Introduction

The Intergovernmental Panel on Climate Change (IPCC) has a mandate for periodically reviewing the scientific literature to assess the current state of knowledge on climate change (CC) and climate actions. The IPCC assessment reports are meant to assess the extensive scientific knowledge in all its complexity, but also, paradoxically, to communicate them in as few words as possible. This requirement is particularly salient in the summary for policymakers (SPM) that prefaces each report and is a standard precis for policymakers.

Short Lived Climate Forcers (SLCFs) present a prime example of the difficulty in abstracting a community’s knowledge. Their atmospheric lifetimes, from hours to a couple of decades, are considered short relative to that of carbon dioxide (CO2). This characteristic allows for rapid climate responses and is important for tailoring mitigation strategies, as SLCFs respond quickly to changes in emissions. Despite this common feature, SLCFs1, which include aerosols (sulfate, nitrate, ammonium, carbonaceous aerosols, mineral dust and sea spray) and chemically reactive trace gases (methane, ozone, some halogenated compounds, nitrogen oxides, carbon monoxide, non-methane volatile organic compounds, sulfur dioxide and ammonia), exhibit a wide spectrum of physicochemical properties, climate forcing, and other environmental effects, and cannot be treated as a single, or even collective, entity. Their volatility in the atmosphere, the complexity of their physical-chemical processes, and their interactions with the biogeochemistry of the Earth system make their study challenging, and result in large uncertainties in their sources, their distributions, and their impacts. In this context, concisely summarizing their role in the climate system relevant to policymaking is a great challenge.

This paper is testimony by the scientists heading the dedicated SLCF chapter in AR6, as well as previous IPCC Assessment Report (AR) chapters covering SLCFs. It is addressed specifically to the scientific community whose research and publications on SLCFs provide the foundation for IPCC assessments, but who are not familiar with the IPCC context. We review key points pertaining to SLCFs in AR6, noting the difficulty of communicating our messages within the context of the whole AR6, and identifying where research and analysis could improve this communication. Unfortunately, we can see that the approved outline of the upcoming 7th assessment cycle of the IPCC will make it more difficult to present a coherent assessment of climate actions relevant to SLCFs.

SLCFs in IPCC history

A collective term for SLCFs was used first in the fifth AR. This term, Near-Term Climate Forcers (NTCFs), was dropped in favor of ‘SLCFs’ in AR6. However, far prior this grouping, statements about individual SLCFs were reported in policymaker or executive summaries from the 1st AR (1990), and findings from the first chemistry model intercomparison for these compounds were presented as soon as the 2nd IPCC AR2. From this stage, the complexity and multi-dimensional nature of SLCFs was evident: some directly impacted the Earth’s radiation balance, some increased other greenhouse gases and accelerated CC; others forced chemical destruction of greenhouse gases and reduced CC. The inapplicability of global warming potentials (GWPs) to SLCFs due to their very uneven distribution was underlined, limiting the capacity to properly capture the tradeoffs associated with SLCFs in the GWP basket. For many governments at the time, there was no interest in including SLCFs in climate policy because of the difficulty and large uncertainties in attributing responsibility for their climate effects that depended on the location and season of their emissions unlike that of long-lived greenhouse gases included in the Kyoto protocol. The intricacies of SLCFs have continued to impede straightforward communication with policymakers and other stakeholders within the IPCC context.

In IPCC AR6 Working Group I (WGI3), a complete chapter (Chapter 64) was allocated to the physical interactions between SLCFs and the climate system, which provided a better link between air pollution and climate. The importance of SLCFs in the climate system was evident in that their discussion was spread across other WGI chapters (2, 4, 5, 7, 8, 10 and 12) as well as the other Working Group ARs and Special Reports of the 6th cycle of IPCC reports3,5,6,7,8,9,10 (Fig. 1). WGI assessed the physical science of SLCFs, encompassing changes in their sources, chemical and physical transformations, and atmospheric distribution. Additionally, it considered their contribution to observed CC, their impacts and feedbacks from CC, as well as their future evolution and climate impacts in response to illustrative socioeconomic scenarios. WGII assessed the impacts of climate- or SLCF emission-driven changes in air pollution on human health and ecosystems, as well as the influence of adaptation options on Sustainable Development Goals (SDGs), some being linked to air pollution. WGIII focused on the assessment of CC mitigation options that decrease SLCF emissions (primarily through those of methane) and their impacts on SDGs. The mitigation of SLCF assessed in WGI and WGIII are complementary. WGI addresses SLCFs from the compound/process level whereas WGIII looks for climate solutions building essentially from simplified modeling of SLCFs in the atmosphere. WGI also assessed the influence of air pollution control (APC) policies as opposed to those solely related to CC mitigation.

