Anthropogenic greenhouse gas (GHG) emissions are the major driver of climate change [1
]. Food supply accounts for 19% to 29% of global GHG emissions, the vast majority of it coming from the agricultural sector [2
]. The livestock sector contributes 15% of global anthropogenic GHG emissions, with pigs accounting for 9% of all livestock-related emissions [3
]. At the same time, climate change threatens global food security and affects livestock production, for instance, through effects on feed production, through livestock diseases or heat stress [4
]. Global temperature has already increased by 0.61 °C (with a 95% confidence interval of 0.55 to 0.67 °C) as compared to the pre-industrial period. Until 2100, global temperatures are expected to increase by about 1.5 °C (0.75–2.2 °C) for a low-emission scenario (emission mitigation) and up to above 4.5 °C (3.4–5.8 °C) for a high-emission scenario [1
]. This will result in more frequent heat stress situations, especially during the summer months.
Commercial, highly productive livestock systems in temperate zones mainly use confined livestock housing systems for pigs [5
]. This entails advantages in terms of the control of thermal environment, i.e., maintaining temperature within a specific range, e.g., by controlling the ventilation rate. Typical ventilation systems in livestock buildings control temperatures by variable air volume flows. However, during the summer months, their capacity is limited. In this regard, the question arises whether cooling systems involving an air treatment will become necessary for pig houses in temperate zones due to climate change in order to maintain livestock performance, including economic performance, and ecological impact at acceptable levels [6
In the past, animal genetics have been modified by breeding in order to increase output. Selection for primary performance traits have put several mechanisms of physiological regulation under pressure. This contributes, among other factors, to a specific vulnerability to heat stress in situations with high ambient temperatures [7
]. High performing livestock have to cope with high metabolic heat production due to increased metabolic rates. The occurrence of high ambient temperatures, increased frequency of diseases and increasing mortality following immunosuppression or increasing pathogen pressure, may result in negative effects on fertility and other performance traits, such as growth rate of pigs [8
Physiological mechanisms that lead to reduced performance include, among others, a reduced feed intake [10
] and a higher (maintenance) energy requirement due to thermoregulation [11
]. In the case of prolonged heat stress, there is a general change in the hormone status and the metabolic rate [10
]. When a decline in the adaptive thermoregulatory mechanisms becomes apparent, reduced well-being occurs [12
]. Below and above a certain temperature range, animals are no longer able to maintain their core temperature on a constant level, and hypothermia and hyperthermia, respectively, will occur. The critical upper limit for the body core temperature is about 42–45 °C [13
]. The limits of the thermoneutral zone (TNZ) for pigs depend on body mass and the age of the animals and vary widely in the literature. It has to be noted that these temperature values are not independent from the coinciding humidity values, which are considered together with the temperature in a temperature humidity index (THI). The majority of publications reviewed by Escarcha et al. [14
] (71%) dealt with climate change impacts (CCI) on ruminants. Monogastric livestock, particularly pigs and chickens, seem to receive less attention in the scientific literature, although their health and productivity may be even more affected by CCI [10
So far, considerations on the adaptation of livestock production systems (PS) to CCI mostly focused on policy advice (national and international climate research panels) or were limited to the technical adjustment of housing systems [15
]. No system analysis assessing the sensitivity to CCI specifically for pig production systems was identified in the literature, which takes into account the multiple relationships between the various elements of a PS.
Quantitative analyses of global warming on production costs and profitability have been conducted [16
] and also the effects of global warming on ammonia and odour emissions have been analysed in recent studies [17
]. However, the repercussions of global warming on environmental impacts and in particular on GHG emissions of confined livestock production systems are hardly addressed in the literature. Feed intake and energy demand for ventilation, heating and cooling in confined housing systems are highly dependent on temperature. Moreover, those inputs strongly contribute to the environmental impact of livestock production: life cycle assessments (LCA) found that the highest contribution to the environmental impact of pig production originates from feed supply, followed by on-farm energy inputs and on-farm animal- and manure-related emissions [19
]. LCA is a well-recognised method for the holistic assessment of environmental impacts, which has already been applied to a variety of pig production systems, see e.g., a review article [20
] or specific case studies, e.g., [19
]. However, climate change adaptation has hardly been addressed.
