The impact of climate agreements on air quality – a comparison of three US scenarios
The Paris Climate Agreement not only protects the Earth's climate but can also significantly improve air quality. This is demonstrated by Dr. Simon Rosanka from the Jülich Institute of Climate and Energy Systems and co-author Professor Ann Marie Carlton from the University of California, Irvine, USA, in a commentary in the Nature Partner Journal Clean Air. They compare three scenarios for particulate matter pollution in the United States of America (US): The first scenario shows the current situation. The second scenario is a hypothetical present without the Clean Air Act, which the US passed in 1970 to reduce air pollution. The third scenario highlights the possible consequences of the USA's withdrawal from the Paris Climate Agreement.
Annual average PM2.5 mass concentration for three emission scenarios. The emission scenarios are: (a) current air quality including CAA policies, (b) no adaptation of any CAA policy, and (c) CAA implemented with net-zero emissions from transport and energy production. The colormap is centered around the annual WHO exposure recommendations of 5 μg m-3, where blue and red colors indicate regions below and above 5 μg m-3, respectively. For the current air quality (a), the annual average of PM2.5 mass concentration reported by the Air Quality System is displayed, which contains data collected by EPA, state, local, and tribal air pollution control agencies. We display all observation stations which have reported data for at least 10 months.Copyright: — Forschungszentrum Jülich/Dr. Simon Rosanka
What was the motivation for the study? Why are you comparing these three scenarios in particular?
The major driver of our research in general is that the exposure to bad air quality, whether chronic or acute, poses a significant health risk, recognized by the World Health Organization as one of the major contributors to premature death. The particular motivation for this research is directly linked to the history of air quality regulations in the US. In the 1960s, the US and many other countries (e.g., Great Britian) experienced an increase in extreme air pollution events associated with elevated death rates. To reduce the occurrence of these events and improve air quality in general, the US passed the Clean Air Act in 1970. Under this act, the US Environmental Protection Agency regulates air pollution through emission limits and performance standards for most controllable sources. The 1970 Clean Air Act and its subsequent amendments have significantly improved air quality, as documented by air quality observation stations throughout the US.
At the beginning of this project, we aimed to demonstrate the Clean Air Act's tremendous success and societal benefits. Therefore, we used a well-established air quality model to simulate current air quality and compared it to a counterfactual scenario in which emissions related to human activities were increased to represent a world without air quality regulations. By comparing these two scenarios, we can demonstrate the benefits resulting from the US Clean Air Act. While working on this project, however, the US administration decided to withdraw from the 2016 Paris Agreement. We wanted to understand how this decision will affect US air quality in the future. Thus, we decided to extend our simulations by a third scenario representing the reduction in emissions as if the US were to fulfill its original pledge in the 2016 Paris Agreement.
What are the results?
This study produced some fascinating results. Since 1999, routine surface network monitors have recorded dramatic improvements in air quality. This overall improvement is evident in our current air quality simulation. Still, we identified many regions in the eastern US as well as at the west coast in which the World Health Organization standard is exceeded.
Our results show that a world without the implementation of the US Clean Air Act would have much worse air quality. The World Health Organization air quality standard would have been exceeded everywhere in the eastern US and in many densely populated areas, such as California, Oregon, and Washington. In such an environment, almost 300,000 additional people would die prematurely annually from health issues related to air quality. To put this number into context, it is approximately half of the yearly US cancer-related deaths and about twice the number of deaths from respiratory system cancers. In short, we should be glad that we were able to avoid such air quality.
Looking to the future and assuming that the US would adhere to its pledge in the 2016 Paris Agreement, we see significantly improved air quality across the entire US, most dramatically in urban areas. We estimate that, in addition to decreasing the impact on the Earth’s climate, improved air quality would prevent 65,000 premature deaths each year. This is about one third of the current excess mortality due to air pollution in the US.
Without the Clean Air Act the increased concentration of fine particulate matter would have led to 300,000 additional premature deaths in the US. How reliable are such estimates and how are they derived?
