What are the CO2 emissions of Cats and Dogs?

#CarbonCatchers #EcoEducation

Despite the increasing awareness of humanity on the effects of carbon emissions and climate change, there is one major contributor which is continually neglected in calculations - pets. Given that there are hundreds of millions of household pets worldwide, their carbon impact has the potential to be very significant. The main source of their emissions comes from their food, which will be discussed in detail in this article, but their waste, impact on the local environment and toys also contribute to their ecological footprint. The combined emissions of cats and dogs in the US released annually as a result of food consumption is 64 ± 16 million tons CO2eq of Methane and Nitrous Oxide (Okin, 2017). In #China, this figure is 2.4-7.5 million tons CO2eq per year (Su et al., 2018). As many more developing countries increase their ownership of pets, and many owners are feeding their pets more high quality, carbon intensive pet food, the carbon emissions of pet ownership may increase considerably. However, this topic is widely overlooked in any calculations of CO2. This article serves to give a brief account of the carbon emissions of the two of the most popular pets worldwide - cats and dogs.

Cats

Cats require an estimated 341 ± 19 kilocalories per day (Okin, 2017). While this may seem relatively small in comparison with the amount of calories us humans consume per day, the contribution of animal-derived food products to cat’s diets is significant at roughly 31% compared with 19% for humans (Okin, 2017). This animal-derived food calculation is based on total consumption of red meat, poultry and fish for humans, and for animals is calculated from the ingredients list on pet food. Beyond their food, cat litter greatly contributes to the environmental impact of owning a cat. The majority of cat litter is made from clay which has been #strip-mined, a highly destructive process which removes the top layer of overlying rock to expose the minerals beneath (Bass, 2016). This greatly disturbs the natural environment, increasing the risk of #erosion in some areas as well as polluting the local water supply and wreaking havoc on local biodiversity (Squillace, 1990). Furthermore, clay litter is not #compostable or #biodegradable and will end up in #landfill.

Cats can also have a significant impact on the local wildlife. They are obligate carnivores, meaning they require meat in their diet for proper nutrition, and are built for hunting. In the US, it is estimated that free-ranging cats kill between 1.4-3.7 billion birds and 6.9-20.7 billion mammals annually. Common garden birds such as the robin, blackbird, house sparrow, goldfinch and wren as well as the ring-necked pheasant were the birds which made up the largest proportion of mortality caused by cats. The most common mammals killed by cats are mice, squirrels, voles and rabbits (Figure 1; Loss et al., 2013). A study of domestic cats in Britain found that on average, a household cat brings home 21 prey items per year (Baker et al., 2005). When this number is multiplied by the 8 million cats that are kept as pets in Britain, and you factor in the prey they capture but do not return home, this is a staggering figure. In fact, due to their impact on local #wildlife, cats have been listed as one the 100 worst invasive species in the world (Lowe et al., 2000). Cats that have been introduced to islands, such as Fiji and the Canary Islands, have resulted in or contributed to the extinction of a minimum of 14% of birds, mammals and reptiles as recorded by the International Union for the Conservation of Nature (IUCN; Medina et al., 2011). For example, the Hawai’ian crow is now extinct in the wild as a result of introduced cats which preyed on young chicks as well as spreading the parasite Toxoplasma gondii which decimated populations (Medina et al., 2011). Cats were introduced to Hawai’i by European settlers to be kept as pets.

Figure 1: Probability distribution of estimated mortality of birds (light grey) and small mammals (dark grey) in millions attributable to cats in the United States. Graph from “The impact of free-ranging domestic cats on wildlife of the United States”, Loss et al., (2013).

Figure 2: Cat approaching small mammal. Image by Katherine Volkoski.

Dogs

Man’s best-friends are unfortunately even worse than cats when looking at their environmental impact. Dogs on average require 1337 ± 106 kilocalories per day. This is not much less than the average requirements for humans (2469 kilocalories for men and 1817 kilocalories for women on average). Their diet also comprises a large amount of animal-derived products, 34% (Okin, 2017), however, unlike cats, dogs are not obligate carnivores and could survive on a vegetarian diet if given proper nutrition. In a book called Time to eat the Dog, the authors lay out the unsavoury fact that the amount of land needed to keep a medium-sized dog fed for a year (also known as its ecological footprint) is 0.84 hectares. This is more than double the ecological footprint of running a 4.6-litre Toyota land cruiser for a full year and driving 10,000 km, say the authors, and shockingly this is more than the ecological footprint of an Ethiopian, which is 0.67 hectares per year (Vale and Vale, 2009). The ecological footprint is a form of ecological accounting which measures human (or in this case animal) demand on the earth’s natural resources, so measuring the amount of land it takes to support a population annually. This way of measuring human impact on the environment enables easy comparison between regions and for the purpose of this article, between species. In terms of their waste, all of the dogs in the U.S. produce 10 million tons of waste annually, equivalent to the waste produced by 90 million humans (Folk, 2019; Schwartz, 2014).

Figure 3: Dog eating off the floor. Image by Matty Coulton.

Another issue with dogs nutrition is that quite often dogs are fed more than their nutritional needs, resulting in increased food waste and worsening their environmental burden. This is especially the case for some larger dog breeds (e.g. Labrador retriever, Golden retriever) (Su et al., 2018). In the United States, as of 2018 it was estimated that 56% of dogs were overweight or obese. A study in Australia found the figure was lower, at around 30% (Bland et al., 2010). In the UK, it is estimated that 1 in 3 dogs (33%) are overweight or obese (Lyons, 2015). Despite these varying figures across countries, one thing that is certain is that diet is the dominant factor in determining a dog’s weight (Heuberger and Wakshlag, 2011). The overfeeding of dogs is both bad for their health and the environment. Like cats, dogs also impact upon local #biodiversity through #predation, although not to the same degree as cats. A study from 2017 states that domestic dogs have contributed to the extinction of 11 vertebrate species, including the New Zealand Quail, and are a threat to a remaining 188 threatened species worldwide. Dogs threaten these species through predation, the spreading of diseases and disturbance to a species' natural habitat such as digging holes (Doherty et al., 2017).