Therefore, the present study will focus on the effect of increased ambient temperatures on the performance of confined pigs. Furthermore, the effect of global warming on inputs for pig production, i.e., feed requirements or energy demand per quantity of product, will be analysed. Accordingly, the LCA reveals the environmental impact of pig production with rising temperatures. LCAs are not sensitive to adaptation options, which do not directly affect animal performance, resource use or emissions from pig housing systems. This will be covered by a semi-quantitative system analysis, which identifies important factors for a comprehensive climate resilience of pig farms.
Four hypotheses are tested: (1) An increasing temperature such as expected until 2100 does not significantly decline important performance traits including mortality in temperate zones in confined housing systems. (2) Rising temperatures will not significantly increase inputs, i.e., feed and on-farm energy requirements per unit of product. (3) The environmental impact of pig production regarding energy use and emissions of greenhouse gases, acidifying and eutrophic substances (see definition in Section 2.2
) does not significantly increase with global warming. (4) Effects of climate change other than those directly related to the performance of pigs, e.g., temperature increase and reduced precipitation in feed production, show a high impact on pig production systems (see Section 2.3
, Section 3.3
and Section 4.3
2. Materials and Methods
2.1. Model Calculations for Livestock Response to Temperature and THI Levels
To test the hypotheses above, different methods were combined. An indoor climate simulation model reveals indoor climate parameters for various adaptation options [15
]. Livestock responses were modelled based on data from various literature sources and were used in order to (i) derive basic functions for important production traits for confined pigs (i.e., body mass gain, feed intake, feed conversion and mortality), and (ii) for characterising pigs’ response to increasing temperature.
For the analysis of the basic functions for important production traits, the literature sources and model assumptions shown in Table S1 in the Supplementary Material
were used. Gompertz functions, linear and polynomic functions were derived from the sources listed in Table S1
. Feed conversion was described by combining results from functions for feed intake and for body mass gain. All basic functions represent traits achieved under good environmental and hygienic conditions, i.e., among others, in absence of heat stress.
Available data from the scientific literature were used to determine the effects of an increasing indoor temperature, mainly data measured in feeding trials with controlled thermal environments (climate chambers; see Table S2 in the Supplementary Material
for references). Responses to heat stress of feed conversion ratio, feed intake, body mass (gain) and mortality were derived for temperature alone and for a temperature humidity index (THI) specifically for the pig-categories sows, piglets, growing pigs and finishing pigs. Only few studies provide responses on humidity (plus temperature, feeding and performance data); hence, the results presented herein mainly focus on increasing temperature and correlated change of performance. For analysis of the THI effect, the THI-model according to National Weather Service Central Region [26
] was used, which has also been used in other studies for pigs [27
THI = ((1.8T) + 32) − (0.55 ∗ (RH/100)) ∗ (((1.8T) + 32) − 58)
T is the ambient temperature in °C and RH the relative humidity, ranging from 0 to 100 and divided by 100 to be expressed in percent. To test differences between performance at normal temperatures (THIs) vs. those under heat stress conditions, a two-sample t-test was used; level of significance was set at p ≤ 0.05.
Furthermore, worst case heat stress responses were identified from single studies: parameters (e.g., feed intake) from [29
] for finishers and from [30
] for growing pigs were included in a scenario “worst case situation”, which simulates a temperature increase by 4.5 °C indoors for a period from March to October.