Fig. 1: Sankey diagram illustrating the co-emissions of long-lived and short-lived climate forcers (on the left), the complex interaction between emitted compounds (as taken into account in the figure WGI SPM.2), and the direct climate forcers (in the middle), leading to environmental issues with impacts on ecosystems and societies (on the right).
figure 1

The scopes of the various IPCC working groups along this chain are indicated at the bottom.

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Cross-WG efforts on SLFCs were limited due in part to the parallel scheduling and isolated management of the WGs. Cross-working group Special Reports or cross-chapter boxes are a way to overcome these difficulties. A good example for SLCFs was the analysis of the solar radiation modification (SRM) in a cross-working group box in AR6 (WGIII Chapter 14). Climate intervention through SRM using aerosols is a complex subject that needs to be assessed not only from the potential of temperature stabilization but also from the perspective of risks for food production, air pollution, biodiversity, regional CC, as well as ethics and governance. Cross WG approach allowed a comprehensive assessment of the multi-faceted implications of SRM in AR6.

Due to its mandate, the IPCC focusses on climate actions, which often means that related environmental goals (e.g., air pollution) are secondary. While synergies and tradeoffs of actions are sometimes evaluated, there is little focus on building multi-target scenarios, despite the potential for lower costs and higher efficacy that these may bring, particularly for SLCFs. Nevertheless, the importance of SLCFs was higher than ever before in the Synthesis Report (SYR) of the 6th cycle. However, the SPM of the SYR only highlights the co-benefits from multiple climate mitigation options on air pollution without reminding governments of the necessity of APC policies in parallel to reach overall public health targets faster. Said differently, while CC mitigation yields important air pollution co-benefits, dedicated air pollution action remains essential and addressing air pollution and CC simultaneously can lower the overall cost of action.

Communicating SLCF science to decision makers

Communicating SLCF science to the public and policymakers is arduous. We explain here this difficulty before explaining some elements relevant for decision making resulting from AR6. Beyond these examples, readers are encouraged to read the original reports for more details including the carefully chosen language on level of confidence and lines of evidence.

Basic SLCF messaging

Elevating clear messages regarding SLCFs for policymakers in the AR6 WGI SPM was particularly difficult despite the established scientific basis. First and foremost, grouping under the term SLCF, which pervades the climate science realm, includes very different compounds in terms of source, lifetime, climate effects, and even chemical and physical forms. It creates an artificial, simplistic collective. Very few characteristics applicable for a subset of SLCFs remain applicable for all, whether in terms of their effects, their sources, or their level of scientific understanding. For many climate scientists, the SLCFs often boil down to just aerosols (or even only sulfate aerosols). Some publications discuss SLCFs only as very short-lived air pollutants without considering methane. Even in AR6 WGI Chapter 6, we failed to integrate all SLCFs in all figures, putting aside the short-lived fluorinated compounds. This fractured nature of SLCF publications is natural and unavoidable, given the extremely large span of domain knowledge encompassed by this collective.

Yet, the complexity surrounding SLCFs, along with the large uncertainties for some, is used during the SPM approval to try to limit the messages on SLCFs. It is essential for the IPCC assessment to allocate sufficient space in the chapters to review the state of science on each SLCF separately, and thus allow the SPM to incorporate the more robust findings regarding specific SLCFs that have the greatest impact on future climate.

Climate change mitigation and air pollution controls go hand-in-hand. Discussion of future SLCFs requires in-depth consideration of the scenario narratives since APC policies are closely coupled to socioeconomic development and to national, political choices. Regional climate outcomes at specific Global Warming Levels (GWL) are now a common way to facilitate communication of climate risk, but these climate outcomes also depend on local actions to reduce air pollution. Consequently, the regional climate responses for a given GWL are sensitive to the scenario assumptions and hypotheses regarding SLCFs and warrant clear caveats that need to be acknowledged and addressed in climate research.

