Following on from the first part of the blog, this second part explains some of the finer details and differences between GWP (global warming potential) and GTP (Global Temperature change Potential). Again to read part one of the blog click here or for the link to the original on the Food Climate Research Network, written by Martin Persson click here.
To recap, the argument for favouring GTPs for livestock products, was that for a distant temperature target, it only matters how much warming is left lingering from an emission today at the point when the temperature target is reached (and not how much warming we get leading up to this point).
If this is the argument for choosing GTPs over GWPs, one must also be aware of the other implications of this choice. First it is not reasonable to adopt a 100 year time horizon in GTP calculations. Instead the time horizon should reflect the time remaining until the temperature target is reached. For the 2 degree C target this is likely to happen sometime between 2050 – 2100. We have shown in a recent paper that if 40-90 years is a reasonable time horizon to use in GTP calculations, the resulting GTP value for methane today is 18, not 4. This is because the temperature impact of releasing a tonne of methane today increases rapidly the closer the point in time it is evaluated.
Second it does not make sense to adopt a constant time horizon for GTPs. (Why should we always be interested in the warming happening at some distant point in the future?). Instead as we approach the temperature target, the time horizon over which we evaluate GTPs should decrease, resulting in a valuation of methane that rise rapidly (since the short – term warming effects become increasingly important) reaching a value of 120 when the target is met.
Consequently, the impact of switching from GWPs to GTPs would have a modest impact on the carbon footprint of beef today: a reduction from 23.5 to 18.9kg CO2 – equivalents per kg carcasss weight, for average EU beef production. By mid-century however the carbon footprint calculated using GTPs could potentially rise to 63 kgCO2eq / kg beef, if it turns out that by then 2 degree C warming is imminent. The choice of metric this will have a large impact on the future role of the livestock sector in climate mitigation (more so than in the present).
Finally the discussion on metrics may partly obscure a key difference between emissions of fossil carbon dioxide and other, shorter lived greenhouse gases, while emissions of the latter will eventually be completely broken down and removed from the atmosphere, part of our carbon dioxide emissions (20-30%) will stay in the atmosphere for more than thousands of years. The practical implication of the latter is straightforward: the only way to stabilise carbon dioxide concentrations in the atmosphere is to bring down emissions close to zero. This is why we talk about a finite carbon budget; a target for cumulative carbon emissions we cannot exceed if we are to limit warming to 2 degrees C.
For other greenhouse gases, however, it is enough to stabilise emissions in order to stabilise atmospheric concentrations (though the higher the emissions, and the longer the lifetime of the gas, the higher the resulting concentration). As a result cone can compare the long – term climate impacts of an emission pulse of carbon dioxide with a constant emission rate for more short-lived greenhouse gases.
Put differently, reducing annual emissions of methane and nitrous oxide will relax the carbon budget compatible with a 2 degree target. Or, conversely for every tonne of carbon dioxide emission mitigated, we can increase annual emissions of shorter lived greenhouse gases forever without affecting long-term warming.
For methane, emitting one kg per year has the same long-term temperature impact as would a one-time emission of around 5 tonnes of carbon dioxide. For nitrous oxide which is both a more potent greenhouse gas and has a much longer atmospheric lifetime, an annual emission of one kg compares to a one-time emission of about 100 tonnes of carbon dioxide.
Using these numbers, the annual emissions from the global livestock sector (some 112 million tonnes of methane and 8 million tonnes of nitrous oxide), if held constant, have the same long-term climate impact as carbon dioxide emissions of roughly 1000 billion tonnes. This is in the same order of magnitude as the total remaining carbon budget under a 2 degree target!
Seen this way, achieving significant cuts in the global greenhouse gas emissions from the livestock sector – through productivity increases, technological development and dietary changes – can substantially raise the carbon budget compatible with the climate target currently agreed by the global community, hence increasing the likelihood that this target will actually be met.
Source: Food Climate Research Network