Esteban Rodríguez Guisado
Head of the Climate Assessment and Modelling Area at AEMET
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.