For example, APC policies, essential for protecting public health, can inadvertently accelerate local effects by rapidly reducing cooling aerosols. Such reductions may trigger localized warming and shifts in precipitation patterns, including extremes, all of which remain highly uncertain in today’s science. Methane emission reductions may offer an opportunity to offset some of the warming associated with declining aerosol burdens and, through a decrease in ozone pollution, would be beneficial for APC. Local policymakers need to be better informed of such tradeoffs and synergies to support effective adaptation planning. Do we have the research and publications to support this option and present it in the next IPCC assessment? A comprehensive scientific perspective on both the climatic and societal consequences of SLCF mitigation is essential.

Key information for decision making

The reduction in SLCF and other GHG emissions observed during COVID-19 lockdowns was reported in AR6 and confirmed our understanding of fundamental Earth system mechanisms. While reductions in CO2 emissions did not prevent increases in CO2 concentrations, reductions in air pollutant emissions led to a rapid decline in some surface SLCF concentrations, such as aerosols and nitrogen oxides, but to a more complex response in others, such as ozone and methane. It illustrated the non-linearities of the ozone production in NOx-saturated areas and emphasized the necessity to target VOC and CO emissions within the framework of decarbonization, especially in the polluted ozone production regions. In these highly polluted regions, climate change will compound the local ozone production. It is critical that the research community characterize regional chemical regimes in a harmonized way at a global scale to guarantee more nuanced messaging on co-benefits and APC needed.

An important AR6 breakthrough for communication to policymakers has been quantifying the individual contributions of SLCFs to the change in global surface air temperature (GSAT) since the beginning of the industrial era (Figure SPM2c11, presented in the WGI SPM12). In previous WGI assessments, only the contributions to radiative forcing were presented in the SPM, whereas policy goals are oriented toward global surface temperature. What analysis and publications are needed to continue to advance/solidify this bringing of SLCFs into the mainstream of SPM thinking?

Misconceptions and false understanding regarding SLCFs are still prevalent in the climate decision-making realm. SLCFs are often thought of as a homogeneous basket of compounds. Worse, for simpler communication to policymakers and for environmental advocacy, the warming SLCFs are sometimes grouped under the name Short Lived Climate Pollutants (SLCP) or “super pollutants”, but this can be dangerously misleading as it hides the more complex consequences of reducing air pollution, which can also cut SLCFs that cool the planet. For example, WGI Figure SPM.2 highlighted the importance of aerosol cooling (mainly due to SO2 emissions) and the complex role played by the reactive trace gases (primarily methane, but also NOx, VOCs and CO) in modulating the overall signal from changes in greenhouse gases and thus the need to consider co-emissions or tightly coupled feedbacks with other SLCFs when assessing regulation.

Another frequent misunderstanding exists between emissions-based11 and concentration-based13 climate influence. For example, Figure SPM.211 is misused, even by scientists, to claim that the observed atmospheric CH4 increase is responsible of half of the current warming. The GSAT changes in SPM.2 are due to CH4 emissions, which have also driven increases in tropospheric O3 and stratospheric water vapor (both direct GHGs), resulting in a contribution greater than that from changes in methane concentration only. If the Figure SPM.2 represents progress from a decision-making point of view by directly linking emitted precursors to their climate effects, the corollary is that the role of secondary GHG like ozone are hidden because they are not directly emitted.

An essential message from AR6 is that methane emission reductions have the potential to limit the near-term warming. If the temperature were to peak and decline thanks to negative CO2 emissions, CH4 mitigation could reduce the magnitude of the peak warming and would limit the required negative CO2 emissions. The co-benefit from such methane reductions on air pollution globally through ozone reduction is also clearly affirmed. Such conclusions are grounded in decades of research and well known in atmospheric science, but were brought to the fore in AR6, particularly in the SYR, because this was a robust area of research and publication ahead of AR6.

