Carbon dioxide equivalent
Carbon dioxide equivalent (CO2e) is a standardized measure used to compare the global warming impact of different greenhouse gases (GHGs) based on their global warming potential (GWP). This metric allows for the conversion of the emissions of various gases, such as methane and nitrous oxide, into equivalent amounts of CO2, facilitating easier comparisons and assessments of their overall effects on climate change. For instance, methane has a GWP of 21 over a hundred-year period, meaning that one metric ton of methane contributes to global warming as much as 21 metric tons of CO2.
The significance of CO2e extends to climate change mitigation strategies, enabling policymakers and researchers to prioritize which gases to target for reduction. It also aids in understanding the total concentration of GHGs in the atmosphere relative to critical threshold levels necessary to avoid severe climate impacts, such as limiting global temperature increases to 2° Celsius. Additionally, CO2e plays a crucial role in emissions trading systems, where the ability to trade emissions credits is streamlined by the existence of a universal measurement standard. Overall, CO2e serves as a vital tool in evaluating and addressing the complexities of global warming and climate policy.
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Carbon dioxide equivalent
Definition
Various greenhouse gases (GHGs) such as methane, nitrous oxide, chlorofluorocarbons, and carbon dioxide (CO2) have different potential effects on global warming. In order to compare GHGs based on their global warming potential (GWP), CO2 is commonly chosen as a reference gas. The carbon dioxide equivalent (CO2e) for a given amount of GHG is defined as the amount of CO2 that would have the same GWP measured over a specified time period, usually one hundred years. More formally, the CO2e for emissions of a given gas is calculated by multiplying the amount of the gas emitted by its associated GWP.
For example, the GWP for methane over a one-hundred-year time horizon is 21. Thus, the CO2e of one metric ton of methane is equal to 21 metric tons. In other words, one metric ton of methane released into the atmosphere would have the same effect on global warming over one hundred years as would 21 metric tons of CO2.
Significance for Climate Change
The CO2e metric provides a universal standard measure by which to evaluate and compare the global warming effects of emissions of various GHGs, as well as to calculate the total effects of the GHGs present in Earth’s atmosphere. Those uses of CO2e have important implications for decisions about climate-change mitigation. On one hand, CO2e measurements of GHG emissions make it easy to compare the various impacts of different plans to prevent such emissions. The fact that the CO2e of methane is 21 times that of CO2 helps determine whether to focus on methane or CO2 reduction in a given context.
On the other hand, the calculation of total effect of GHGs expressed in CO2e concentration can provide a way to compare existing concentrations of GHGs in the atmosphere with theoretical critical threshold levels. Recent studies have confirmed that one such threshold is represented by a global temperature rise of 2° Celsius during the twenty-first century. In order to prevent surpassing this threshold, the total concentration of all GHGs must be less than 450 parts per million in CO2e. However, GHG concentration is already beyond that level.
In addition to facilitating global climate study, the CO2e metric as a reference for evaluating GHGs also plays an important role in emissions trading. Various national and international regulatory structures allow polluters that reduce their emissions below a certain level to trade emissions credits (the right to pollute) to other entities whose emissions are still above the maximum permitted level. The existence of a universal standard of emission measurement, CO2e, allows these credit systems to function much more effectively and efficiently.