Nathan Borgford-Parnellis staff attorney at the Institute for Governance and Sustainable Development.Johan C I Kuylenstiernais policy director of the Stockholm Environment Institute (SEI).Steffen Kallbekkenis research director at the Center for International Climate and Environmental Research.Harro van Asseltis senior research fellow at SEI’s Oxford centre.Kevin Hicksis deputy director of SEI’s York centre.Drew Shindellis professor of climate sciences at the Duke University Nicholas School of the Environment.

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 CO₂equivalent (CO₂e) pledge.

Glossary

CO₂equivalent:温室气体可以表达的of carbon dioxide equivalent, or CO₂e. For a given amount, different greenhouse gases trap different amounts of heat in the atmosphere, a quantity known as the global warming potential. Carbon dioxide equivalent is a way of comparing emissions from all greenhouse gases, not just carbon dioxide.

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CO₂equivalent:温室气体可以表达的of carbon dioxide equivalent, or CO2e. For a given amount, different greenhouse gases trap different amounts of heat in the atmosphere, a quantity known as…Read More

The use of CO₂e is a significant barrier to reaching the goal of theParis Agreementof keeping warming “well below 2C” above pre-industrial levels while also recognising 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-yearglobal warming potentials, to show their “bottom line” commitment.

Focusing on CO₂e sends the message that actions to reduce CO₂and 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 anew studythat 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 whatother effectsthey have beyond warming.

For example, CO₂and methane both cause climatic changes, such as higher maximum temperatures and precipitation changes, that can hinder plant growth. However, high CO₂水平还可以促进植物生长,部分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 tosequester carbon.

Vulnerable systems

Reducing near-term warming is an important goal in itself. Even at levels well below 2C, 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 CO₂emissions are crucial to stabilising the climate in the long term, but will take decades to be felt, because of the slow response of climate systems to changes in CO₂levels.

This means thataddressing near-term warmingalso 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 concentrations of fine “PM2.5” particulates 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 ofpledges, this often meant highlighting near-term sustainable development objectives: but all 190 quantified commitments as CO₂e. Mexico was the only country to include aseparate pledgefor a non-greenhouse gas, black carbon, given in CO₂e and tonnes.

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

1. 承诺减排的个性化的物质lly (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 recognising 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 CO₂mitigation. We believe the opposite is likelier. Addressing each substance separately would clearly show how much each country is doing to reduce CO₂emissions 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-CO₂emissions could lead towider cooperation on CO₂mitigation.

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) hasdeveloped guidelinesfor estimating and reporting emissions of carbon monoxide (CO), nitrogen oxides (NO_x), sulphur dioxide (SO₂) and volatile organic compounds (VOCs). These guidelines are 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.

Main image: CCFAS’s ‘future climates’ project in Nepal in 2012. The project involves a series of farmer exchanges, in which farmers from Beora will visit their “future climates”, to help them learn about how communities cope with higher temperatures and different rainfall patterns. Credit: Neil Palmer/CIAT viaFlickr.