The synergies of simultaneous consideration of air pollution, development goals and climate actions are also underlined in AR6. Earth system models (ESMs) and chemistry-transport models with high resolutions and detailed chemistries are necessary to model the fate of SLCFs in the atmosphere and thus evaluate their climate forcing. Such a level of scientific assessment is critically needed to assess tradeoffs or co-benefits for policies coupling local APC and CC. AR6 used simplified modeling tools to investigate rapidly large numbers of scenarios. Such simplified tools (e.g., emulators and simple parameterizations of source–receptor relationships for surface air pollution) have to be applied with caution for this task. since they are not equipped to handle the complexity of chemical interactions and spatial scales involved. Considering their intrinsic simplicity, we need a more robust assessment of the performance and limitations of these tools to better inform policymakers.

AR6 reminds that no simple metric can perfectly convert SLCF emissions into CO₂-equivalent emissions and fully capture the temporality of the effects of SLCF emissions changes on temperature. Sophisticated metrics have been developed in recent years to be applied in very specific cases, but they can be readily misused to support nonsensical mitigation, such as cases arguing the status quo regarding methane emissions is a good climate choice14. In AR7, a better characterization of misuse of metrics and methodologies to analyze multi-compound mitigation strategies15, including SLCFs, would be useful.

The co-benefit of decarbonization on air pollution is highlighted throughout the report and synthesis. However, IPCC assessments rely on existing literature, whereas assessment of the overall, and even unanticipated, environmental impacts caused by deployment of new technologies is usually delayed. The consideration of air pollution is distributed across several SDGs and likely contributes to the difficult traceability of the synergies and tradeoffs associated with adaptation and mitigation options regarding air pollution.

Key research progress that benefited AR6

Essentially, only high-confidence statements can land in SPM. Such confidence results from decades of efforts to observe, understand, and simulate atmospheric chemistry and its connections with climate. Decades of scientific advances have enabled more accurate estimates of the past spatial and temporal evolution of emissions for model intercomparisons prior to AR6 and have enhanced representation of chemistry and climate processes in global Earth System models (ESMs).

Chemistry-climate models can now better account for the rapid adjustments that occur within the atmospheric column, and in clouds in particular, before the climate (i.e., sea surface temperatures) change. This ability to derive the effective radiative forcing (ERF) from SLCFs gives us a more accurate measure of GSAT and other climate impacts with substantial changes relative to the fifth assessment (AR5). In AR6, the total aerosol radiative forcing is estimated to be larger than in AR5: The forcing caused by aerosol-cloud interactions (ERF_aci) is larger, but that due to direct aerosol-radiative interaction (ERF_ari) is smaller. The uncertainty in aerosol ERF is thus smaller than in AR5, but it is still the largest forcing uncertainties. The global total aerosol forcing (negative) is dominated by the effect of sulfates (with a larger share than in AR5). Globally, the effect of carbonaceous aerosols is estimated to be very small due to a cooling effect of organic carbon counteracting the effect of black carbon.

The AR6 WGI has been enriched by a better characterization of the physical changes at regional and local scales, with one-third of the report dedicated to regional aspects. AR6 clearly highlights the necessity of considering regional changes in SLCFs to properly characterize and understand the observed trends in temperature and precipitation.

Considering the importance of SLCFs on climate, in particular over the short-term horizon, relevant characterizations of SLCFs in the future scenarios are necessary. The Representative Concentration Pathways (RCPs) assessed in the AR5 presented an insufficiently large range in the SLCF future emissions trajectories with assumptions of systematically strong air pollution reduction due to strong economic growth in all regions. The Shared Socioeconomic Pathways (SSPs) assessed in AR6, on the other hand, considered a wider range of socioeconomic narratives, CC mitigation options, and APC assumptions, resulting in more diverse SLCF emissions trajectories. The SSP scenarios and additional scenarios used in the literature facilitated a more relevant assessment of near-term air quality outcomes as well as SLCF-induced climate effects in AR6.

A few limitations in the AR6 and ways forward

The reliability of our assessments of SLCFs strongly depends on the accuracy of their representation in the ESMs. However, the high computational cost due to the large number of tracers and the cost of chemical solvers in the climate models limits the representation of atmospheric chemistry and the number of simulations that can be run, meaning that many conclusions suffered from being based on very limited ensembles. This limit is inevitable, and so it would be prudent for the community to focus research efforts on some SLCFs and their impacts with the most accurate model configurations and ensembles.

