Figure 1: a) The effect of constant CO2 emissions on warming showing that warming increases. b) The effect of constant emissions of methane on warming showing that warming is elevated but stable.11-15% of anthropogenic greenhouse gas emissions (GHG) are a result of raising livestock (Yusuf et al. 2012; Gerber 2013). The discussion surrounding the value of low-meat diets, such as vegan and vegetarian diets, in reducing global warming through reduced GHG emissions is ongoing. Livestock produce methane and CO2 emissions, which are currently defined by their 100-year or 20-year Global Warming Potential (GWP) where all gases are treated as equivalent to CO2 and the way it acts in the atmosphere.
This approach has limitations due to the variation between long-lived gases like CO2 and short-lived gases like methane, in the way that they are broken down in the atmosphere and their resulting effect on global warming (Shackley & Wynne 1997). New methods to define the global warming effect of greenhouse gases that take into account these differences are changing the way that we look at livestock emissions.
One way to visualise the differences in short-lived and long-lived GHGs is to imagine pouring water into a cup with a hole in the bottom at the same rate as the water was falling out the hole. In this scenario, the amount of liquid in the cup would remain constant. If the cup did not have the hole, the amount of water in the cup would increase. This is the same for GHGs.
Methane released into the atmosphere is like pouring water into the cup with the hole: the ongoing process of methane conversion means that it does not inevitably accumulate over time in the same way as CO2. However, CO2 is long-lived in the atmosphere; therefore, as more CO2 is emitted it accumulates, like pouring water into an intact cup.
This allows stark differences to be seen with the effect of these gases. So, how can a single metric of measurement give a true picture of their effects on global warming? Conventionally, GWP100 (100-year Global Warming Potential) measurements are standardised to the equivalent of a single burst of CO2 emissions (Epa 2017). This does not take into account the difference in the heating effect of short-lived gases like methane compared to long-lived gases such as CO2. A new metric that can encompass the warming effect of both short-lived gases and long-lived gases has been developed, GWP* (Allen et al. 2018).
There are two parts to the understanding of the effect that this has on defining methane emissions, one more complex than the other. The simple observations that can be made relating to methane emissions is as follows. Methane has a more potent warming effect than CO2; however, due to its short-lived nature, with constant emissions there is no increase in temperature as the amount in the atmosphere reaches an equilibrium where it is broken down at the same rate that it is added. The effect is an elevated but constant temperature (Figure 1b). In contrast, CO2 concentration has a practically linear effect on warming meaning that temperature increases with constant CO2 emissions (Figure 1a).
Figure 1: a) The effect of constant CO2 emissions on warming showing that warming increases. b) The effect of constant emissions of methane on warming showing that warming is elevated but stable.
However, as previously mentioned this does not fully explain the extent to which methane is broken down in the atmosphere because methane degrades into a small amount of CO2, which is long-lived. Therefore, short-lived methane emissions have an influence on the CO2 levels so that even when the warming effect of the original methane emissions have gone, the effect of the resulting CO2 is still prevalent (Figure).
Why is this important?
Livestock have been linked to emissions of both CO2 and methane. They directly produce methane as a by-product of their digestion and in their manures. A single molecule of methane has a stronger warming effect compared to a single molecule of CO2. Therefore, it has been widely claimed that reducing intake of meat would have a huge positive effect on the climate warming issue (Yusuf et al. 2012). If everyone became vegan overnight so that livestock associated methane emissions stopped, the temperature would decrease, and the warming caused by livestock methane would be undone in a relatively short time (Figure 2), discounting the warming caused by the resulting CO2.
Figure 2: Effect of reducing livestock numbers so that they are producing no methane emissions and the associated change in warming when discounting the resulting CO2produced when methane is degraded.
However, this new definition of methane emissions that takes into account that it is short-lived gives a different perspective of the potential effects of stopping meat intake and therefore its associated methane emissions. After this initial reduction, the temperature would level out again at a lower level. Also, while emissions are not increasing, no further addition in temperature as a result of methane would occur. In contrast, all CO2 emissions cause an increase in temperature, regardless if they are comparatively less than before because they will still cause an overall increase in the amount of CO2 in the atmosphere.
Therefore, with the aim to reduce global warming, reducing methane emissions (e.g. through reduction of meat intake) is helpful to provide an immediate reduction in temperature, but long-lived gases such as CO2 still need to be reduced in order to have a long-term effect on global temperature (Allen et al. 2018).
References
Allen, M. R. et al. (2018). A solution to the misrepresentations of CO2-equivalent emissions of short-lived climate pollutants under ambitious mitigation. npj Climate and Atmospheric Science, 1 (1), p.16.
Epa, U. S. (2017). Understanding global warming potentials. Recuperado el, 8. [Online]. Available at: https://www.epa.gov/ghgemissions/understanding-global-warming-potentials.
Gerber, P., Steinfield, H., Henderson, B., Mottet, A., Opio, C., Dijkman, J., Falcucci, A., Tempio, G. (2013). Tackling Climate change through livestock: a global assessment of emissions and mitigation opportunities.FAO, Rome.
Yusuf, R. O. et al. (2012). Methane emission by sectors: A comprehensive review of emission sources and mitigation methods. Renewable and Sustainable Energy Reviews, 16 (7), pp.5059–5070.
Shackley, S., Wynne, B. (1997). Global Warming Potentials: ambiguity or precision as an aid to policy? Climate Research, 8, p89-106.
Posted by Justine_Wickman on Monday, 4 May, 2020
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