Releasing a ton of carbon dioxide into the atmosphere has quite a different effect on the global average temperature than releasing a ton of methane. Have you ever wondered how the effects of different greenhouse gases are compared? Designing appropriate metrics is nontrivial but essential for setting standards and defining abatement strategies to limit anthropogenic climate change, as was done for example in the Kyoto Protocol. Yes, we are talking about a “carbon footprint.” Do you know how it is defined?
The standard unit for measuring carbon footprints is the Carbon dioxide equivalent (CO2e), which is expressed as parts per million by volume, ppmv. The idea is to express the impact of each different greenhouse gas in terms of the amount of CO2 that would create the same amount of warming. That way, a carbon footprint consisting of lots of different greenhouse gases can be expressed as a single number.
Standard ratios are used to convert the various gases into equivalent amounts of CO2. These ratios are based on the so-called Global Warming Potential (GWP) of each gas, which describes its total warming impact relative to CO2 over a set period of time (the “time horizon,” usually 100 years). Over this time frame, according to the standard data, methane scores 25 (meaning that one metric ton of methane will cause the same amount of warming as 25 metric tons of CO2), nitrous oxide comes in at 298 and some of the super-potent greenhouse gases score more than 10,000.
The adequacy of the GWP has been widely debated since its introduction. The choice of a time horizon is a critical element in the definition. A gas which is quickly removed from the atmosphere may initially have a large effect but for longer time periods becomes less important as it has been removed. Thus methane has a potential of 25 over 100 years but 72 over 20 years; conversely sulfur hexafluoride has a GWP of 23,900 over 100 years but 16,300 over 20 years. Relatively speaking, therefore, the impact of methane – and the strategic importance of tackling its sources, such as agriculture and landfill sites – depends on whether you’re more interested in the next few decades or the next few centuries. The 100-year time horizon set by the Kyoto Protocol puts more emphasis on near-term climate fluctuations caused by emissions of short-lived species (like methane) than by emissions of long-lived greenhouse gases. Since the GWP value depends on how the gas concentration decays over time in the atmosphere, and this is often not precisely known, the values should not be considered exact. Nevertheless, the concept of the GWP is generally accepted by policy makers as a simple tool to rank emissions of different greenhouse gases.