amount of chemicals applied by aerial spraying; or simply by minimising the number of cultivation passes over the land.); 2. using biofuels (ethanol, methane) derived from farm produce, and 3. attempting to reduce the use of fertilisers or replacing them with organic sources.
Additionally, agriculture can help alleviate the Greenhouse Effect by storing carbon on the farm in the form of either living biomass or organic carbon in the soil. Agricultural activities usually lead to a reduction in the amount of carbon stored on the farm either above or below ground. Tree farming, either in the form of permaculture or just the provision of shelter belts, can store considerable amounts of carbon in the trunks of trees. Frequent cultivation and the burning of stubble, on the other hand, all lower the amount of carbon retained in the soil organic matter, while mulching, zero tillage or use as pasture all tend to lead to a build up of the amount of soil organic matter.
Carbon storage on the farm can be quantitatively very significant. A hectare of forest may contain 100 t (or more) of carbon in tree trunks and 100 t of carbon in the soil organic matter. A paddock with wheat, on the other hand, may contain only 5 t of carbon above ground and lose 1% of soil organic matter per year of cultivation. By clearing a 1,000 ha farm, a single farmer may thus cause the immediate release of 100,000 t of carbon into the atmosphere, and cause the loss of another 1,000 t of soil carbon per year. Conversely a switch from annual soil preparation to zero tillage may arrest the carbon loss, and a switch to permaculture may lead to an increase of 2,000 t per year on the 1,000 ha farm. This compares with the average of about 5 t carbon emitted by the average Australian in one year. These figures are, of course, only indicative and would differ between regions, soil types and management systems.
methane
The major agricultural sources of methane are enteric fermentation in the guts of herbivores, biomass burning and anaerobic organic matter breakdown, mainly in flooded rice paddies.
Scope for limiting methane production from cattle lies firstly in a switch in diet in consumers from beef, which produces large amounts of methane per unit of protein to other animal proteins such as from chickens or fish, or to a more vegetarian diet, which would have health in addition to environmental benefits.
There is scope for limiting methane production per unit beef produced. Methane production is significantly increased with poorer nutrient content of the type of fodder ingested by the animals. Animals receiving a high-protein diet may release half as much methane per day as animals on a poor diet and may grow twice as fast.
Most cattle in Australia unfortunately graze in northern regions of Australia on very poor fodder. This can be artificially improved by providing non-bloat capsules or urea and molasses licks to animals. This may be already economical in many farming enterprises, and could get a significant further boost due to its environmental benefits.
About one third of Australian methane emissions are released during biomass burning. Of that, about 60% originate from grassland fires in Northern Australia and 15% from natural unpreventable bushfires2. The remaining 25% are released as part of agricultural and forestry management, and part could be minimised by phasing out stubble and sugar cane burning. Rice paddies also release significant amounts of methane, and that could be avoided by a switch to dryland rice production.
nitrous oxide
Nitrous oxide emissions originate largely from agricultural activities, and it is principally activities that involve the turn-over of soil nitrogen. During any process where nitrogen is converted from one chemical form to another, some of the nitrogen escapes into the atmosphere in the form of nitrous oxide.
As nitrous oxide is chemically very inert, it may remain in the atmosphere for 150 years before eventually being broken down by photochemical decomposition in the stratosphere.
The chemistry of nitrous oxide is still surprisingly poorly understood, and it is not entirely clear what its major sources are.
Large amounts of nitrous oxide might be released from nitrogen fertilisers, with estimates of the conversion of fertiliser N to nitrous oxide ranging from 0.01 - 2.0%4. The actual release is believed to depend on a whole range of factors such as fertiliser type, soil moisture and temperature, and farming practise such as the frequency of soil disturbance.
Some reductions in nitrous oxide release might be achievable through a switch from organic to inorganic fertilisers or greater reliance on legumes, as that would lower the concentration of inorganic nitrogen in the soil and give less opportunity for the escape of gases. However, while this is plausible, concrete experimental evidence in its support is still not available.
Reducing the release of nitrous oxide might only be possible through a reduction in nitrogen turn-over in the soil. As this would be associated with a decline in soil fertility, it would be a costly way to achieve reductions in nitrous oxide release. All in all, the paucity of information about the sources of nitrous oxide release make it difficult to recommend specific steps to reduce its emissions.
conclusions
Agriculture is reliant on a predictable and more or less benign climate. If the climate changes toward a harsher regime, productivity will decrease. If the climate becomes unpredictable farmers will find it increasingly difficult to make the correct managerial decisions to match species and management to their climatic conditions. A shift in climatic zones may also cause considerable hardship for many individual farmers in one region even if, nationally averaged, the inherent productivity of the country were to remain the same.
While being the biggest likely loser out of Greenhouse-induced changes, agriculture is also one of the main contributors. Methane and nitrous oxide, in particular, are chiefly of agricultural origin, and for methane the scope for modification to agricultural practises to minimise its release is significant. The control of nitrous oxide is more difficult, as its most significant sources, and the factors controlling these, are still only very poorly understood.
Some steps to limit the emission of Greenhouse gases may be warranted and economic in their own right, such as cessation of stubble burning or conversion to zero tillage regimes. Others may be justified by other environmental concerns, such as a shift to more organic farming methods, especially permaculture, or cessation of the growing of flooded rice because of concerns over rising ground water.
Other steps may not be economically attractive to farmers, but they could constitute an important contribution to limiting emissions as a national responsibility. For example, the provision of urea and molasse licks for cattle in the north of Australia would have considerable potential to limit the emission of Greenhouse gases. It may not be economic for individual cattle station managers to use licks, but it might be warranted to support such a move as a nationally supported and financed initiative to combat the Greenhouse Effect.
Perhaps a fundamental way that farmers can prepare for the future changes is by preserving their preeminent resource, the land, from further degradation. By protecting our land, water and biological resources and maintaining the biodiversity of ecosystems, farmers will have a broader resource base which can help them adjust to climate change.
references
1. Cicerone, R. and Oremland, R.; Biochemical aspects of
atmospheric methane; Global Biochem. Cycles 2; 1988.
2. Greene, D., Gavin, G., Armstrong, G., O'Dwyer, A.J., Braddock,
P.; Reducing greenhouse gases: options for Australia; Report
prepared for the Australian and New Zealand Environment Council;
1990.
3. Mitchell, J.F.B., Manabe, S., Tokioka, T. and Meleshko, V.;
Equilibrium climate change; Scientific assessment of climate
change (Houghton, J., Seck, M., Moura, A. eds), Intergovernmental
panel on climate change, working group I; 1990.
4. Watson, R. Rodhe, H., Oescheger, H. and Siegenthaler, U.;
Greenhouse gases and aerosols; Scientific assessment of climate
change (Houghton, J., Seck, M., Moura, A. eds), Intergovernmental
panel on climate change, working group I; 1990.
5. Wilkenfield, G. and Associates; Greenhouse gas emissions from
the Australian energy system: the impact of energy efficiency and
substitution; National Energy Research, development and
