The differences between those studies and the study at hand can be mainly attributed to three reasons: Firstly, the existing studies used outdated conversion factors for the global warming potential of methane and nitrous oxide. This results in lower carbon intensities for all product groups compared to the study at hand. In addition, this underestimation is more pronounced for animal-based products (methane being the main contributor). This is also why results increasingly diverge the more animal-based products were included (see [ 107 ]). Secondly, excluding fertiliser production from emission inventories lowered CFs [ 19 ]. Thirdly, CFs were lower because food consumption surveys were used for the calculation [ 16 107 ], which generally underestimate food quantities compared to statistical data [ 48 ]. In one case [ 16 ], individual variations in food consumption were translated to the variation in the diet CFs. So, their study was hardly comparable to the study at hand even though they calculated diet CFs with an MC simulation.
Comparing our diet CF results directly to other studies is challenging as there is a wide variety of possible methodological choices. Even if methods are similar, distinct data sources as well as the inherent uncertainty and variability may lead to substantially different results and, therefore, limited comparability [ 106 ]. For the average French diet, results are lower (1522 kgCOe cap 16 ]) than our result for O_medium. Calculations for the average diet in the European Union underscored our vegan diet (605 kgCOe cap 19 ]). The outcomes in a study regarding diet CFs of average Dutch women were much lower than our results for dietary patterns with higher meat consumption, while their vegan and vegetarian diet CFs were more in line with the modelled diet CFs [ 107 ].
The increasing confidence interval widths from Vegetarian CF to O_high indicate that the more animal-based products were prevalent in a diet, the larger the 90% confidence interval ( Figure 2 ). This is because only distributions for animal-based product GHGEs were used in the calculation. If value ranges for plant-based GHGEs are also fed into the model, the CF widths would increase for all diets. However, this effect would be much smaller compared to animal-based products since GHGE ranges for plant-based products are in general far lower than for animal-based products [ 61 ].
The diet CF probability distributions’ shapes evolved due to the log-normal probability distributions for animal-based product GHGE values that fed into the model. The literature and the model (when medians are regarded) agree that per weight unit beef, cheese, and other red meat cause the highest carbon emissions and that particularly beef displays by far the highest GHGEs [ 4 48 ]. The outcomes of this study confirm findings [ 61 ] in which beef displayed the largest GHGE range, followed by fish/seafood. Equivalent to our study, they found much narrower ranges for cheese, pork (comparable to other red meat), poultry, eggs, and other dairy. Additionally, another study [ 103 ] found particularly highly varying carbon intensity ranges for ruminant meat as well as fish/seafood and lower variations for other animal-based products. The reasons for the magnitude of GHGE ranges in animal-based products are very difficult to trace since the effects may level out or reinforce each other. Large ranges for both beef and fish/seafood GHGEs can emerge from high variability of emissions due to different production systems and, in the case of fish/seafood, distinct products included in these ranges. Aquaculture and trawling fishery for instance emit more GHGEs than non-trawling fishery, while more extensive beef production systems display far larger GHGEs than intensive systems or coupled dairy-beef systems [ 103 ]. So, ranges become narrower when disaggregated into product categories and/or production systems [ 61 103 ].
Emissions from meat consumption arise directly from livestock farming, especially methane and nitrous oxide from cattle breeding. Also feed requirements contribute to high GHGEs. For one calorie of meat, multiple calories of feed have to be provided [ 9 ]. With few exceptions, plant-based products have lower GHGEs than those animal-based products with the lowest GHGEs, e.g., milk and eggs. Plant-based products with relatively high GHGEs are some vegetables grown in heated greenhouses, alcohol (which has a high processing intensity), and rice, whose wet cultivation causes high methane emissions [ 65 103 ]. Nevertheless, plant-based foods have comparatively low GHG intensities. Even if plant-based products with comparatively high GHGEs are consumed more in diets when switching from O_high to Vegan, they do not outweigh the effect caused by reducing animal-based foods.
Even though O_low still contains some meat and fish/seafood ( Table 1 ), it yielded substantial emission savings and displayed a similar CF to Vegetarian. Therefore, food categories must not be excluded entirely to achieve substantial emission reductions, confirming conclusions in the literature [ 18 102 ]. However, the findings of our calculations must be regarded cautiously since O_low displays very small amounts of meat consumption, translating to only occasional consumption, e.g., approximately one small beef steak per month.
5.2. Benchmarking Dietary Patterns’ Adherence to Global Warming Thresholds
The benchmarking displays that a higher prevalence of animal-based products reduces the likelihood of limiting global warming to 1.5 or 2 degrees. Our study is the first to show that all dietary patterns exceed the goal for the 1.5 degrees global warming target, even when quantifying uncertainty. This includes the vegan diet, which generated the lowest GHGEs. Therefore, even when 100% of the population follows a vegan diet right now, the food system’s emissions would still exceed the available threshold to limit climate change to a temperature rise of 1.5 degrees. O_medium and O_high for the most part exceeded both 2 degrees thresholds, while Vegan was below both limits. Vegetarian and O_low mainly adhered to both the 67% and 50% chance limits. Even though vegan and vegetarian diets could potentially offset high-emission diets, their current share in the population is too low. For instance, in Germany, the share is 8% [ 108 ].