Another difficulty lies in the assessment of the robustness of the tools used. Linking the community studying processes in regional or global chemistry transport models with global climate modelers remains a major unsolved challenge. It is necessary to relate the ability of models to simulate SLCFs in the current atmosphere before we attribute their role in climate change. The comparison of simulated and observed abundances often is inadequate to test the fit-for-purpose of the models unless accompanied by an in-depth analysis of the representation of processes in the various models. This difficulty will grow with growing number of processes and feedbacks represented in models. A fit-for-purpose analysis would require protocols to properly assess the sensitivity of climate effects to emission change with a methodology enabling the detection of error compensation. Progress in ERF characterization needs to continue, especially to characterize the ERF of small forcings typical of SLCFs that would currently require too many ensemble calculations. The prospect of regional ERFs should be investigated, but we already know that the RF from many SLCFs is regionally and seasonally dependent. A major challenge for AR7 will also be the ability to project the detailed regional changes in SLCFs with better high-resolution models and with more detailed and resolved trajectories for their emissions.

From the second assessment report, shared simulation protocols enabling coordinated climate model intercomparison projects (CMIP) have been established. For atmospheric chemistry, such intercomparisons were based on chemistry transport models (CTMs) initially and then transitioned to more organized CMIP style MIPs more recently (ACCMIP, AerChemMIP)16. These multimodel exercises have played a pivotal role in particular to characterize uncertainties. Based on these exercises, the climate community has developed climate emulators, allowing translation of these complex chemical results into more actionable metrics, which in part facilitated to quantify individual contributions in the WGI AR6. However, atmospheric chemistry is complex and couples different species, yielding unexpected, non-intuitive results. This chemical coupling leads to several challenges when evaluating the role of SLCFs in climate. The contribution of each precursor to GSAT change is contingent not only on its emissions rate but also on the abundance of other reactive compounds. The inability to unambiguously separate each SLCF’s impact in an atmosphere means that there will always be structural limitations in the scope of attribution of CC to individual SLCFs. Effectively, the impact of an SLCF depends on where and when it was emitted, as well as the atmosphere into which it was placed. Ex-ante quantification of benefits from SLCF mitigation needs to be conducted with models able to capture this complexity to properly inform the decision.

The AR6 WGI SPM also reflects the more robust assessment of the opposing effects of aerosols versus GHG in driving changes in precipitation. It underlined how increasing aerosol emissions contributed to a weakening of African and Asian monsoon precipitation over the twentieth century (this effect having declined by then with decreases in cooling aerosols and the growing influence of rising GHGs). Nevertheless, the SPM also recognizes that the remaining uncertainties in aerosol forcings and model uncertainties limit skillful projections of precipitation.

The AR6 assessed for the first time the magnitude of many feedbacks involving atmospheric chemistry in a consistent framework, thanks to progress in the representation of biogeochemical cycles in the ESMs. While the magnitude and sign of many feedbacks involving SLCFs remain uncertain, and the lifetimes and emissions of many SLCFs remain of low confidence, AR6 provided a unified framework for better quantification in the future.

The response of natural systems to CC (emissions, such as lightning NOx, biogenic VOCs, organic and inorganic aerosols, and large-scale atmospheric circulation, such as Brewer-Dobson circulation) thus remains a known unknown, with a challenge to better characterize them in the ESMs but also to assess the robustness of their sensitivities. This limits the assessment of future air pollution and the climate feedbacks. Some aspects, like the effect of air pollution and more generally SLCF deposition on crops (and biogeochemical cycles) have been investigated by a very limited number of studies at the global scale with varying protocols leading to low confidence in the assessment. More generally, very few ESMs with dynamical SLCFs have produced simulations to enrich the model ensembles, which limits the scope of analysis. The tropospheric and stratospheric modeling communities often work separately in designing MIPs and run with different model versions. It would be a step forward to better merge these two efforts and streamline the modeling effort regarding chemistry climate interactions.

The effect of climate change on air pollution remains insufficiently characterized. The Climate Impact Drivers (CIDs), introduced in the AR6, offer an analytical framework to characterize the physical changes that occur with CC and are relevant to specific impacts. In this framework, ‘air pollution weather’ was proposed as a CID in AR6 but needs to be now better characterized by determining a limited number of physical metrics that can be provided by ESMs and could capture how CC affects air pollution (e.g., stagnation, precipitation, wildfires, temperature, humidity).

