The use of CO2 equivalent (CO2e) is a significant barrier to reaching the goal of the Paris Agreement of keeping warming “well below 2C”
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This article was first published in Carbon Brief on May 12, 2016

Authors: Nathan Borgford-Parnell*, Johan C.I. Kuylenstierna, Steffen Kallbekken, Harro van Asselt, Kevin Hicks and Drew Shindell

The Ad Hoc Working Group on the Paris Agreement (APA), which is meeting for the first time in Bonn next week, should recommend that countries pledge emission reduction targets of each substance separately as part of their Nationally Determined Contributions (NDCs), instead of combining them into a single CO2 equivalent (CO2e) pledge.

The use of CO2e is a significant barrier to reaching the goal of the Paris Agreement of keeping warming “well below 2°C” above pre-industrial levels while also recognizing the “intrinsic relationship that climate change actions, responses, and impacts have with equitable access to sustainable development and eradication of poverty.”

Allowing countries to set separate pledges would send a powerful message that countries need not choose between their near-term sustainable development priorities and long-term climate goals; they can and should achieve both.

Bottom Lines

In their commitments leading up to the Paris Agreement, countries pledged to address emissions of a diversity of different substances, but then followed a longstanding practice of grouping all emissions together as CO2e, using 100-year global warming potentials, to show their “bottom line” commitment.

Focusing on CO2e sends the message that actions to reduce CO2 and any short-lived emissions are interchangeable, when they are actually complementary. We are not the first to highlight the problem of CO2e; Professor Myles Allen and colleagues argued in a new study that CO2e complicates efforts to reduce peak warming.

CO2e also obscures crucial differences between substances: how they act in the atmosphere, how long they stay there, and what other effects they have beyond warming.

For example, CO2 and methane both cause climatic changes such as higher maximum temperatures and precipitation changes that can hinder plant growth. However, high CO2 levels can also enhance plant growth, partly offsetting the damage from climate change.

In contrast methane acts in the opposite direction by contributing to the formation of tropospheric ozone, which harms plants and reduces crop yields. Reducing tropospheric ozone through methane reductions can actually give a triple benefit, as it is also harmful to human health and has been shown to suppress the ability of terrestrial ecosystems to sequester carbon.

Vulnerable systems

Reducing near-term warming is an important goal in itself. Even at levels well below 2°C, warming is already having an impact on vulnerable Earth systems such as semi-arid areas, that are drying further, and the melting cryosphere.

Prompt reductions in CO2 emissions are crucial to stabilizing the climate in the long term, but will take decades to be felt, because of the slow response of climate systems to changes in CO2 levels.

That means that addressing near-term warming also requires implementing strategies to reduce emissions of substances with short lifetimes but powerful climate impacts, such as methane, black carbon (soot), and hydrofluorocarbons (HFCs), known collectively as short-lived climate pollutants (SLCPs).

Such SLCP strategies also provide large health benefits as some of the measures significantly reduce the PM2.5 concentrations that cause the largest impact on human health and premature mortality, which are significant near-term sustainable development concerns.

Climate context

Distinguishing between near- and long-term concerns will help countries to frame climate action in the context of their national goals and objectives. This is one of the most valuable aspects of the Paris Agreement: through the NDCs, it has encouraged countries to define their contributions in the ways that are most meaningful to their citizens.

In the first round of pledges, this often meant highlighting near-term sustainable development objectives: but all 190 quantified commitments as CO2e. Mexico was the only country to include a separate pledge for a non-greenhouse gas, black carbon, given in CO2e and tonnes.

BP Table 2.jpg

Source: INDC texts as posted on the UNFCCC web portal for INDCs, available at:

We urge the APA to advise Parties in its guidance for NDCs to:

(1) pledge emission reductions of substances individually (e.g. as tonnes of each substance) rather than in CO2e;

(2) report on progress made in implementing and achieving their NDCs substance by substance, in tonnes;

and (3) voluntarily include pledges for non-greenhouse gas emissions, including aerosols that affect the climate, health, and ecosystems.

Transparent progress

Taking this approach would provide clear, unambiguous data for the United Nations Framework Convention on Climate Change (UNFCCC) to track progress towards global climate goals while also recognizing the distinct benefits of SLCP strategies.

It would make the system more precise and transparent, and would facilitate a more holistic approach to climate action, aligning closely with social, economic, and environmental priorities. This would also allow developing countries to focus on climate and development benefits of particular concern to them, such as changes in rainfall patterns, sea-level rise, human health, and agriculture.

Some may worry that this might result in a reduced focus on CO2 mitigation. We believe the opposite is likelier. Addressing each substance separately would clearly show how much each country is doing to reduce CO2 emissions in particular, instead of obscuring it within a CO2e calculation.

Moreover, successful near-term measures with immediate and visible benefits are likely to increase the willingness to push for emission reductions, and successful cooperation on mitigation of non-CO2 emissions could lead to wider cooperation on CO2 mitigation.

Another possible concern is that some countries may wish to retain flexibility to trade across substances through emissions trading systems that include multiple substances. The APA should address this issue, allowing flexibility but laying the ground rules of trading to safeguard environmental integrity by allowing countries to carefully evaluate the advantages and drawbacks of trading emissions with very different lifetimes and impacts.

Fortunately, much of the infrastructure and guidance is already in place to support separate NDC pledges and progress reporting. Countries already report substance-by-substance emissions, in tonnes, in their national greenhouse emissions inventories under the UNFCCC.

Although aerosols and precursor gases are not covered by the UNFCCC, the Intergovernmental Panel on Climate Change (IPCC) has developed guidelines for estimating and reporting emissions of carbon monoxide (CO), nitrogen oxides (NOx), sulphur dioxide (SO2) and volatile organic compounds (VOCs) based on methodologies for the Convention on Long-Range Transboundary Air Pollution (CLRTAP), which includes black carbon, to ensure consistent reporting.

Critical choice

In building a new architecture for the climate regime, Parties face a critical choice. They can continue to use a single CO2e metric that, in the past two decades, has often obscured critical differences between long-term and near-term climate change mitigation.

Or they can adopt a more accurate accounting system that highlights synergies between climate change mitigation, air quality, and sustainable development. If we are to achieve the vision of the Paris Agreement, the choice is clear.

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