Considering Co-benefits: Counting the role of non-CO2 climate pollutants in the near and long term

The first principle governing the ICCT’s work, established in the 2001 Bellagio Memorandum, was to “design programs and policies that reduce conventional . . . pollution and greenhouse gas emissions in parallel.” Persistent air quality and health impacts in the UK and elsewhere in Europe resulting from policies that promote diesel vehicles for CO2 reductions and allow diesel vehicles to pollute more provide an example of why that principle continues to be so important.

The ICCT has completed viable policy roadmaps both for ambitious greenhouse gas and conventional pollutant emissions standards around the world. While the health co-benefits of fuel economy standards are relatively minor in comparison with direct emissions standards, the climate co-benefits of emissions standards can be significant. This is because the technologies that they require also control black carbon and other short-lived climate pollutants (SLCPs), which are a subset of the gases and particles that conventional pollutant standards directly targeted. We wanted to be able to put those co-benefits in perspective by adding up the near-term climate potential of both policy pathways. We found that in the near-term, currently enacted emissions standards add an additional 50% to the CO2-equivalent reductions from on-road vehicles in 2030, a climate impact that could be doubled by extending stringent emission standards throughout the rest of the world.

We are working with the Climate and Clean Air Coalition to help support adoption of stringent standards and policies to accelerate retirement of older vehicles, both to reduce SLCPs and the local air pollutants that are impacting health and quality of life in cities around the world. Diesels are the most deadly source of pollutants from the transportation sector, and are specifically targeted as a highly potent source of black carbon, the pollutant that is second only to CO2 in total climate impact. In comparison with other sources of black carbon, diesel vehicles emit a higher ratio of black carbon to other particulates that have a cooling counter-effect, so controlling diesel emissions has immediate and unequivocal benefits for climate.

ICCT’s policy roadmap to reduce conventional pollutant emissions from vehicles would achieve an additional 900 million metric tons (Mt) of CO2-equivalent reductions from on-road vehicles in 2030, if calculated over a 20-year time horizon. This would is a 75% increase over the potential 1,200 Mt of CO2-equivalent reductions estimated for greenhouse gas and fuel economy standards. Looking along a longer time horizon, the 100-year climate potential for emissions standards is much more modest. In other words, we can’t let up any slack on CO2, but here we see a clear example of how the immediate and pressing interests of local communities are very well aligned with the global imperative. As several members of the National Academy of Sciences wrote in 2009, pursuing policies that dramatically reduce short-lived climate forcers “can complement strategies for adapting to the effects of climate change by delaying warming for several decades, reducing adaptation costs, and mitigating risks to ecosystems and economic prosperity.”

The two figures below show the climate impacts of emission standards combined with fuel economy/GHG standards using both near-term time horizon (global warming potential over 20 years, GWP-20) and the longer term (GWP-100). The gray baseline divides the impacts from policies that have already been adopted (the light green and light blue wedges) and the potential impact of expanding and strengthening standards worldwide (darker green and blue wedges) as described in the ICCT global transportation energy and climate roadmap and global health roadmap. (The results presented here differ slightly as they have been updated to reflect policy changes since the publication of those reports.)

GWP-20: Near-term effects of policy regulating CO2 and SLCP from on-road vehicles.

GWP-100: Long-term effects of CO2 and SLCP from on-road vehicles.

The charts above show the CO2 and non-CO2 emissions reduction potential associated with GHG standards (blue) and emissions standards (green). CO2 emissions are all included in the blue wedges but both types of policies provide non-CO2 benefits. Thus the magnitude of both blue and green wedges changes, depending on if you are considering a 20-year or 100-year time horizon. Regardless of time horizon, GHG policies adopted in the last decade have contributed 2 Mt of CO2 reductions and extending these standards to other markets and beyond current rulemakings could prevent an additional 1.2 gigatons (Gt) by 2030. On a 20-year time horizon, non-CO2 emissions account for 45% of the climate emissions reduction potential: adopted GHG standards achieve an additional 0.3 Gt of CO2-e from reductions in N2O, CH4, and HFCs, and have the potential for 0.2 additional Gt; and adopted emission standards have achieved a 1.2 Gt CO2-e reduction and have potential for 0.9 Gt further reductions (this includes the climate benefits of reducing BC and the counter-effects of simultaneously reducing OC and SO4). Over a 100-year time frame, the total non-CO2 reduction potential drops to 16% of the total climate mitigation potential from on-road vehicles.