The remaining carbon budget (RCB), i.e., the future emissions of CO2 compatible with a temperature goal, is now central in the PA discussion. Nevertheless, AR6 underlined the importance of the SLCF pathways in determining the RCB levels compatible with the PA goal. In AR6, a single trajectory of reduced SLCF emissions was assumed, but different trajectories can modulate by ±44% the uncertainty range in the RCB needed to have a 50% probability of remaining below 1.5 °C. Better incorporation of SLCF trajectories in the RCB budget is necessary in the next assessment. Another related piece of information needed by policymakers is the estimate of fair share emission reductions according to the PA frame for SLCFs.

For the assessment of tradeoffs (WGII and WGIII) in climate actions with SDGs, a better link between environmental chemists and the IPCC could be beneficial. Similarly, the evaluation of tradeoffs and synergies with air pollution for the adaptation options, e.g., tree planting in urban areas, road constructions to facilitate the displacement of population, often falls short in capturing a more nuanced and complex reality involving the full life cycle. A recent approach for deriving the risk of tradeoffs in aviation-climate decisions17 relies on the confidence level of the uncertainties, however large. It illustrates how the development and assessment in uncertainties in the climate components of a tradeoff would be valuable for AR7.

The assessment of the economical benefit of air pollution reduction relies on the concept of “value of life” suited to compare various environmental threats. The cost-benefit of such policies can be evaluated regionally, but can be meaningless if used at a global scale. More generally, the fit-for-purpose of the tools used to produce integrated assessments of policies in the literature or to directly inform policymakers18 would benefit from an in-depth assessment in the IPCC reports, similar to the critical assessment applied to physical climate models.

The attribution of changes in air pollution mortality in the future (WGII) could be improved by a disentangling of the effect of CC from the effect of other drivers such as straight physical stress (heatwaves) and, in particular for SLCFs, APC and aging of the population. In addition, there is a need to better characterize the compounding effects of combined air pollution (particles and ozone) and heatwaves on health.

Other challenges have to be tackled, first, the weak representation of urbanization and local air pollution policies in the emission inventories and projections, and second, the scarcity of data in the global South for observations as well as the global modeling of atmospheric composition. SLCFs should benefit from a denser monitoring network, but the funding and training for such an observing system are daunting. Lastly, changes in the regional SLCFs have to be systematically measured and documented if we are to attribute regional CC. Considering the important role played by SLCF changes at the regional scale, the span of regional climates for a given GWL should be appropriately qualified.

Challenges for AR7

Participation of the scientific community in the draft review process is an effective way to ensure that up-to-date knowledge on SLCFs is well reflected in the IPCC reports. In the AR719, a special report on climate change and cities will be produced by mid-2027. Regarding SLCFs, this SR is an opportunity to inform local policymakers and practitioners on air pollution and climate interactions. In parallel, a methodological report will guide the countries to report the SLCF emissions in their national inventories. In WGI AR7, the assessment of SLCFs will be distributed in many chapters. Unfortunately, the place to assess the recent trends in SLCFs' drivers seems limited despite their strong regional heterogeneities and uncertainties underlined in AR6. Space appears limited as well to assess how climate change can impact air pollution directly or through changes in the emissions from perturbed natural systems (e.g., fires, biogenic VOCs, lightning NOx). It is important to have in mind that the mention of “air quality” in the chapters was strongly challenged by some governments during the approval of the outlines, which would have weakened the possibility to elevate messages about this important topic in the SPM of the AR7. It reflects a tension already notable in the AR620. The WGII outline does not explicitly mention SLCF or air pollution, but could discuss their effect on agriculture and health chapters. In the WGIII report, the emissions of “GHG and non-GHG” are mentioned in the chapter dealing with past and current anthropogenic emissions. Air pollution could be addressed under the synergies/tradeoffs and sustainable development framework on Sustainable Development and Mitigation. Cross-WG efforts would strengthen the assessment of the effect of CC on air pollution (WGI-WGII) and the characterization of the co-benefits and tradeoffs of CC mitigation on air pollution (WGI-WGIII). Considering the spread of information regarding SLCFs in the reports, a cross-working group effort would ensure a more multidisciplinary and useful synthesis for policymakers.