Key messages
- The successful reduction of air pollution has brought significant public health benefits in several regions, but has also reduced the net cooling effect that aerosols exert on the climate. Hence, the reduction of aerosols in the atmosphere has partially “demasked” the full warming impact of accumulated greenhouse gas emissions.
- Changes in aerosol loading in the atmosphere are expected to have regionally differentiated effects on temperature, rainfall, and the incidence of extreme weather events. The nature of these climatic impacts depends on the location of the emission sources, with both local and remote effects.
- Advances in scientific understanding of how aerosols shape global and regional impacts of climate change should inform mitigation targets and adaptation plans.
Aerosols are tiny particles of air pollution from road traffic, fires and other sources, and have a huge influence on global and regional climate. Airborne particulate matter of certain size and origin is also considered the greatest environmental health threat around the world. Recent research shows that the rapid change of aerosol emissions in the lower few kilometres of the atmosphere (i.e., the troposphere) is influencing observed climate change via pathways distinct from GHG (Figure 2).
Broadly, the climate effects of GHG during the industrial era amounts to an increase of the global mean temperature and of total precipitation, while aerosols act as a counterplayer and slightly cool and dry the planet. The details, however, reveal that the effects of aerosols are much more complex than those of GHGs. Aerosols exist in various types and undergo numerous interactions with other atmospheric variables, so are able to either cool or warm the atmosphere. Their atmospheric residence times are considerably shorter than that of CO2, leading to a distinct fingerprint of effects both near the location of emissions and further afield. Aerosols therefore have a much greater impact than CO2 at the regional and local levels, and their cooling effect is more short-term than the warming induced by the well-mixed CO2.
New findings show the extent to which aerosols can modify global warming effects. Depending on the region of aerosol changes and the type of aerosol, they can influence global and regional temperature, precipitation and circulation. Consequently, they can have severe social impacts, including extreme weather events.
In past decades, aerosol loading has decreased in many regions of the world due to successful air pollution mitigation policies, such as over East Asia; in others, e.g., South Asia, they continue to rise (Figure 2). Research on the pattern of recent aerosol emissions changes has revealed much about the remote effects of aerosols on the climate, for example via changing circulation patterns. It is clear that altered aerosol emissions do not only matter for the regions where the sources are located, but can also influence climate risks in remote areas.
Beyond these regionally differentiated effects, research based on satellite data helped quantify the impact of aerosol changes on the radiation balance and relate it to the radiative effects of changes in CO2 for the same period. Concurrently, the rate of global warming has increased, and the reduction of aerosol loading is suspected of partially “demasking” the full warming impact of accumulated GHG. The recent findings support what was already known about this (de)masking effect, and have added quantification to narrow uncertainties.`
For future climate change, the new results imply that the positive effects of reducing CO2 in net-zero scenarios can be undermined by the demasking from mitigating aerosol emissions. Global warming could temporarily progress even more rapidly, with corresponding societal impacts.
Other new research investigated ship emissions, black carbon, delayed demasking effects for polluted regions, and the possible underestimation of anthropogenic aerosol loading, which to some degree had entered public debates. In short, the studies all support the overall picture that, to fend off global warming, anthropogenic GHG emissions must be drastically and quickly reduced.
Additional research is needed to further improve the quantification of the climate effects of aerosols, especially on the interactions between aerosols, cloud particles and precipitation. Despite the complexity of their roles in the climate system and remaining uncertainties, there is consensus that aerosols decisively influence how fast and in which ways climate change will take shape in different regions. Through their influence on extreme and compound events, there are vital implications for adaptation pressure and for the discussions on loss and damage. Decision-makers should thus take the most recent findings on aerosols into account, using the best available regional climate data and drawing on expert knowledge.
Further reduction of anthropogenic aerosol emissions will reduce health impacts and directly save lives, and is beneficial for climate and the environment. It will, however, amplify climate warming, and can also strengthen precipitation change and extreme events in many regions. It is essential that climate action takes the effects of aerosol changes into account in both adaptation and mitigation strategies.
Policy implications
- Advances in the scientific understanding of aerosols’ influence on climate, specifically on surface temperature, have implications for the level of ambition for GHG emissions reduction goals aligned with the Paris Agreement goals. While the Mitigation Work Programme (MWP) does not explicitly address the effects of changing patterns of aerosol loading, emerging science suggests this is an important consideration for comprehensive mitigation planning, and hence within its mandate. The MWP could play a key role in raising awareness and sharing knowledge about this issue.
- Relatedly, while Nationally Determined Contributions (NDCs) do not account for the effects of changing patterns of aerosol loading, the emerging scientific understanding could justify its inclusion on future updates. The knowledge of aerosols’ impact on regional climate change (including patterns of precipitation and extreme weather events) should also inform adaptation planning, reflected in National Adaptation Plans (NAPs).
- Given the complexity of aerosol–climate interactions, a coordinated international science-action effort, similar to what has been achieved for methane, would be important for effectively incorporating aerosols into comprehensive climate action planning. An initial step could be a focused initiative and/or specialised task force to provide recommendations for integrating aerosol considerations into more comprehensive plans. The IPCC-AR7 Methodology Report on Short-lived Climate Forcers expected in 2027 will provide essential guidance on this matter.
- Mitigation of GHG emissions and air pollution can be addressed simultaneously. For example, promoting soil carbon sequestration by adding biomass can reduce agricultural residue burning. Similarly, shifting away from burning fuelwood and charcoal to cleaner cooking methods has been incentivised effectively through subsidised, cleaner cookstoves. The joint benefits for human health and the climate should be communicated clearly, to enhance the adoption of these practices.
- Ongoing research will improve projections and quantifications of the regional climate effects of changing aerosol emissions and loading, which can inform decision-making. Establishing and strengthening air quality monitoring and enhancing procedures for data sharing and transparency can benefit both scientific research and decision-making.
Recent changes in aerosol amounts (difference between 2014–2023 and 2004–2013 period averages), quantified as Aerosol Optical Depth (AOD). Main sources of aerosol emissions, responsible for the observed changes. Impacts (local and remote) of changes in aerosol loading over Europe, East Asia, and South Asia are depicted in the top and bottom windows. Modified from Persad et al. (2023).
