Contrasted greenhouse gas emissions from local versus long-range tomato production

3.1 Comparison of the production systems in the three study regions

Table 2 records the global warming potential of tomatoes provided by four key supply systems at the point of sale in an average Viennese supermarket. Figure 3 presents the results in an aggregated form. The cultivation of organic tomatoes in plastic tunnels is associated with the lowest greenhouse gas emissions at the point of sale from all four systems, with absolute numbers of 180 g CO₂e. Austrian tomatoes grown in Venlo houses are associated with the highest carbon emissions from all four supply chains investigated in the study, corresponding to 1,397 g CO₂e per kg product. Both imported tomatoes from Spain and canned tomatoes from Italy, at 759 and 868 g CO₂e, respectively, produce nearly 1.5 to nearly 2 times lower greenhouse gas emissions than tomatoes from heated Austrian systems (Fig. 3).

Table 2

Global warming potential for 100 years in gram CO₂e for 1 kg of tomato product along the four key systems at the point of sale in an average Viennese supermarket

Item	System 1: Venlo, Austria	System 2: tunnel, Austria	System 3: multi-tunnel, Spain	System 4: open field, Italy
	Conventional	Organic	Conventional	Conventional
Greenhouse infrastructure (subtotal)	114	8	70	0
Steel	6	3
Aluminum	80
Glass	18
Plexiglas	2
Plastic		5	43
Iron			19
Concrete	8		8
Fertilizing and pesticides (subtotal)	27		61	38
Mineral fertilizer N	21		44	15
Mineral fertilizer P	0.74		1.41	1.13
Mineral fertilizer K	5.14		10.65	2.92
Manure compost		0.04	0.07	0.20
Organic fertilizer		0.11
Pesticides	0.39		5	19
Soil (subtotal)		8	59	36
N2O emissions direct		5	40	22
N2O emissions indirect		4	19	14
Humus sequestration		−0.75		0.34
Greenhouse management (subtotal)	1,144	5	103	12
CO2 fertilizing	280
Machine use	8	5	24	12
Desalination plant			79
Rockwool	8
Heating requirement	848
Transport (subtotal)	7	85	392	196
Transport short distance	7	85	18	18
Transport long distance			374	178
Storage	3	3	3
Processing and packaging (subtotal)	71	71	71	586
Processing				139
Packaging	71	71	71	447
1 kg packed product	1,367	180	680	868
1 kg fresh loose product	1,296	109	609	281

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The production of greenhouse elements such as steel and glass accounts for not more than 9 % of the total global warming potential in the tomato production systems studied. In system 1, the production of aluminum constructions is largely responsible for the 80 g CO₂e per kg tomatoes. The figure of 1 ton of aluminum employed per hectare of greenhouse area is remarkable. The production of plastic foil for the tunnels generally has a small effect on the total global warming potential (4 g in system 2), rising up to 43 g CO₂e per kg tomatoes in Spanish tunnels. The latter is equivalent to 2,624 kg plastic foil.

The production of fertilizers makes a contribution of about 27 g CO₂e to the total greenhouse gas emissions associated with conventional Venlo houses. Carbon emissions arising from the production of organic fertilizers such as vinasse and compost are negligible in the case of organic cultivation. System 3 in Almería has the highest global warming potential from nitrogen fertilizer input, with 44 g CO₂e, representing 10 % of the total carbon emissions per kg product. The results show very high greenhouse gas emissions from fertilizer input in the Spanish systems compared to Austrian production. At the same time, this leads to remarkable nitrous oxide emissions in this study, 59 g CO₂e per kg product, which are likely to be underestimated although we used the average standard leaching default value 0.3 in accordance with the IPCC guidelines (2006). The IPCC standard (2007) does not explicitly report on the fraction of leaching from drip irrigation. The data collected for the inventory report substantially higher amount of nitrogen fertilizer input for system 3 (Table 1). An explanation for the high absolute amounts of nitrogen fertilizer input in the Spanish productions systems is certainly the presence of inefficient management practices with large manure applications at greenhouse construction and insufficient consideration of the nitrogen amounts subsequently (Thompson et al. 2007). Nevertheless, in this study, we analyze a great heterogeneity of production systems and site-specific management decisions. According to the ranges of emission factors quoted in the IPCC guidelines (2006), it is very likely that the results provide uncertainties.

