A straightforward way of reporting how livestock contribute to global warming

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A straightforward way of reporting how livestock contribute to global warming


by Dr John Lynch, LEAP, University of Oxford


02 April 2020


Reporting emissions the way we currently do obscures some key differences between the greenhouse gases emitted by agriculture, and we cannot use these figures to estimate temperature changes over time. New LEAP research demonstrates how an alternative means of relating different gases to carbon dioxide (CO2) can provide a straightforward means of overcoming these limitations.

When we hear about the climate impacts of agriculture and livestock, we are generally told the amount of ‘carbon dioxide equivalent (CO2e)’ greenhouse gas emissions these activities emit, either totalled per country or across the world, or the ‘carbon footprint’ of individual products. But current reporting methods cannot tell us if we will meet our climate change commitments. Our latest research, in the journal Environmental Research Letters, demonstrates an alternative and uncomplicated way of overcoming these limitations. 

It might be assumed that the challenge in providing a simple link between emissions and global temperatures is complexity: that the physical science is so complicated it requires a high level of expertise and access to supercomputers. The details of exactly how climate change will manifest across the world, changing local weather patterns and increasing the frequency of extreme events, for example, are indeed very complex. 

But at the level of global average temperature increases, we can distil the full complexity models down to some fairly straightforward principles. For many purposes this simplified picture will provide a sufficient level of detail, as global average temperature increases are generally agreed to be a good indicator of the more specific impacts through which climate damages are realised. This is why the Paris Agreement’s overarching goal is keeping global temperature increases  below a 1.5-2°C threshold.

For CO2, the biggest contributor to global warming, a surprisingly simple picture emerges. Each individual CO2 emission adds an essentially permanent amount of warming that is only undone if we manage to actively remove past emissions from the atmospheric carbon cycle. Other greenhouse gases, with much shorter atmospheric lifetimes than CO2, do not work in the same way. If a gas only has a short atmospheric lifetime, then we have to consider natural removals of the gas balanced against ongoing emissions.

The standard way of communicating ‘Carbon dioxide equivalent’ emissions is to use the ‘100-year Global Warming Potential’, or GWP100. The GWP100 imagines a one-off pulse emission of different gases, and scales the total climate impact this emission would have relative to CO2, for the first 100-years after the emission of either gas. This can provide one indication of the effect of individual emissions, but because it comes down to weighting each gas by a single number, the GWP100 essentially means that you have to treat each gas in exactly the same way. 

If we have a single reported ‘CO2e’ emission total, we don’t know how much is CO2, where each emission will add a permanent increment to global temperature, or how much is, for example, methane (CH4), where the gas will break down and much of its temperature impact will automatically be undone after a few decades. In turn, if we want to know how different emission scenarios contribute to global temperature increases over time, we cannot use the GWP100.

To overcome this, we suggest an alternative conceptual framework, denoted ‘GWP*’, which we use to derive ‘CO2-warming-equivalents (CO2-w.e.)’ because emissions reported using this approach correspond directly to global warming in the same way as CO2 emissions.

If a greenhouse gas has a short atmospheric lifetime, then if we continue emissions beyond this period, eventually the emissions will be balanced by natural atmospheric removals. Sustaining emissions of a short-lived gas at a stable rate will primarily maintain an elevated level of warming, in contrast to the ever-increasing warming that will occur if we continue emissions of a long-lived gas. For short-lived gases such as methane, then, an ongoing sustained rate of emissions can largely be thought of as equivalent to a single emission of a long-lived gas (e.g. CO2): both will add a relatively stable amount of long-term warming. This is the logic behind GWP*.

Under GWP*, long-term, stable methane emissions are reported as a fairly small CO2-warming-equivalent, as once the balance between emissions and removals described above is reached, there is relatively little additional warming. The catch is that changing the rate of methane emissions has a very strong effect, so increasing methane emission rates is reported as a large CO2-w.e. emission, while decreasing methane emission rates is reported as a negative CO2-w.e.

An alternative way of understanding this novel concept of equivalence can be gained by returning to our pulse emission of the short-lived gas: with GWP*, we no longer describe this as a single pulse-emission of CO2. Instead, the initial release is described as large CO2-w.e. emission, but after a couple of decades much of this CO2–w.e. is then subtracted, representing the natural removal processes occurring for the short-lived gas. By treating long-and short-lived gases separately, GWP* enables us to capture some of the dynamic differences between them. This is essential if we are to understand how different gases contribute to global warming and what needs to be achieved with each gas in order to stabilise global temperatures.

Appreciating these details is important if we want to know the role of food system emissions in global temperature change, as the short-lived gas methane is responsible for a large portion of the warming from agriculture. This is particularly the case for ruminant livestock: cows, sheep and goats, as they directly belch out significant amounts of methane as part of their digestive process. GWP* can provide insight into what different scenarios for livestock production will mean for our climate commitments. Upcoming work from LEAP will further explore the implications of GWP* for questions around livestock and global warming.

For an overview and further discussion of agricultural methane, see the Food Climate Research Network (FCRN) explainer ‘Agricultural methane and its role as a greenhouse gas’.