Global Warming and Climate Changes

Climate models predict that a doubling of current atmospheric carbon dioxide (CO2) levels will cause a global increase of 1.4-5.9°C in mean surface air temperature by 2080 (Houghton et al. 2001). This increase in temperature is also likely to be accompanied by an increase in temperature variance. Moreover, extreme weather events that were previously rare for example, heavy precipitation or long droughts may become more frequent (Hulme and Jenkins 1998; Houghton et al. 2001). However, changes in temperature, precipitation and atmospheric CO2 levels could lead to mistaken conclusions about the magnitude and direction of environmental impacts (Abler et al. 2002).

Nevertheless, such changes have implications for pest, disease and weed outbreaks in agroecosystems (Risch 1987) through effects on physiological development, migration and dispersal. Although external inputs such as chemical fertilizers, pesticides and genetically modified varieties may provide some buffering against climate change in conventional agriculture, organic agriculture is far more dependent on internal resources within the system (Stacey 2003) and this has important economic implications for both conventional and organic farmers.

Atmospheric concentrations of the greenhouse gases carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) contributing to climate change are increasing at a rate of approximately 0.4, 0.6 and 0.25% per year, respectively (IPCC 1997). There is a growing interest in quantifying the significant sources and sinks of these trace gases and the international community has taken steps to reduce these emissions (Flessa et al. 2002).

The greenhouse gases and atmospheric loading due to agricultural production may be strongly influenced by the type of farming and land management system used (Flessa et al. 2002; Dalgaard et al. 2003). Agriculture plays a major role in the global fluxes of these greenhouse gases (Robertson et al. 2000; Flessa et al. 2002) and is assumed to be one of the major sources (Figs. 4.1 and 4.2), particularly of N2O and CH4. Nitrous oxide emissions from agriculture are estimated to account for more than 75% of the total global anthropogenic emission (Duxbury et al. 1993; Isermann 1994), the major part being produced in soils as an intermediate during nitrification and denitrification (Hutchinson and Davidson 1993). Overall, agriculture accounts for approximately one fifth of the annual increase in radiative forcing (IPCC 1997), which is a measure of the change in balance between incoming and outgoing radiation at the earth's surface.

Forestry

Waste wasterwater 2.8 %

Agriculture 13.5 %

Forestry

Waste wasterwater 2.8 %

Agriculture 13.5 %

25.9 % Energy supply

13.1 % Transport

19.4 % Industry

7.9 % Buildings

Fig. 4.1 Greenhouse gas emission such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) converted to CO2 equivalents, by sector in 2004 (Barker et al. 2007). Agriculture and forestry together play a major role in the global fluxes of the greenhouse gases carbon dioxide, nitrous oxide and methane

19.4 % Industry

25.9 % Energy supply

13.1 % Transport

7.9 % Buildings

Fig. 4.1 Greenhouse gas emission such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) converted to CO2 equivalents, by sector in 2004 (Barker et al. 2007). Agriculture and forestry together play a major role in the global fluxes of the greenhouse gases carbon dioxide, nitrous oxide and methane

11% Rice production (CH4)

12% Biomass burnin (CH4 + N20)

11% Rice production (CH4)

12% Biomass burnin (CH4 + N20)

Enteric fermentation

Fig. 4.2 Main sources of greenhouse gas emissions in the agricultural sector in 2005 (Smith et al. 2007). Soil emission and enteric fermentation are the main sources and agriculture is assumed to be one of the major sources particularly of nitrous oxide (N2O) and methane (CH4)

Enteric fermentation

Fig. 4.2 Main sources of greenhouse gas emissions in the agricultural sector in 2005 (Smith et al. 2007). Soil emission and enteric fermentation are the main sources and agriculture is assumed to be one of the major sources particularly of nitrous oxide (N2O) and methane (CH4)

Composting and biogas production are often suggested as measures for mitigating climate change. In this context, benefits of aerobic fermentation of manure by means of composting are ambiguous: while a shift from anaerobic to aerobic storage of manure can reduce CH4 emissions, nitrous oxide emissions increase by a factor of 10 (Kotschi and Müller-Sämann 2004).

4.3.3.1 Carbon Dioxide

Agriculture can help to mitigate climate change by either reducing emissions of greenhouse gases (GHGs) or by sequestering CO2 from the atmosphere in the soil. The global warming potential (GWP) of agricultural activities can be defined as GHG emissions in CO2 equivalents per unit land area or per unit product. The global warming potential of organic farming systems is considerably smaller than that of conventional or integrated systems when calculated per land area. This difference declines, however, when calculated per product unit, as conventional yields are higher than organic yields in temperate climates (Badgley et al. 2007). Under dry conditions or water constraints, organic agriculture may outperform conventional agriculture, both per crop area and per harvested crop unit. Typically, conversion from conventional to organic farming leads to a lower total fossil energy use (Flessa et al. 2002). Organic farming practices may result in a lower amount of CO2 production per area of agricultural land; but in most cases the reductions in the energy input were higher than the reductions in CO2 output from the production. Consequently, there are reports that energy efficiencies, defined as output per energy input, are higher in organic than in conventional farming. Reductions in fossil energy use lead to similar reductions in the emissions of CO2, which cause less GHG contribution (Dalgaard et al. 2003). Artificial nitrogen fixation for synthetic fertilizer manufacture and use in conventional agriculture consumes large amounts of non-renewable energy supplies responsible for CO2 emissions and contributes to the greenhouse effect. The same is true for emissions of N2O, which is approximately 300 times more powerful than CO2 in its contribution to the greenhouse effect (Vetterli et al. 2003).