Comparison

While dogs are responsible for more emissions than cats (based on their higher calorie requirement), the combined emissions of cats and dogs is staggering. In the same study that quantified the carbon emissions of cats and dogs in the USA, the authors also state that they consume 19% of the number of calories Americans do in a year. This figure is equivalent to the amount of calories consumed by the entire population of France in one year (Okin, 2017). In fact, if you ranked pets against other countries based on their meat consumption, they would be fifth following Brazil, Russia, China and the U.S. (Folk, 2019).

In comparison to other pets, smaller pets such as hamsters produce far less emissions, mainly as a result of their smaller size and thus reduced calorie requirements. Furthermore, smaller pets do not have the same effect on local biodiversity as cats and dogs do. They are also omnivores, so can have a mixture of plant-based and non-plant based foods, further reducing their environmental impact. The feed used to maintain fish also has a much lower environmental cost than that of cats and dogs, however, something to be conscious of with fish is the electricity required to run the tank and keep the water aerated. The bigger the tank, the more energy that is required. The amount of energy used to aerate a normal sized tank (30” x 12” x 18”) is typically 200 kWh per year. This will be greatly increased if using a heater for tropical species however, and could be closer to 2000 kWh per year (Kase, 2019). But undoubtedly the most negative aspect of keeping fish as pets, is the trend of keeping rare, ornamental fish in private home aquariums. These fish are often endangered and captured from the wild, thus posing a real threat to biodiversity (Papavlasopoulou et al., 2013).

Figure 4: A yellow tang, a tropical reef fish that is extremely common in home aquariums. Photo by Craig Lovelidge.

How to become an eco-friendly pet owner

In order to reduce your pet’s #ecological footprint, the most important factor to look at is their food and nutrition given that this makes up the bulk of their #emissions. Feeding pets nutritionally recommended portions and not overfeeding them will greatly reduce the amount of food consumed as well as waste, thus reducing their overall environmental impact. Reducing meat consumption for cats is not really an option but as dogs can live on a more plant-based diet, maybe consider looking into this as an option. However, be sure to consult with your vet before attempting any changes to your pet’s diet. Researchers have also pointed out that smaller cats and dogs require less nutrition and so have less of an environmental impact than larger animals (Okin, 2017). This is another potential way to reduce a pet’s ecological footprint if you are concerned about it.

Another way to be a more environmentally responsible pet owner is to use #biodegradable poop bags. Compostable cat litter is also available and greatly reduces the environmental impact of a pet cat. Additionally, these plant or paper based litters are safer for your cat as ingesting clay litter leads to intestinal damage (Bass, 2016). Additionally, in order to reduce your dog or cat’s ecological burden, researchers recommend keeping your cat indoors where possible and keeping your dog or cat contained when outdoors so as to reduce the chances of them killing local #wildlife (Legge et al., 2020).

In conclusion, there is a substantial secret environmental cost to having a cat or a dog as a pet. While researchers are not recommending we do away with companion cats and dogs, they implore that we take their ecological footprint into consideration and make efforts to reduce their impact where possible.

Trends

• Pet ownership is increasing in developing countries, particularly in Asian countries such as China, where it has nearly doubled over the past 7 years. This is expected to increase by a further 50% in the coming years.
• The standard of pet-food is continually increasing, with many luxury brands on offer. More and more pets are consuming meat, and some pets even consume more meat than their owners, leading to a larger ecological footprint.
• The number of cats and dogs which are obese is increasing. This is a trend which vets have attributed to poor diet choices and overfeeding.

Calculations

• The USA has the highest ownership of cats and dogs worldwide with an estimated 70 million dogs and 74 million cats. A close second is China with 27 million dogs and 53 million cats as pets.
• The combined annual emissions of all domestic cats and dogs in the US is the same as the annual emissions of the entire population of France.
• Invasive cats have contributed to the extinction of 14% of mammals, birds and reptiles from the islands of the world. Domestic dogs have contributed to the extinction of 11 vertebrate species including the New Zealand Quail.

References:

Baker, P., Bentley, A. J., Ansell, R. J., Harris, S., 2005. Impact of predation by domestic cats Felis catus in an urban area. Mammal Review, 35(3-4), 302-312.

Bass, O., 2016. Cat litter is big business and has a big environmental impact. RecycleBlu, accessed May 2020, https://www.recycleblu.com/news/cat-litter-is-big-business-and-has-a-big-environmental-impact/

Bland, I. M., Guthrie-Jones, A., Taylor, R. D., Hill, J., 2010. Dog obesity: Veterinary practices’ and owners’ opinions on cause and management. Preventive Veterinary Medicine, 94(3-4), 310-315.

Doherty, T. S., Dickman, C. R., Glen, A. S., Newsome, T. M., Nimmo, D. G., Ritchie, E. G., Vanak, A. T., Wirsing, A. J., 2017. The global impacts of domestic dogs on threatened vertebrates. Biological Conservation, 210(A), 56-59.

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Papavlasoupoulou, I., Vardakas, L., Perdikaris, C., Kommatas, D., Paschos, I., 2013. Ornamental fish in pet stores in Greece: a threat to biodiversity? Mediterranean Marine Science, 15(1), 126-134.

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