Long Term Environmental Impacts of Cultured Meat

Introduction

Fig. 1: The first beef patty grown directly from animal cells. (Source: Wikimedia Commons)

It is well known that livestock production is a contributing factor to anthropogenic climate change. [1] The greenhouse gas (GHG) emissions associated with the animals themselves are predominately those of methane (CH₄) and nitrous oxide (N₂O) which are produced by their manures. Additional N₂O and carbon dioxide (CO₂) emissions are linked to livestock production processes through pasture conversions and the application of fertilizers to these pastures. When measured by 100-year global warming potentials, agriculture accounts for 15-25% of global GHG emissions. [2] A disproportionate amount of these environmental costs are incurred by the production of beef because CH₄ results in significantly greater radiative forcing per molecule than CO₂. [3]

The high environmental costs associated with beef have prompted research into the development of meat production methods that can circumvent the wider biological processes of livestock cultivation and thereby reduce GHG emissions (particularly CH₄ emission) per kg of consumption. In 2013, researchers at Maastricht University were able to produce a burger patty grown directly from animal cells (see Fig. 1), demonstrating proof-of-concept for one potential mode of agriculture-free beef.

Since 2013, the process of growing large muscle cultures from animal cells in a controlled laboratory or factory setting has gained a large amount of attention and prompted the development of several modes of production. The terms synthetic, lab grown, clean, and in vitro meat have all previously been used to describe such generated tissue. These terms have been deemed unsavory by consumers and the equally vague but more palatable term cultured meat is more widely used today. As of 2020, cultured meat is not available for commercial consumption but proponents of cultured meat often point to the large scale reduction of CH₄ and N₂O emissions as a sufficient reason for the eventual phasing out of livestock production in favor of cultured meat production methods.

However, it is important to remember that GHG emissions are not equally damning: one must compare not only the radiative forcing per molecule but also the atmospheric lifespan of the molecules. CO₂ remains in the atmosphere for thousands of years while CH₄ and N₂O have atmospheric lifespans of approximately 12 years and 100 years respectively. So, though it is true that CH₄ and N₂O cause much higher radiative forcing per molecule when compared to CO₂, the drastically different lifespans make the long-term environmental impacts of each difficult to compare, even under the condition of constant emission rate. [4]

The most common method for equating the environmental impact of different GHGs is through the use of a carbon dioxide equivalence metric, the standard of which is the 100-years Global Warming Potential (GWP₁₀₀). The GWP₁₀₀ value of each type of emission is computed by integrating the amount of radiative forcing a single emission pulse would produce over a 100-year period. The efficacy of GWP₁₀₀ values is therefore questionable when one tries to use these values to compare long-term impacts of sustained emission. In particular, the use of GWP₁₀₀ overstates the warming impact of short-lived gasses like CH₄. For this reason, we will follow the methods of Pierrehumbert and compare environmental impacts based on an atmospheric modeling approach rather than using GWP₁₀₀ comparisons. [5] Previous studies have used this model to compare projected temperature changes based on different livestock production models through equivalent mass of cumulative CO₂, CC_eq, (the relevant value in the model) comparisons. [6] But calculations of this value for cultured meat production systems has not been done.

Production Systems

Culture 1: The first cultured meat production system is based on a 2011 study by Tuomisto and Teixeria de Mattos. [7] In this system embryonic stem cells are grown in a stirred bioreactor. The medium for the growth is mainly composed of cyanobacteria hydrolysate but also contains vitamins and genetically engineered animal growth factors. The quantities of vitamins and animal growth factors are considered to be negligible and therefore do not contribute to the environmental impact of the system. The main environmental impact comes from the growth of the cyanobacteria and transportation of the cyanobacteria to the bioreactor. Additionally, the energy needed to stir the reactor for the 60-day growth cycle contributes to the cumulative footprint. In the most optimistic scenarios, the average emissions for this production system are 1.69 kg CO₂ per kg of cultured meant, with 0 emissions of CH₄ or N₂O. [5] The GWP₁₀₀ CO₂ equivalence value for this scenario is 1.69 per kg of cultured meat.

