Although reducing aerosol emissions improves air quality and public health, a study shows that it contributes to an acceleration of global warming

Is the study of good quality?

“Yes, it is a study of a high scientific standard, which uses robust, validated data.”

Does it have any limitations that should be taken into account?

“In principle, no. It addresses an issue that has been highlighted in climate change studies, and particularly in analyses of the planetary energy balance, which is key to understanding what has been happening to the Earth’s climate in recent decades. The reduction in aerosols as a result of restrictions on the use of chemical components in marine fuels is causing an increase in solar irradiance reaching ocean basins, particularly. This further enhances the current process of climate warming.”

What implications does it have, and how does it fit with existing evidence?

“There is a paradox in that a regulation aimed at improving air quality is resulting in a further intensification of the current warming process. In particular, the accumulation of energy in ocean basins, whose thermal behaviour is fundamental to the regulation of the Earth’s climate.”

How is it possible that the intended reduction in atmospheric emissions contributes to global warming?

“This is an intensification of the warming process with an anthropogenic cause (the reduction of chemical components in marine fuels to improve air quality) which has a natural effect: an increase in solar irradiance reaching the Earth’s surface. The planet’s energy balance is mainly disrupted by the presence of greenhouse gases emitted in large quantities by humans since the 1960s, which prevent infrared radiation from escaping into outer space. However, this is now compounded by the increased arrival of solar radiation at the Earth’s surface due to this reduction in aerosols, which previously had a reflecting effect on that radiation and are now doing so to a much lesser extent due to their reduced presence in the lower atmosphere. This means, in short, that the current climate warming process is being reinforced and that in the short and medium term the prospects for solving this global problem are becoming increasingly distant.”

What consequences could these findings have for the development of policies on atmospheric pollutants?

“The measure taken to reduce ship emissions pollution is a good one, as it improves air quality, which is another serious environmental problem we face on our planet. However, in the short and medium term, as we continue to emit greenhouse gases on a large scale (through the burning of fossil fuels), the effect of maintaining or even increasing warming will become more pronounced. The solution would be for all countries in the world, and especially the largest emitters, to drastically reduce their emissions in order to achieve a natural functioning of the Earth’s climate. This would more than offset the increase in solar irradiance that has been recorded in recent decades and which this study rigorously quantifies.”

The study uses two climate models to assess the impact of emission reductions between 2013 and 2023 on the acceleration of global warming experienced over the past decade. The methodology is robust, employing emission inventories and running long simulations with different forcings in each model in order to constrain uncertainty. The results are in line with other published studies: the reduction in aerosol emissions, which has had a positive impact on public health, has contributed to the acceleration of warming in recent years.

The study adds value by carrying out a global-scale analysis and by quantifying the impact of emission reductions from different sources: Europe, North America, China, and the reduction in sulphur content in fuel used by cargo ships, with the largest contributions coming from the latter two factors. It also highlights that the impact is not only due to the direct effect of aerosols in suspension, which intercept part of the incoming solar radiation, but also to their effect on clouds in some regions: a high concentration of aerosols increases the number of cloud droplets, which helps clouds reflect more radiation. A reduction in aerosols leads to fewer droplets and larger droplet sizes, which reduces their efficiency as a ‘reflector’ of solar radiation.

The study is robust and the results from the two models used are highly consistent; however, the uncertainty in the results is significant. The figure of 52% contribution to the acceleration of warming represents the midpoint of the uncertainty range (between 14% and 90%), so it would be preferable to communicate this uncertainty to the general public, using phrasing such as ‘approximately half’, rather than very precise figures. Despite the consistency between models, and the clear evidence of an impact of emission reductions on the acceleration of warming, it would be useful to use a larger number of models to increase the robustness and precision of the results.

Measures to reduce emissions have been aimed at improving public health (for example, their impact on respiratory diseases in population centres has been very positive). Although some sectors, seeking to discredit them, may frame them otherwise, these are not measures intended to reduce warming, but to improve the quality of the air we breathe. The acceleration of warming has been a collateral effect and, in reality, the way to interpret this is not that the measures implemented have caused the opposite of the intended effect, but rather that particle emissions, which had been increasing until corrective measures were introduced, were masking the warming caused by greenhouse gas emissions.

The study explores the evolution of the impact of these measures in the future and finds that, although they may continue to contribute to additional warming in the coming years, most of this impact has already occurred in recent years, meaning their contribution to warming can largely be considered already realised and will not continue (or will do so to a much lesser extent) in the future.

On the other hand, for the scientific community and policymakers, the study provides a clear example of two key points. Firstly, it shows the extent to which anthropogenic emissions affect the climate system and the Earth’s radiative balance. Secondly, it demonstrates the capacity of internationally coordinated measures to mitigate such impacts, as was the case with the Montreal Protocol. Therefore, rather than being seen as a failure, this should be viewed as a success: if greenhouse gas emissions are effectively addressed through coordinated international action, we have the capacity to significantly reduce warming and, consequently, global impacts. We should avoid framing this as a trade-off between public health and warming, and instead focus our efforts on reducing emissions that drive warming, without rolling back measures that have already proven beneficial.

The study indicates that approximately 50% of the recent acceleration in global warming can be attributed to the reduction of atmospheric aerosols in recent years. These aerosols, primarily generated by industrial pollution and maritime transport, exerted a cooling effect by reflecting solar radiation and altering cloud properties. As these emissions decrease due to air quality policies and public health considerations, that ‘masked’ cooling effect is reduced, contributing significantly to the observed warming.

It is important to continue reducing these pollutants, as they have a very significant impact on human health; however, at the same time, it is essential that they are properly represented in future climate models. In addition, a substantial part of the climate response is still unfolding due to the thermal inertia of the oceans, which delays the full manifestation of warming for decades. All of this underscores the need for more ambitious mitigation efforts.

It is well known that while the increase in atmospheric greenhouse gas concentrations is responsible for the global warming we are currently witnessing with apparent passivity, atmospheric aerosols (microscopic particles, whether solid or liquid, suspended in the air) generally tend to partially offset this warming, either by reflecting or scattering incoming solar radiation or through their contribution to increased cloud formation. Aerosol sources can be natural (Saharan dust, volcanic ash, or salt-rich marine droplets) or anthropogenic. Among the latter, aerosols from industrial processes, power plants, and urban pollution associated with the use of fossil fuels are predominant.

Although it is known that the recent acceleration of global warming is closely linked to the reduction in anthropogenic aerosol emissions, the originality of this study lies in the fact that it quantifies, using two climate models, the role of the three main sources of these aerosols: air pollution control in China, stricter regulations on shipping emissions, and the control of aerosol emissions in other countries. This work estimates that 52% of the observed acceleration in warming is due to the reduction in emissions from these three anthropogenic aerosol sources.

The reduction of anthropogenic aerosol emissions is essential in terms of public health, as it substantially reduces cardio-respiratory conditions, with the associated high mortality caused by air pollution. This study once again emphasises that, in order to control global warming associated with increased greenhouse gas concentrations in the atmosphere and climate change, while also reducing the serious public health impacts of air pollution, policies must prioritise — and make compatible — decarbonisation measures and air pollution controls.