The proportion of time spent under heat stress in a confined pig house (indoor climate) was derived from the indoor climate simulation model presented in [15
]. It is based on hourly meteorological data between 1981 and 2017 and considers thermal properties of the building, the energy release of the pigs as well as the ventilation system. The data were calculated for a site north of the Austrian Alps, which is representative for confined livestock houses. The altitude of the province in Upper Austria is around 300 m above sea level, the average annual precipitation in the period 1971 to 2000 amounts to 980 mm and the average mean annual temperature is 8.8 °C [31
]. According to the climate classification of Köppen and Geiger, the site is classified as “warm temperature, fully humid, warm summers (Cfb)” [32
]. Accordingly, the site is representative for large areas in Central Europe excluding the Alps [15
]. Additionally, the site is highly representative for intensive pig production, as it represents the highest class of pig density with >250 pigs per km2
The outdoor and indoor temperatures—the latter for typical houses with mechanical ventilation systems—and proportions of time spent under heat stress (times above critical temperatures) were calculated for the period 1981 to 2010. A comparison between the coldest year in this period (1984) and the warmest year (2003) shows that mean outdoor temperatures differ by 1.5 °C (8.2 vs. 9.7 °C), while the mean indoor temperatures differ by 1.0 °C (18.4 vs. 19.4 °C). The comparison of the heat stress potentials of these two years 1984 and 2003 is thus well suited to illustrate an effect of increased global temperatures by 1.5 °C until 2100 following the low emission scenario [4
For the heat stress effect on animal performance and on LCA impacts of pig production, proportions of the total annual time with occurrence of heat stress and with exceedance of the temperature limits were analysed. The indoor climate conditions were calculated for the housing systems without any technical changes over the time series. The following scenarios were compared for their heat stress effects:
For further methodological and technical details concerning the housing system (completely confined, fully slatted floors) and its mechanical ventilation system, see [15
2.2. LCA Method: Impact Categories, System Boundaries and Inventories
To assess the environmental impact of temperature increases on confined pigs, an LCA for the following impact indicators was used:
Cumulative energy demand (CED; method v.1.10; in MJ)
Global warming potential (GWP; in CO2
equivalents according to IPCC [4
Acidification potential (AP; CML-IA non-baseline V3.04/EU25; in SO2-eq)
Eutrophication potential (EP; aggregating freshwater and marine eutrophication from the method ILCD 2011 Midpoint+ V1.10/EC-JRC Global, equal weighting; N-eq and P-eq in PO4−-eq).
These environmental impact categories were identified as most important for pigs [20
]. The assessment was conducted using SimaPro v8.5.0 and included provision of infrastructure. Using Monte-Carlo simulations in SimaPro v8.5.0, a probability distribution was generated with 10,000 iterations for every LCA result. This can be used to identify the uncertainty range of a result and to test differences between results for their significance.
As shown in Figure 1
, the system boundaries for the inventory include material and energy inputs and emissions connected with livestock housing (manure management and enteric fermentation). Sow and piglet rearing were covered within the pig LCA. The main functional unit is 1 kg body mass of a finisher pig before slaughter, a further functional unit considered is 1 kg of reared piglet of 30 kg body mass at the farm gate.
2.2.1. Inventory Inputs
Feed was identified as the most relevant input in the LCA studies mentioned above. The feed intake per kg body mass gain was calculated from equations in Table S1
. For the feed ingredients, we used conventional default diets. The data for the average diet for sows (over the whole production cycle) including weaned piglets was derived from unpublished farm survey data and [33
]; the average diet for growing-finishing pigs according to [34
] can be seen in Table S3 in the Supplementary Material
. The heat stress scenarios do not account for specific dietary modification as an attempt to reduce heat stress. Theoretical concepts exist for potential benefits of specific feeding strategies or single feeding measures for adapting to heat stress [35
]. Some of these concepts are, however, questioned and generally applicable recommendations are lacking [36
]. Therefore, dietary modifications are hardly implemented by pig producers in the analysed site.
Other major inputs in pig production besides feed are: (1) electric energy for feed and water supply, lighting, ventilation, cooling and heating systems, etc., which was assumed with the Austrian electric energy mix; (2) the operation of lorries and/or tractors for transports of feed, manure etc.; (3) water.