Culture 2: The second cultured meat production system we will consider is less optimistic. This system is based on a 2015 report by Mattick et al. [8] In this system muscle stem cells are initially placed in a bioreactor and allowed to proliferate for a 5-day period. The reactor is then drained, cleaned with sodium hydroxide, and filled with a new medium consisting mainly of peptides and amino acids from soy hydrolysis, glucose, and cornstarch. The cells remain in this medium for 72 hours to allow for mass gain and cell differentiation. The building size and energy footprint for the facility in this scenario is similar to that of a pharmaceutical plant. The final emissions produced in this scenario are 6.65 kg CO₂, 0.019 CH₄, and 0.0013 kg N₂O per kg of cultured meat. [4] These values equate to a GWP100 CO₂ equivalence value of 7.5 per kg of cultured meat.

The livestock production systems considered represent the best and worst case scenarios in terms of GHG emission totals. Note that the CO₂ emissions quoted here are emissions associated with the production cycle not with the animals themselves. These emissions result from tractor use in maintaining pastures and from the transport of grains and fertilizer.

Beef 1: The best scenario is based on production by an organic Swedish ranch. [6] This system does not involve using pesticides or fertilizers for pasture growth. Animal birth rates are limited to one per year and fast weight gain (limited CH₄ emission due to manure) is assumed. The estimated emissions are 0.90 kg CO₂, 0.80 CH₄,0.02 kg N₂O, with a 28.6 GWP₁₀₀ CO₂ equivalence value per kg of boneless beef. [4] The worst case scenario for livestock production is demonstrated by USA midwestern pastures. [6]

Beef 2: In this scenario fast weight gain is still assumed but extensive pasture fertilization occurs. This leads to average emissions of 5.4 kg CO₂, 0.8 CH₄, 0.06 kg N₂O, with a 43.7 GWP₁₀₀ CO₂ equivalence vale per kg of beef. [4]

Average emissions for each production system are summarized in Table 1.

Atmospheric Model

The atmospheric model used by Pierrehumbert is a two-box energy balance model that translates radiative forcing into temperature change. [5] The model accounts for a shallow mixed layer of gas and water that responds quickly to changes in emission rates. The shallow mixed layer gains energy at a rate proportional to the radiative forcing and loses energy to the atmosphere at a rate proportional to its temperature. The second box considered by the model is the deep ocean which is assumed to have a heat capacity much larger than that of the shallow mixed layer. The effect of the deep ocean layer can be seen when one considers the effect of reducing the emission rate of a gas from some large constant value to zero in a short amount of time.

We start by defining the temperature of the deep ocean as 0°C. Under the conditions of constant radiative forcing the shallow mixed layer will settle into equilibrium after a few years (a relatively short period of time). Because the heat capacity of the deep ocean is much greater than that of the shallow mixed layer the temperature after a few years is still approximately zero (no change). If the emission rate of gas being considered is suddenly reduced to zero the radiative forcing will be non-zero for several years beyond this time (due to non-zero the atmospheric lifetime of GHG emissions). After a long period of time (the meaning of "long" here depends on the lifetime of gas) the radiative forcing will tend to zero. The temperature of shallow mixed layer will quickly respond to the change in the forcing and rapidly drop but, by this point, the temperature of the deep ocean has increased to some non-zero value. This means that the shallow mixed layer will only decrease in temperature until it reaches the temperature of the deep ocean. The trend back to zero then occurs over the slow deep ocean time scale.

The key advantage of this model is the ability to account for different time scales. It is shown that for relatively short lived gasses like CH₄ and N₂O radiative forcing at any time is roughly proportional to the emission rate, whereas long-lived gasses result in radiative forcing proportional to the cumulative emission. Thinking in terms of warming cased by short lived gasses, this means that for on time scales longer than the lifetime of the gas, the warming is proportional to emission rate, so a constant rate of emission is equivalent to a fixed amount of a longer lived gas like CO₂. Using this line of reasoning an equivalent mass of cumulative CO₂ carbon, CC_eqcan be defined for short lived gasses so one can compare the long-term warming effects of these gasses for situations with a steady emission rate.

