There are numerous interlinked effects of climate change on livestock rearing. This activity is both heavily affected by and a substantial driver of anthropogenic climate change due to its greenhouse gas emissions. As of 2011, some 400 million people relied on livestock in some way to secure their livelihood.Kerr R.B., Hasegawa T., Lasco R., Bhatt I., Deryng D., Farrell A., Gurney-Smith H., Ju H., Lluch-Cota S., Meza F., Nelson G., Neufeldt H., Thornton P., 2022: Chapter 5: Food, Fibre and Other Ecosystem Products. In Climate Change 2022: Impacts, Adaptation and Vulnerability [H.-O. Pörtner, D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke, V. Möller, A. Okem, B. Rama (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, US, pp. 1457–1579 |doi=10.1017/9781009325844.012 The commercial value of this sector is estimated as close to $1 trillion. As an outright end to human consumption of meat and animal products is not currently considered a realistic goal, any comprehensive adaptation to effects of climate change must also consider livestock.
The observed adverse impacts on livestock production include increased heat stress in all but the coldest nations. This causes both mass animal mortality during heatwaves, and the sublethal impacts, such as lower quantity of quality of products like milk, greater vulnerability to conditions like lameness or even impaired reproduction. Another impact concerns reduced quantity or quality of animal feed, whether due to drought or as a secondary impact of CO2 fertilization effect. Difficulties with growing feed could reduce worldwide livestock headcounts by 7–10% by midcentury. Animal parasites and vector-borne diseases are also spreading further than they had before, and the data indicating this is frequently of superior quality to one used to estimate impacts on the spread of human pathogens.
While some areas which currently support livestock animals are expected to avoid "extreme heat stress" even with high warming at the end of the century, others may stop being suitable as early as midcentury. In general, sub-Saharan Africa is considered to be the most vulnerable region to food security shocks caused by the impacts of climate change on their livestock, as over 180 million people across those nations are expected to see significant declines in suitability of their rangelands around midcentury. On the other hand, Japan, the United States and nations in Europe are considered the least vulnerable. This is as much a product of pre-existing differences in human development index and other measures of national resilience and widely varying importance of pastoralism to the national diet as it is an outcome of direct impacts of climate on each country.
Proposed adaptations to climate change in livestock production include improved cooling at animal shelters and changes to animal feed, though they are often costly or have only limited effects. At the same time, livestock produces the majority of greenhouse gas emissions from agriculture and demands around 30% of agricultural fresh water needs, while only supplying 18% of the global calorie intake. Animal-derived food plays a larger role in meeting human protein needs, yet is still a minority of supply at 39%, with crops providing the rest. Consequently, plans for limiting global warming to lower levels like or assume animal-derived food will play a lower role in the global diets relative to now. As such, net zero transition plans now involve limits on total livestock headcounts (including reductions of already disproportionately large stocks in countries like Ireland), and there have been calls for phasing out subsidies currently offered to livestock farmers in many places worldwide.
Heat stress in livestock Projected worldwide increases
In general, the preferred ambient temperature range for domestic animals is between . Much like how climate change is expected to increase overall thermal comfort for humans living in the colder regions of the world, livestock in those places would also benefit from warmer winters. Across the entire world, however, increasing summertime temperatures as well as more frequent and intense heatwaves will have clearly negative effects, substantially elevating the risk of livestock suffering from heat stress. Under the climate change scenario of highest emissions and greatest warming, SSP5-8.5, "cattle, sheep, goats, pigs and poultry in the low latitudes will face 72–136 additional days per year of extreme stress from high heat and humidity".
In Jamaica, considered representative of the Caribbean region, all livestock animals besides layer hens are already exposed to "very severe" heat stress in the present climate, with pigs being exposed to it at least once per day during the 5 summer and early autumn months, while ruminants and broilers only avoid daily exposure to very severe heat stress during the winter. it has been projected that even at of global warming, "very severe" heat stress would become a daily event for ruminants and broilers. By , it would be felt for a longer duration, and extensive cooling systems would likely become a necessity for livestock production in the Caribbean. At , only layer hens would avoid daily exposure to "very severe" heat stress during the winter months.
Studies of heat stress and livestock had historically focused on cattle, as they are often kept outdoors and so are immediately exposed to changes in climate. On the other hand, a little over 50% of all pork production and 70% of all poultry production worldwide originated from animals kept entirely in confined buildings even around 2006, and the raw numbers were expected to increase by 3–3.5 times for pigs, by 2–2.4 times for layer hens and 4.4–5 times for broilers. Historically, livestock in these conditions were considered less vulnerable to warming than the animals in outdoor areas due to inhabiting insulated buildings, where ventilation systems are used to control the climate and relieve the excess heat. However, in the historically cooler midlatitude regions, indoor temperatures were already higher than the outdoor temperatures even in summer, and as the increased heating exceeds these systems' specifications, confined animals are left more vulnerable to the heat than those kept outdoors.