Table S4 in the Supplementary Material
shows the quantities required per kg body mass gain and the sources used. Vaccinations and drugs were not accounted for in the inventory due to very low amounts applied and hence a very low influence on the impact categories addressed (CED, GWP, AP and EP). All environmental impacts related to energy and material inputs (e.g., feed) were calculated based on Ecoinvent data (v3.4) [37
], following an attributional LCA approach.
In intensive livestock production systems, feed is frequently produced off-farm, purchased at international markets and imported into the system [5
]. As a consequence, CCIs for feed crop cultivation are situated outside the system boundaries of the LCA; however, they are addressed in a semi-quantitative system analysis (see Section 2.3
and Section 3.3
2.2.2. Inventory Outputs
As shown in Figure 1
, inputs and outputs were allocated to the functional unit of 1 kg piglet or 1 kg finished pig. Livestock produces manure, which replaces synthetic fertilisers for an integrated production of feedstuffs on-farm, and which contributes to undesired losses. The latter consist of ammonia (NH3
), nitrogen oxides (NOX
) and dinitrous oxide (N2
O) emissions arising from manure management. Furthermore, methane (CH4
) emissions result from both enteric fermentation and manure management.
For animal-related emissions, emission factors from [38
] were utilised and typical slurry systems were assumed, with 50% of slurry stores being covered. The CH4
emissions were assumed to rise with increasing temperatures due to higher methane conversion factors [38
emissions from housing and manure management systems were modified for high temperature according to [17
]. The potential emissions for the low temperature scenario (based on the year 1984) are provided in Table S5 in the Supplementary Material
Slurry-N (reduced for N-losses from manure management), P2
O were accounted for as potential fertilisers that substitute a market mix of conventional mineral fertilisers according to [37
]. For approximately 0.03 kg N per kg finishing pig body mass gain, a potential utilisation efficiency was assumed of 70% of the nitrogen applied. The amounts of P2
O were assumed as 50% and 100%, respectively, of those for N.
2.3. Semi-Quantitative System Analysis
CCI effects on productivity parameters of the PS, which are in most cases outside the LCA system boundaries, e.g., CCI impacts on feed production, and interactions between PS elements, were estimated by a comprehensive system analysis. Besides the literature, e.g., [3
], experts’ knowledge was used for the system analysis, which was derived from a workshop with 13 experts and from additional interviews with selected experts.
First, system elements directly related to pig production (animals and housing) address parameters that could be affected by CCI or help to increase resilience against CCI. These are, for instance, health plans and measures to improve animal welfare, water supply of livestock, availability of affordable feed, feed losses, energy supply, access to CCI-related information and investments in CCI-relevant infrastructure. Second, other system elements for on-farm or off-farm feed production cover development of yields, locally adapted nutrient management, irrigation or landscape structures for greater biodiversity and more favorable conditions in terms of evaporation (hedges, trees, etc.). Third, further elements address soils or infrastructure, the type of tillage (e.g., conventional, conservation or reduced), the degree of using home-grown feed and feed storage capacities, farms’ water and energy supply or a potential impact on the water supply of neighboring farms. Fourth, an additional group of system elements covers economic issues that may contribute to resilience: cooperation of farms, knowledge exchange or social networking.
Finally, 97 potential system elements were identified in literature and from experts for pig PS, including the upstream chains for e.g., feed. The whole system was divided into the two subsystems “Climate change impacts and feed crop production” and “Climate change impacts and livestock”. The 13 and 12 most important system elements out of each approximately 50 elements were categorised to these subsystems, respectively (see Tables S6 and S7 in Supplementary Materials
). The interaction of each factor with all other factors was evaluated by the experts within the two subsystems. The intensity of the relationship between elements varied from 0, i.e., no or ambiguous impact, up to 2, i.e., a strong direct impact. These expert knowledge-based numbers were introduced into the software Systaim SystemQ [42
]. By using this software, both the direct and indirect feedback loops between the system elements were evaluated. The results of the analysis quantify the contribution of each element to achieve a climate resilient pig production system.