The results of our study show that regional production must not be equated with low greenhouse gas emissions under all circumstances, e.g., if regional production requires the use of heating systems, when production abroad can proceed without heating. Even substantial emissions from long-distance transport or processing and packaging may be lower than those associated with producing vegetables in the cold season in central Europe. The analysis suggests that any evaluation of the total climate impacts of food products needs to consider the entire supply chain, including agricultural production, distributional transport, and packaging.
Based on the results of such studies, relevant strategies of the Council of the European Union (2006) that aim to support the development of rural areas with its regional production and shorter transport routes can be examined. The results, presenting greenhouse gases arising from each life cycle stage, help to understand the climate performance of options for overcoming seasonal production restrictions in colder climates. The results highlight that a differentiated approach is required for reducing the global warming potential of products.
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).
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.
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.
A relevant hotspot is the transport stage in unheated tomato production systems. The results of the study show that transport is a key parameter in unheated tomato production systems regarding the total global warming potential of 1 kg tomatoes at the point of sale (Table 2, Fig. 3). In unheated Austrian plastic tunnels, the carbon emissions from transport contribute with 85 g CO2e to the total greenhouse gas emissions. In system 1, transport accounts for 7 g CO2e per kg tomato product. The highest greenhouse gas emissions from transportation arise in the case of Spanish tomatoes with 392 g CO2e per kg which corresponds to 58 % of the total carbon emissions per kg product. In system 4, transport accounts for 196 g CO2e, which equates to 23 % of total greenhouse gas emissions per kg product at the point of sale in average supermarkets in Vienna.
The utilization of different data sources has a considerable impact on transport emissions. Here, the transport distance was 70 km, whereas Lindenthal et al. (interim report 2009; unpublished) found similar transport emissions for Austrian tomatoes, although the distance was 270 km from the production site to the point of sale.
Weber and Matthews (2008) and Hawkins and Dente (2010) found that, in general, carbon emissions from transporting agricultural products represent 0.5 % of total greenhouse gas emissions. This number correlates to the results of system 1. In contrast, when tomato production systems are less intensive (system 2), emissions from transport are higher per kilogram product because transport emissions refer to a lower yield. Attention must be paid to the fact that the studies mentioned have used aggregated data for different production sections on national per capita consumption and have not exclusively investigated tomato products.
However, transportation of tomatoes from Spain or Italy does not raise the related greenhouse gas emissions to a level above that of heated greenhouse tomatoes; these results accord with results from a comparison of tomato production in Spain and Great Britain by Williams et al. (2008) or as recorded by Müller-Lindenlauf and Reinhard for lettuce and other products (2010), where expenditures for the transport of Mediterranean vegetables in winter months do not outweigh the disadvantage of regional production.
Another focal point of discussion is the impact of agricultural upstream transportation from, e.g., mineral fertilizers or compost to the production site. Based on the results of their investigation, Weber and Matthews (2008) arrive at the proposition that upstream transportation requirements remain more important than final delivery, when analyzing global warming potential with data of food consumption by US households. However, this aspect depends upon several assumptions regarding, e.g., location of production plant, distance, means of transport, and storage, and is usually included in the calculations from LCA databases such as EcoInvent (2008). However, the present study concludes that upstream transports related to agricultural production of tomatoes does not make an influential contribution to the total global warming potential for 1 kg tomatoes (amounting to less than 1.5 %, assuming a 200-km transport distance). Furthermore, we based our calculations on an average load factor of 80 % and one single transport distance from the location of production to the supermarket in Vienna was assumed and empty return trips were not taken into consideration. Although this could lead to uncertainty, there is evidence that load factors do not influence total greenhouse gas emissions per kilogram product (O’Donnell et al. 2009).
Packaging was identified to be a major hotspot regarding greenhouse emissions along the supply chain of tomatoes. The influence of packaging upon the total global warming potential of a product depends on the type of packaging material and the tare weight. Furthermore, determining the amount of greenhouse gases arising from other life cycle stage is fundamental in considering whether the packaging stage represents a hotspot of a supply chain or not.
With reference to 1 kg tomatoes, the production of plastic containers made of PET causes greenhouse gas emissions of 71 g CO2e, whereas the production of the tins accounts for 447 g CO2e. Additionally, the processing stage preparing the fresh tomatoes accounts for 139 g CO2e per kg canned tomatoes.
The results show that plastic packaging represents a hotspot in the supply chain of unheated organic tomatoes (system 2) but not in heated cultivation in Austria (system 1). Furthermore, tin packaging is clearly a hotspot in supply chains. In system 4, carbon emissions resulting from packaging and processing are up to seven times higher than emissions resulting from the farm stage.
Greenhouse gas emissions from heating glasshouses used in vegetable production are a central issue in northern countries. Williams et al. (2006) calculated energy inputs in stand-alone boiler-heated British greenhouses, where heating represented 97 % of the total global warming potential of tomatoes. Due to the different forms of heating energy available, e.g., district heating with notably lower carbon emissions, our study finds that the proportion of the total global warming potential in this system is little more than 60 %. Compared to other studies, the heat requirement seems to be low in our study (Boulard et al. 2011; Torrellas et al. 2012). One reason is the reference unit used, in this case 1 kg packed tomatoes. Another major reason is the heating pause of 1 to 2 months usually in December and January and highly likely the favorable climate found in Vienna’s suburbs.
In urban Vienna, district heating is prevalent now, but there are still houses using natural gas as primary energy carriers. Furthermore, glasshouses with older technologies are still in use. Under these circumstances, the proportion of total global warming potential from heating can be 80 %, when using natural gas is used without re-using the fumes for fertilizing (Theurl 2008). Note, however, that our assumptions on heating energy requirements were conservative, so that the related carbon emissions might be higher than the values reported in Table 2. The use of renewable energy sources in the form of wood chip heating would reduce total greenhouse gas emissions by 50 % (Theurl 2008). In Austria, keeping plastic tunnels frost free gains importance in for soil-grown cultures and is partly allowed according to organic standards. Organic tunnels held frost free by the use of oil heating have significantly higher carbon emissions than glass house systems using district heating (Theurl 2008). Even where year-round heating is achieved through wood chips, higher carbon emissions arise in the case of plastic tunnel systems than in the case of Venlo or glass house systems. One reason for this is a higher energy demand due to a lower heat transfer coefficient of plastic material and another reason is the lower yields in soil systems (Theurl 2008).
In Austria, the market for organic products is still growing and the demand for domestically produced organic vegetables is rising accordingly. Therefore, heating options could become of a major focus for organic production, although increasing costs are certainly a limiting factor.
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).