Carbon emissions from pesticide production on one hand emerge as negligible in system 3 and may not need to be accounted for, as the system relates to integrated pest management and the data on pesticide input refer to applications in exceptional cases. On the other hand, relevant carbon emissions stemming from the production of pesticide do arise in the open field production system.

According to the results, the contribution of organic farming to carbon sequestration in tomato cropping is minimal. Despite intensive open field production, the results show only a slight humus decrease in system 4. Regarding these low numbers, it is questionable whether carbon sequestration should be considered in the calculation of carbon footprints of high productive vegetable systems and whether compensation mechanisms should be taken into account at all (Schmidt 2009).

A further relevant contributor to the agricultural production stage is CO₂ fertilizing from stand-alone tanks, which accounts for 280 g CO₂e per kg tomatoes in system 1.

The major share of the emissions from the agricultural stage in soil-less Venlo systems originates from “greenhouse management” that includes heating and CO₂ fertilization, whereby carbon emission from heating is the dominant factor. A consistent result for unheated soil-grown tomato systems is found, i.e., that emissions from the agriculture are considerably lower and are dominated by emissions from nitrogen fertilizer application and production. Using 1 kg tomatoes as the functional unit enables a reliable comparison of production systems but does not refer to a difference in fruit quality. In comparison with greenhouse gas emission arising at farm stage from previous studies, carbon emissions differ slightly in the respective production systems (Table 11, Boulard et al. 2011).

Energy consumption from storage of the fresh tomatoes is associated with very low greenhouse gas emissions. Even in system 2, where the total global warming potential appears to be low, storage contributes only about 3 % to the total carbon emissions.

3.2 Identifying hotspots in tomato production systems

Interestingly, when analyzing different tomato production systems, there is no unique “hotspot” valid for all the systems studied (Fig. 3). Depending on the production system, the distance from the consumer, the prevalence or absence of heating, or even the packaging of the final product, different stages in the supply chain may be most important in terms of their contribution to total greenhouse gas emissions at the point of sale.

3.3 Consumer’s choice and policy

The aggregated results of the study presented in Fig. 3 show that it is important to take an integrative view when analyzing the global warming potential of different tomato productions systems: the global warming potential of organic production outweighs that of conventional cultivation, but organic regional production has the lowest global warming potential in the growing season, yielding from June to September in Austria. A surprising result of the study is that the seasonally grown Italian canned tomatoes have a relatively high global warming potential due to processing and canning, which is nearly three times higher than the long-distance transport from Italy to Vienna. Interestingly, CO₂ savings may be made in winter months in Austria by eating fresh tomatoes from Spain or canned tomatoes from Italy. A considerable CO₂ reduction potential exists in the substitution of energy sources, replacing current district heating with wood chips heating in Austrian Venlo houses. Interestingly, the global warming potential of Austria’s organic tomato tunnels requiring year-round heating with wood chips would outweigh that of the production of processed canned organic tomatoes from unheated tunnels (Theurl 2008).

In contrast to studies from Hauwermeiren et al. (2007), the present study found lower CO₂ emissions in the case of locally grown tomatoes produced without heating in summer, which can be explained by the differences in the tomato production systems of the regions studied in each case. Decision makers should not simply implement existing policy strategies because this study demonstrates that supporting regional producers as a single measure is clearly not an effective strategy for reducing climate impacts. Strategies for reducing greenhouse gas emissions should therefore include encouraging individuals to refrain from eating fresh tomatoes from energy-intensive tomato production from greenhouses in northern countries or to eat fresh or canned tomatoes from southern parts of Europe during the winter months.

Furthermore, CO₂ labeling of food products represents another important instrument in influencing consumer choices and this continues to be of growing interest also in terms of a broader implementation in the near future. In this context, it is very important to find ways to include production and transport to the retailer within the calculation used to ensure that labeling retains validity. From the policy perspective, consumers alone are not likely to be drivers for a transition from the existing food system to a decarbonized one. Rather, a targeted and coherent labeling policy is needed, which cannot succeed without establishing a robust linkage between policy and stakeholders, such as retailers (Gadema and Oglethorpe 2011; Schmidt 2009). Nevertheless, the instrument of life cycle accounting is already available and can help both to improve understanding of the sources from which greenhouse gas emissions arise and to identify where the optimization potential of product systems can be found (Schmidt 2009).