No further studies are known that benchmark dietary pattern CFs which regard different shares of animal-based products against both the 1.5 and 2 degrees global warming target for the food system. Therefore, comparability remains limited. A study [ 37 ] benchmarked global dietary recommendations (World Health Organization, USA, Australia, Germany, China, and India) to total per capita emission thresholds available in 2050 for reaching the 1.5 and 2 degrees global warming targets. All recommendations were below the 2 degrees limit and all recommendations except the national Indian guideline, which was low in meat quantities, exceeded the 1.5 degrees global warming threshold. Another study [ 2 ] projected the global food system GHGEs under different emission reduction scenarios and assessed their compatibility with total GHGEs available to remain below the 1.5 and 2 degrees global warming targets. Their business-as-usual diet remained below the 2 degrees threshold and trespassed the 1.5 degrees global warming target. A mainly plant-based diet was able to cut emissions by half and thus did not trespass the 1.5 degrees emission limit. In contrast to our results, in both studies, dietary patterns with a much higher meat content than O_low are within the 2 degrees target (for comparison, most national recommendations include a meat content that falls between O_low and O_medium). However, both studies compare the emissions caused by the food system with the total available emissions threshold. The food sector already consumes a large portion of the budget, leaving little room for GHGEs arising from other sectors, such as mobility and housing.
2 e a−1) emissions. A further study [ 2 e per year in 2050 (business-as-usual scenario: 16.1 GtCO 2 e). The authors, however, did not relate the scenarios’ GHGEs to food system-specific thresholds to achieve the 2 or 1.5 degrees global warming targets. Willet et al. [ 35 ] downscaled a 2 degrees global warming limit to the food system and assessed for a business-as-usual case and three other dietary patterns (pescetarian, vegetarian, and vegan) whether a healthy universal reference diet complies with this limit. In their model, all diets except the business-as-usual case were able to meet the target. The average amount of meat in this “planetary health diet” is twice as high as in our O_low scenario, while the amount of milk and milk products is 10 kg higher. The 2 degrees limit is still complied with as the authors assume a much higher threshold for food system GHGEs (5 GtCOe a) emissions. A further study [ 36 ] proposed a scenario of a diet switch towards the planetary health diet [ 35 ]. This scenario leads to substantial emission reductions of the food system of up to 6.9 GtCOe per year in 2050 (business-as-usual scenario: 16.1 GtCOe). The authors, however, did not relate the scenarios’ GHGEs to food system-specific thresholds to achieve the 2 or 1.5 degrees global warming targets.
Even if these studies are not fully comparable with our approach, they clearly reflect what our results demonstrate: The reduction of animal products in the diet leads to drastic GHGE reduction potentials. Dietary shifts to more plant-based diets are necessary to achieve the global climate goals, but will not suffice. There is still a gap that must be closed. Firstly, even though one-fifth of all Europeans already consume animal-based products in moderation [ 18 ], it is not possible that the entire Western European population will switch to a vegan diet. Individual as well as cultural acceptance and the difficult shift in habits often prevent changing dietary patterns [ 109 ]. Secondly, emissions should be kept well below the 2 degrees threshold. To avoid adverse consequences of climate change it is crucial to adhere to the 1.5 degrees threshold, rather than the 2 degrees goal [ 1 ]. Thirdly, the thresholds themselves are optimistic estimates because other sectors were expected to reach net zero emissions in 2050 [ 2 ]. Fourthly, for each year between 2020 and 2100 in which GHGEs trespass the average annual limits for this timeframe, the emission reductions in the years to follow must be even larger. Fifthly, countries in the Global North are required to reduce their GHGEs more ambitiously than countries in the Global South due to “ the principle of common but differentiated responsibilities and respective capabilities”, stated in the Paris Agreement ([ 110 ], p. 22). Thus, the GHGE thresholds would be lower for Western Europe than the global GHGE thresholds we used in this study.
In addition to changing consumption patterns, therefore, a significant transformation on the production side is necessary. Reducing food waste as well as increasing yields and agricultural efficiency are further key drivers for approaching climate targets in the time from 2020 to 2100 [ 2 ]: Halving food waste can reduce total food system emissions by one quarter. Narrowing yield gaps and growing genetically modified crops are expected to reduce emissions by 15%. A decrease of 40% can be achieved by enhanced agricultural production (e.g., more efficient input application). However, Poore & Nemecek [ 6 ] show that producers face limitations on how extensively they can reduce the impacts of their production. They conclude that the impact of dietary changes exceeds those of technological improvements. Measures abating mainly carbon dioxide emissions, such as obtaining renewable energy and more efficient use of (fossil) energy sources, only have limited impact. This is because the food system primarily emits other GHGs than carbon dioxide [ 111 ]. Combining these findings with our results shows that diversified strategies in consumption and production are needed to achieve global warming targets. Future research should examine to what extent these mitigation measures offer pathways for individual dietary pattern carbon footprints to approach food system-specific climate targets.