These estimates are the result of a two-step process. First, we simulate the ambient air quality for the three scenarios mentioned. To ensure the reliability of our predictions, we use the well-established and frequently evaluated Community Multiscale Air Quality model. Developed by the US Environmental Protection Agency, this atmospheric chemistry model is the primary tool used by the Environmental Protection Agency and other state environmental agencies to evaluate compliance with the regulations established in the US Clean Air Act, alongside observational networks. Furthermore, the US National Weather Service uses this model to produce air quality forecasts for the US twice daily. The model provides us with snapshots of US air quality for each of the three scenarios. In a second step, we calculate excess mortality based on these snapshots. Like the atmospheric chemistry model, we use the Benefits Mapping and Analysis Program, which is also developed by the US Environmental Protection Agency. This program uses risk functions to calculate excess mortality. These risk functions mathematically relate six different mortality endpoints, such as lung cancer, to changes in ambient air quality. This allows us to derive excess mortality for each scenario.
As with any scientific research, our research is associated with uncertainties. In our case, these uncertainties and variabilities originate from predicted air quality, US population data, county-level mortality rates, and risk functions. To address these uncertainties, we implement multiple measures in our study. First, we compare simulated air quality to that observed by the network of surface stations. We find that the simulated air quality agrees very well with these observations, demonstrating the robustness of the modeling framework. Second, we perform a mathematical analysis and estimate the impact of these uncertainties on our estimates.
How are air quality and climate targets related?
This is an interesting question. In general, the sources of emissions related to human activities that contribute to global warming are the same ones that lead to poor air quality. For example, a combustion process like that of a power plant or motor vehicle emits carbon dioxide, which leads to global warming. At the same time, the process emits nitrogen oxides, which can lead to poorer air quality, as they are air pollutants by themselves and can promote the formation of airborne particles.
In our study, we decided to focus on the 2016 Paris Agreement since the US administration recently announced its withdrawal. The main objective of the agreement is to limit global warming to well below 2°C, preferably to 1.5°C, above pre-industrial levels. To achieve this goal, the signatory states have committed to gradually reducing emissions (e.g. CO₂, methane). This measure also reduces nitrogen oxide concentrations, as these are emitted by the same processes. We demonstrate that, in addition to its undeniable benefits to the Earth’s climate, adherence to the 2016 Paris Agreement significantly improves air quality and ultimately saves lives.
The EU has adopted a more stringent air quality threshold since the end of 2024. The limit for 2030 is twice as high as the WHO limits. Doesn't that mean the US is actually in a better position?
In 2024, the United States and the European Union adopted new annual limits for fine particulate matter. These limits are very similar: the European Union set the limit at 10 μg m-3 and the United States adopted the slightly lower value of 9 μg m-3, both of which are above the World Health Organization's guideline of 5 μg m-3. In reality, there is no documented safe level of exposure to fine particulate matter. Ultimately, it is crucial that the set limits are adhered to. The commitment of all European members to the 2016 Paris Agreement will result in overall emission reductions and thus improved air quality, in addition to existing limits on particulate matter. For example, in Paris, driving restrictions on some center streets and other measures drastically improve air quality in recent years. The sooner we achieve ambient air quality in accordance with World Health Organization guidelines, the greater the benefits for our health and the environment will be.
Are there any comparable scenario studies for Europe/Germany?
If we talk about an identical study for Europe, then no. We are not aware of any study evaluating such a scenario in Europe. However, there are studies that evaluate the impact of future changes in emissions on European air quality. One such study, led by colleagues at my institute, was published last year in the journal Elementa (https://doi.org/10.1525/elementa.2023.00127). They evaluated the impact of different European future emission scenarios with respect to adherence to the World Health Organization guidelines and their effect on excess mortality. My colleagues found that the emission reductions currently planned until 2030 will result in 10 million fewer Europeans being exposed to air pollution above the World Health Organization's recommended guidelines.