Where, τ is the atmospheric life time of the gas in years. The radiative efficiency of the gas a' is defined as the radiative forcing per unit mass of the gas in atmosphere in units of W m^-2 kg^-2. In the limit of sustained emission at a rate E in kg yr^-1 the radiative forcing will asymptotically approach a'τE (W m^-2). Two additional parameters are introduced: λ a climate sensitivity parameter and Γ a carbon sensitivity parameter.

Discussion

Following Pierrehumbert CC_eq values were computed for each production system and are summarized in Table 2. [5] The values listed assume a time frame of 1000 years and a steady production rate of 1 kg of bone-free meat per year. The values were computed using λ = 1.2 W m^-2 °C^-1 and Γ = 2 x 10^-15 °C kg^-1. [6,9] (Note that Γ has units of °C per kg of C not kg of CO₂). The first column gives the direct CO₂ emissions in kg of carbon per kg of CO₂. Columns two and three show the kg of CC_eq for CH₄ and N₂O respectively. The last column of the table shows the total CC_eq in kg for each production system over the 1000 year period.

Keeping in mind that in the model we are considering, kg of CC_eq is directly proportional to warming, we see the problems associated with cultured meat production processes as they stand today. Though the Culture 1 production system has the lowest environmental cost, it is abnormally low when compared to the other production systems considered. This likely due to the idealized conditions assumed in the study and the numbers are likely to be unrepresentative of a real world scenario. When comparing the remaining production systems we see that the Swedish ranch model actually has the lowest warming potentials over the 1000 year period. This contrasts the GWP₁₀₀ CO₂ values where the Culture 2 process has drastically lower environmental cost than either livestock production system. This discrepancy, as discussed previously, comes from the fact that GWP₁₀₀ values over emphasize long-term warming effects of short lived gasses like methane. Though cultured meat processes may one day improve, as it stands today neither livestock production nor cultured meat production are in keeping with long-term climate goals.

© Linsey Rodenbach. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] R. T. Pierrehumbert and G. Eshel, "Climate Impact of Beef," Environ. Res. Lett. 10, 085002 (2015).

[2] A. M. Dudley et al., "Uncertainties in Life Cycle Greenhouse Gas Emissions From U.S. Beef Cattle," J. Clean. Prod. 75, 31 (2014).

[3] N. Pelletier, R. Rasmussen, and R. Pirog, "Compartive Life Cycle Impacts of Three Beef Production Strategies in Upper Midwestern States," Agr. Syst. 103, 380 (2010).

[4] J. Lynch and R. Pierrehumbert, "Climate Impacts of Cultured Meat and Beef Cattle," Front. Sustain. Food Syst. 3, 5 (2019).

[5] R. T. Pierrehumbert, "Short-Lived Climate Pollution," Annu. Rev. Earth Planet. Sci. 42, 342 (2014).

[6] R. T. Pierrehumbert and G. Eshel, "Climate Impace on Beef: an Analysis Considering Multiple Time Scales and Production Methods Without Use of Global Warming Potentials," Environ. Res. Lett. 10, 085002 (2015).

[7] H. L. Tuomisto and M. J. Teixeira de Mattos, "Environmental Impacts of Cultured Meat Production," Environ. Sci. Technol. 45, 6117 (2011).

[8] C. S. Mattick et al., "Anticipatory Life cycle Analysis of In Vitro Biomass Cultivation for Cultured Meat Production in the United States," Environ. Sci. Technol. 49, 11941 (2015).

[9] M. R. Allen et al., "Warming Caused by Cumulative Carbon Emisions Towards the Trillionth Tonne," Nature 458, 1163 (2009).