Carbon dioxide equivalents

Guest contribution by Dr Chris McGrath

There is considerable confusion surrounding climate stabilization targets based only on atmospheric carbon dioxide (CO₂) levels and targets that group together all greenhouse gases and other factors using the term carbon dioxide equivalents (CO₂-e or CO₂-eq).

Because of this confusion some authors avoid using carbon dioxide equivalents altogether when discussing targets for stabilizing anthropogenic climate change.

The major advantages of carbon dioxide only targets are that they are simple to understand and can be measured directly with precision as illustrated by the famous Keeling Curve. The level of carbon dioxide is also the most relevant measure when considering ocean acidification, the “evil twin” of climate change.

However, because carbon dioxide is not the only greenhouse gas, policy targets based on stabilizing the combined effect of all greenhouse gases and other factors affecting the climate are important.

“Carbon dioxide equivalents” is a unit of measurement that allows the effect of different greenhouse gases and other factors to be compared using carbon dioxide as a standard unit for reference. The term is defined and used in slightly different ways in the context of emissions and atmospheric concentrations of greenhouse gases.

In the context of emissions of greenhouse gases, “carbon dioxide equivalents” refers to the amount of carbon dioxide that would give the same warming effect as the effect of the greenhouse gas or greenhouse gases being emitted. It is normally used when attributing aggregate emissions from a particular source over a specified timeframe. It is used in this way at national and international levels to account for greenhouse emissions and reductions over time. For instance, Article 3 of the Kyoto Protocol states targets for emissions reductions in terms of “aggregate anthropogenic carbon dioxide equivalent emissions of the greenhouse gases”. For example, Canada’s net greenhouse gas emissions across all sectors in 1990 totaled 593,998,462 tonnes of carbon dioxide equivalents.

In the context of atmospheric concentrations of greenhouse gases, the term “carbon dioxide equivalents” is used in two main ways:

• Some authors use “carbon dioxide equivalents” to refer to the concentration of carbon dioxide that would give the same warming effect as the collective effect of all of the greenhouse gases in the atmosphere. This approach excludes the cooling effect of aerosols (e.g. Stern 2007).
• Some authors define carbon dioxide equivalent concentrations as the net forcing of all anthropogenic radiative forcing agents including greenhouse gases, tropospheric ozone, and aerosols but not natural forcings. This approach includes the cooling effects of aerosols (e.g. Meinshausen et al 2006).

Targets for stabilizing temperature rises between 450-550 ppm “carbon dioxide equivalents” generally refer to the term in the latter usage, thereby considering all components of the atmosphere affecting global temperature rises. However, Gavin Schmidt noted the confusion surrounding the term on RealClimate in the context of comments by Tim Flannery that “we are already at 455 ppm carbon dioxide equivalents.”

The differences between the usages of these terms are very significant and care must be taken not to confuse them. The terms are very different quantitatively as is evident from the following points draw from IPCC (2007: 102):

• Atmospheric carbon dioxide reached 379 ppm in 2005 and was increasing by around 2 ppm per year.
• Including the effect of all long-lived greenhouse gases such as methane, the total concentration of atmospheric greenhouse gases was around 455 ppm carbon dioxide equivalents in 2005 (range 433-477 ppm).
• However, the cooling effects of aerosols and landuse changes reduce radiative forcing so that the net forcing of human activities was about 375 ppm carbon dioxide equivalents for 2005 (range 311-435 ppm).

While net forcing of all greenhouse gases, aerosols and landuse changes is roughly equal to the effect of carbon dioxide alone at the present time, this may not remain the case in the future particularly if atmospheric pollution levels decrease (thereby reducing the cooling effects of aerosols). Stabilizing at 450-550 ppm carbon dioxide alone could mean significantly higher levels of stabilization when measured in carbon dioxide equivalents, leading to much higher levels of temperature rise than 2-3°C.

There is a real potential for governments and policy-makers to misunderstand and mis-apply information based on using either carbon dioxide only figures or carbon dioxide equivalents. However, there is no doubt that these terms will continue to be used in the future. In such circumstances, the best solution is to clearly flag the differences in the usage of these terms.

Authors writing about policy targets for climate change and ocean acidification can minimise the potential for confusion by including in their introduction a statement similar to the following:

“Here we use atmospheric carbon dioxide concentrations to discuss policy targets for anthropogenic climate change and ocean acidification. The use of carbon dioxide alone should not be confused with stabilization targets based on ‘carbon dioxide equivalents’, which includes the effects of all long-lived greenhouse gases and, in some contexts, the effects of other radiative forcing agents such as aerosols and landuse changes (IPCC 2007).”

Posted by Chris McGrath on Wednesday, 1 September, 2010

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