Organically farmed soils are likely to be a larger sink for CO2 compared to many conventionally farmed soils (Jareckia et al. 2005). This is mainly because of their higher biomass levels fixed in the form of root material. Restoration of soil organic carbon (SOC) in arable lands represents a potential sink for atmospheric CO2. Strategies for SOC restoration by adoption of recommended management practices include conversion from conventional tillage to no-till, increasing cropping intensity by eliminating summer fallows, using highly diverse rotations, introducing forage legumes and grass mixtures in the rotation cycle, increasing crop production and increasing carbon input into the soil (Jareckia et al. 2005).

Arable cropland and permanent pastures lose soil carbon through mineralization, water and wind erosion and overgrazing. Global arable land loss is estimated to be 12 million hectares per year, which is 0.8% of the global crop land area or 1,513 million hectares (Pimentel et al. 1995). This rapid loss is confirmed by experimental data from Bellamy et al. (2005) in England and Wales. Between 1978 and 2003, they found carbon losses in 92% of 6,000 soil samples. Annual CO2 emissions from intensively cropped soils were equivalent to 8% of national industrial CO2 emissions. Therefore, if agricultural practices remain unchanged as it is in current intensive production systems, the loss of organic carbon in typical arable soils will continue and eventually reach a lower level than present. The application of improved agricultural techniques, e.g. organic farming, conservation tillage and agroforestry, however, stops soil erosion (Bellamy et al. 2005) and converts carbon losses into gains (Reganold et al. 1987) particularly due to the use of green and animal manure, conserving crop rotations with intercropping and cover cropping and composting techniques. Long-term field trials showed that organically managed soils have significantly higher organic matter content (Foereid and H0gh-Jensen 2004). Consequently, considerable amounts of CO2 may be removed from the atmosphere.

4.3.3.2 Nitrous Oxide

The global warming potential of conventional agriculture is strongly affected by the use of synthetic nitrogen fertilizers and by high nitrogen concentrations in soils. The primary reasons for enhanced N2O release from cultivated soils are increased N inputs by mineral fertilizers, animal wastes and biological N fixation (IPCC 1997). A constant emission factor of 1.25% for the amount of N applied to agricultural land is recommended for calculating global and national emissions from fertilized soils (IPCC 1997). Global nitrogen fertilizer consumption produced by fossil energy in 2005 was 90.86 million tonnes (International Trade Centre and FiBL 2007), which required approximately 90 million tonnes of diesel equivalents fossil fuel to produce or about 1% of global fossil energy consumption (Cormack 2000). Emissions of nitrous oxide are directly linked to the concentration of easily available mineral nitrogen in soils. High emission rates are detected directly after fertilization and are highly variable. For example, denitrification is additionally enhanced in compacted soils. According to IPCC, 1.6% of nitrogen fertilizer applied is emitted as nitrous oxide. In organic agriculture, the ban on the use of mineral nitrogen and the reduced livestock units per hectare considerably reduce the concentration of easily available mineral nitrogen in soils and thus N2O emissions. Immediate application of manure and slurry from dairy, beef, pig and poultry farms have also become an environmental problem because nutrients are often available in excess and over-fertilization of forage and arable crops occurs during its disposal. Emissions of carbon dioxide, nitrous oxide and methane are likely to be very high and water pollution may also occur when manures are not properly matured before application. Composting of farm manures and vegetable wastes according to the organic standards and regulations can thus help to reduce the global warming potential of food production.

4.3.3.3 Methane

Methane accounts for about 14% of the greenhouse gas emissions of which two thirds are of anthropogenic origin and mainly from agriculture (Duxbury et al. 1993; Barker et al. 2007). Even in highly industrialized countries such as Germany, the agricultural sector belongs to the most important national sources of CH4 and N2O emissions (Flessa et al. 2002). Biological CH4 production in anaerobic environments such as enteric fermentation in ruminant animals, animal waste processing and flooded rice fields are the principal sources (IPCC 1997). In addition, agricultural practices may also influence atmospheric concentration of CH4 by affecting its consumption in aerated soils. To a large extent CH4 emissions are directly proportional to livestock numbers. In Western Europe around 17% of CH4 emissions come from animal excrement. Organic animal husbandry methods commonly use straw for bedding and feeding, which becomes a component of manure, but much less is used in intensive conventional systems where liquid manures or slurries present great emission potential for methane and ammonia (Vetterli et al. 2003). Avoidance of CH4 emissions of anthropogenic origin and especially of agricultural origin is of particular importance for mitigation. Organic agriculture has a potentially important impact on reduction of CH4 emissions, as the overall population of livestock on organic farms is relatively small and breeding animals are replaced less frequently than in conventional systems (Kotschi and Müller-Sämann 2004; Olesen et al. 2006; Weiske et al. 2006). On the other hand, lower milk yields of organic cows and a higher proportion of roughage in the diet might increase CH4 emissions per unit of yield.

Organic Gardeners Composting

Organic Gardeners Composting

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