In reporting on climate change, the carbon, carbon dioxide (CO2), greenhouse gases, radiative forcing, and CO2-equivilent (CO2-eq) are often used almost interchangeably to refer to the human contribution to recent warming.

However, this proliferation in terms and their subtly different meanings leads to a good deal of confusion among policy makers and scientists alike, to say nothing of the general public. In particular, the conflicting use of CO2 and CO2-eq across different documents and reports has muddied the waters, especially in the context of discussing levels of atmospheric concentrations associated with particular mean expected warming.

With all the focus on carbon footprints, carbon trading, carbon taxes, etc. it’s important to remember that there are other important greenhouse gases, such as methane, nitrous oxide, and various halocarbons, that also contribute to warming – though not so much as carbon dioxide. The increasing atmospheric concentrations of most of these gasses over the past few decades are also the result of human emissions, though measurements of specific sources often are plagued by considerably more uncertainty than is true for carbon. Additionally, there are aerosols emitted into the atmosphere that have a cooling effect. Figure One, from the latest Intergovernmental Panel on Climate Change (IPCC) report, shows the major climate forcings and their magnitude, and also the respective uncertainty ranges and level of scientific understanding.

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Figure One: Taken from the IPCC Fourth Assessment Report Working Group One Summary for Policy Makers.

Carbon dioxide equivalence is a simple way to normalize all these greenhouse gases and other climate influences in standard units based on the radiative forcing of a unit of carbon dioxide over a specified timeframe (generally set at 100 years).

For example, one ton of methane would be equal to 25 tons of CO2-eq, because it has a global warming potential 25 times that of CO2.

One major source of confusion surrounding the use of CO2-eq is that there are two different ways in which CO2-eq can be interpreted. In one interpretation, it is simply the sum of all positive greenhouse gas forcings. This approach pegs current atmospheric CO2-e concentrations at slightly more than 455 parts per million (ppm) CO2-eq.

The second interpretation sums both positive (greenhouse gas and land use changes) and negative (aerosol) forcings. In this case, atmospheric concentrations of CO2-eq are calculated by taking current CO2 concentrations, adding in other greenhouse gasses, and subtracting the cooling effect of aerosols. Conveniently enough, the mean expected negative forcing of current aerosol concentrations roughly cancels out non-CO2 gases, leading to a situation where both CO2 and CO2-eq concentrations are around 380 ppm.

These two different interpretations have led to quite a bit of confusion over the past few years. For example, the Australian biologist Tim Flannery, told the press last year that the then-upcoming IPCC report would reveal that atmospheric CO2-eq concentrations had reached 450 ppm 10 years ahead of schedule. Similarly, the influential Stern Review used the first interpretation of CO2-eq when discussing current atmospheric concentrations and the second interpretation when discussing stabilization scenarios.

The second interpretation of CO2-eq, where positive and negative forcings are summed, is becoming far more common. The coincidence of aerosol forcings effectively canceling out non-CO2 greenhouse gas forcings makes it easy to conflate CO2 and CO2-eq without causing much confusion. However, this becomes quite problematic when discussing future targets associated with specific atmospheric concentrations of greenhouse gases. It is very unlikely that effective atmospheric concentrations of CO2-eq and CO2 will coincide in the future, because emissions of methane and nitrous oxide are likely to increase while aerosol emissions will decrease.

As described in a recent article in The Yale Forum, on aerosols, the push by rapidly developing countries to improve health and local environmental quality is expected to reduce aerosol emissions significantly over the next century. In addition, steps to reduce carbon emissions will often have the unintended effect of reducing aerosol emissions, because the dirtiest sources of power generation (e.g., coal combustion) are also the greatest source of aerosol emissions. With aerosols having a short atmospheric lifetime, any change in aerosol emissions will result in an immediate change in CO2-eq.

These expected changes in aerosols and non-CO2 greenhouse gases are taken into account in the 2007 IPCC report. Figure Two shows the different stabilization scenarios developed for the Fourth Assessment Report and the respective CO2 concentrations and CO2-eq concentrations associated with each.

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Figure Two: Taken from the IPCC Fourth Assessment Report Working Group Three Summary for Policy Makers.

Ferene Toth from the International Atomic Energy Agency explains how this generates confusion in setting targets:

It is important to note a common confusion concerning concentration targets. It is somewhat not clear whether CO2 only or CO2-equivilent GHG concentration is meant. The former ignores the radiative forcing of non-carbon GHGs and the actual climate change which might be higher, corresponding to about an additional 100 ppmv increase in CO2 concentration. The latter raises the problem of GHG accounting in terms of CO2-equivilence. Confusion prevails even in high-level policy pronouncements, like the 1996 European Union declaration that global average temperature should not exceed the pre-industrial level by more than 2 degrees C and the CO2 concentration levels should not increase above 550 ppmv.

Because units of CO2 and CO2-eq have the same effective forcing by definition, excluding non-CO2 factors can lead to significant underestimations of actual warming. For example, if atmospheric concentrations of CO2 are capped at 450 ppm (the level generally associated with 2 degrees C warming), but concentrations of CO2-eq turn out to be closer to 550 ppm when non-CO2 forcings are taken into account, the world might end up with a 3 degree C warming rather than the 2 degrees expected by the policymakers setting the target.

Given the confusion surrounding the user of CO2-eq, a number of people and groups are pushing to make the terminology used more consistent or to change it entirely. NASA scientist James Hansen has suggested that a focus on CO2-eq is more trouble than it is worth. Hansen says the whole issue has “caused great confusion, to no benefit – we can and should talk about CO2 … the other GHGs are important, as it is a lot better if they reduce the CO2 requirement by 10 or 20 ppm, rather than exacerbate it by that much – but whoever started this should NOT have introduced CO2-equivalent, which just confuses everybody … when I talk about CO2 amount, I mean CO2 amount – that is the best way to go.”

Not all agree, saying we should drop the use of CO2 in isolation, or develop a tacit understanding that the use of carbon or CO2 always refers to the broad mix of climate forcings. Some question the merit of CO2-eq as a metric, pointing out that the idea of negative tons of CO2-eq generated by aerosol forcings is deeply counterintuitive. They advocate using raw units of radiative forcing as the common metric for discussing different gasses and factors influencing the climate.

Regardless of which approach prevails in the literature, journalists should understand the distinction between CO2 and CO2-eq, especially in the context of reporting on atmospheric concentrations of greenhouse gases associated with future mitigation targets.

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