Health impacts of heat stress
Once the body temperature of livestock animals is above normal, this soon leads to "heat stroke, heat exhaustion, heat syncope, heat cramps, and ultimately organ dysfunction". Livestock mortality rates are already known to be higher during the hottest months of the year, as well as during heatwaves. During the 2003 European heat wave, for instance, thousands of pigs, poultry, and rabbits died in the French regions of Brittany and Pays-de-la-Loire alone.
Livestock can also suffer multiple sublethal impacts from heat stress, such as reduced milk production. Once the temperatures exceed , cattle, sheep, goats, pigs and chickens all begin to consume 3–5% less feed for each subsequent degree of temperature increase. At the same time, they increase respiratory and sweating rates, and the combination of these responses can lead to metabolic disorders. One examples is ketosis, or the rapid accumulation of ketone bodies, caused by the animal's body rapidly catabolizing its fat stores to sustain itself. Heat stress also causes an increase in antioxidant enzyme activities, which can result in an imbalance of oxidant and antioxidant molecules, otherwise known as oxidative stress. Feed supplementation with antioxidants like chromium can help address oxidative stress and prevent it from leading to other pathological conditions, but only in a limited way.
The immune system is also known to be impaired in heat-stressed animals, rendering them more susceptible to various infections. Similarly, vaccination of livestock is less effective when they suffer from heat stress. So far, heat stress had been estimated by researchers using inconsistent definitions, and current livestock models have limited correlation with experimental data. Notably, since livestock like cows spend much of their day laying down, comprehensive heat stress estimation needs to take account of ground temperature as well, but the first model to do so was only published in 2021, and it still tends to systematically overestimate body temperature while underestimating breathing rate.
Economic impact and adaptation
In the United States alone, economic losses caused by heat stress in livestock were already valued at between $1.69 and $2.36 billion in 2003, with the spread reflecting different assumptions about the effectiveness of contemporary adaptation measures. Nevertheless, some reviews consider the United States to be the least vulnerable nation to food security shocks caused by the negative impacts of climate change on livestock, as while it rates in the middle of the pack in terms of exposure of its livestock and the societal sensitivity to that exposure, it has the highest adaptive capacity in the world due to its GDP and development status. Japan and the nations in Europe have low vulnerability for similar reasons.
Meanwhile the exposure of Mongolian livestock to climate change is not very different from that of American livestock, but the enormous importance of pastoralism in Mongolian society and its limited capacity to adapt still renders it one of the most vulnerable countries in the world. Nations in sub-Saharan Africa generally suffer from high exposure, low adaptive capacity and high sensitivity due to the importance of livestock in their societies, with these factors particularly acute in Eastern African countries, where between 4 and 19% of livestock-producing areas are expected to suffer "significantly" more "dangerous" heat stress events after 2070, depending on the climate change scenario. There is high confidence that under the most intense scenario, SSP5-8.5, the net amount of land which can support livestock will decline by 2050 as heat stress would already become unbearable for them in some locations.
A range of climate change adaptation measures can help to protect livestock, such as increasing access to drinking water, creating better shelters for animals kept outdoors and improving air circulation in the existing indoor facilities. Installing specialized cooling systems is the most capital-intensive intervention, but it may be able to completely counteract the impact of future warming.
Difficulties in feeding livestock Climatic impacts on feed and forage
Livestock is fed either by letting them directly graze forage from pasture, or by growing crops like corn or soybeans for fodder. Both are highly important; the majority of soybeans are grown for fodder, while a third of croplands worldwide are devoted to forage, which feeds around 1.5 billion cattle, 0.21 billion buffalo, 1.2 billion sheep and 1.02 billion goats. Insufficient supply or quality of either leads to a decrease in growth and reproductive efficiency in domestic animals, especially in conjunction with the other stressors, and at worst, may increase mortality due to starvation.
This is a particularly acute issue when livestock herds are already of an unsustainable size. For instance, two-thirds of animal feed requirements in Iran come from its rangelands, which cover around 52% of its land area, yet only 10% have forage quality above "medium" or "poor". Consequently, Iranian rangelands support over twice their sustainable capacity, and this leads to mass mortality in poor years, such as when around 800,000 goats and sheep in Iran perished due to the severe 1999 − 2001 drought. This was then exceeded by millions of animal deaths during the 2007–2008 drought.