Natural Short-Lived Halogens: A Missing Piece in Climate Models
Rafael Pedro Fernandez
Institute for Interdisciplinary Science (ICB) - National Research Council (CONICET), Argentina
Winning article: Natural Short-Lived Halogens exert a radiative cooling effect on climate (Nature, 2023)
“Our results are crucial, not just for atmospheric chemistry but for Earth science as a whole. They reveal a key connection between natural chemical, physical, and radiative processes that are essential for understanding Earth’s climate evolution.”
Whether one considers basic or applied scientific perspectives, several atmospheric chemistry phenomena have sustained a distinguished place at the forefront of Earth’s research. The thinning of the ozone (O3) layer due to man-made emissions of ozone-depleting substances and the future projections of its recovery following the implementation of the Montreal Protocol are systematically evaluated by leading experts worldwide (WMO, 2022). Similarly, scientists, governments, and consulting agencies and organizations have focused their efforts on developing and implementing different mitigation strategies to avoid crossing the 2.5 ºC threshold; driven by widespread socio-political concern regarding global enhancement of anthropogenic greenhouse gases (particularly carbon dioxide (CO2) and methane (CH4)) and its implications for the Earth’s radiative balance (IPCC, 2023) and the resulting global warming. Furthermore, future worsening of urban and regional air pollution, driven by complex interactions between natural sources and human activities, poses growing risks to public health and will certainly have differing impacts on developing and non-developing countries.
All of these topics are either directly or indirectly linked with natural Short-Lived Halogens (SLH), a group of organic halogenated compounds and inorganic halogen species which are naturally emitted from the oceans, polar ice, and the biosphere following either biological (phytoplankton and macroalgae) or abiotic (heterogeneous recycling) routes. Due to their fast reactivity, SLH sources rapidly suffer photochemical degradation, which releases very reactive halogen atoms (Cl, Br, and I) which participate in a myriad of catalytic reactions affecting the abundance of the main chemical components of the atmosphere, particularly ozone, OH (which is known as the atmospheric “detergent”) and methane. Therefore, for the past 15 years, our research has focused on improving the understanding of SLH sources, their chemistry, and the coupled process-based connections with the main components of Earth’s climate system.
Despite observational evidence of the ubiquitous presence of natural SLH from the tropics to the polar regions, and from the Earth’s surface to the stratosphere, followed by the unambiguous theoretical and experimental results demonstrating the importance of SLH chemistry both under pristine and polluted environments (Saiz-Lopez, A. & von Glasow, R.,2012), the ‘baseline’ contribution of naturally emitted SLH compounds to the atmospheric chemical background is currently not accounted for in climate models. This omission is highly significant not only within the atmospheric chemistry community but also for the broader Earth sciences, as it represents a direct link between physical, chemical, and radiative processes, which could enhance our understanding of the evolution of Earth’s climate.
Accurately predicting the future evolution of Earth’s climate requires precise estimates of net radiative forcing, not only from major radiatively active components like ozone, methane, and aerosols, but also from smaller reactive species such as SLH. These minor species can influence climate through complex direct and indirect interactions with the main atmospheric components, leading to non-linear effects on the Earth's radiative balance. Our research aims to determine, for the first time, the natural climate baseline associated with SLH by quantifying their radiative effects and assessing how this influence changes across past, present, and future climates.
Specifically, our research allowed us to estimate the net contribution of naturally emitted SLH species to the Earth's radiative balance and demonstrated that natural SLH induces a natural cooling effect on the climate system (Saiz-Lopez, A., Fernandez, R.P., et al., 2023). In quantitative terms, we showed that short-lived halogens exert a substantial indirect cooling effect at present (−0.13 ± 0.03 W/m2) that arises from strong halogen-mediated radiative perturbations (in some cases with opposite signs) of the main atmospheric components, including tropospheric and stratospheric ozone (−0.24 ± 0.02 W/m2) and methane (+0.09 ± 0.01 W/m2), plus additional minor and highly uncertain contributions from tropospheric aerosols and stratospheric water vapor.
Figure 1: Scheme highlighting the influence of SLH on Earth’s radiative balance. Schematic representation of the influence of short-lived halogens on the Earth’s atmospheric composition and radiative balance. For a detailed description of the figure, see the original article (Saiz-Lopez et al., 2023) in the following link (https://www.nature.com/articles/s41586-023-06119-z/figures/5).
Most notably, this substantial cooling effect has significantly increased since pre-industrial times by −0.05 ± 0.03 W/m2, driven by the anthropogenic amplification of natural halogen emissions, and is projected to change further moving into the future, depending on climate warming projections and socioeconomic developments. Given that the natural radiative cooling caused by SLH is of equivalent magnitude to other radiative forcings typically considered in climate projections, like those induced by the increase of dust emissions or the combined contrail-cirrus forcing produced by aircraft, we have called the scientific community to incorporate SLHs into chemistry-climate models to achieve a more realistic natural baseline of Earth’s climate system.
Understanding the complex connections between SLH and the dominant radiatively active climate forcers controlling the Earth’s radiative balance was possible thanks to a detailed and comprehensive representation of SLH sources and chemistry in a coupled chemistry-climate model. These include not only a detailed representation of gas-phase and heterogeneous phase atmospheric chemistry, but also the bi-directional exchange of gases and particles across the ocean, the sea-ice, the biosphere, and their dynamic connections with the overhead stratosphere. Equivalent methodologies should also be applied to other natural components of the Earth’s system to reduce current uncertainties related to the natural climate baseline.
Most notably, our research underscores a crucial yet often overlooked aspect of climate science: before we can accurately assess anthropogenic impact on planetary boundaries, we must first establish the contributors to the natural baseline of the system, in this case, for SLH. While most current research focuses on identifying the safest pathways to avoid crossing planetary boundaries, we emphasize the fundamental need to quantify how the natural background operates. Only by understanding these underlying atmospheric interactions can we improve climate projections, especially throughout different socioeconomic developments.
Improving our understanding of the natural climate baseline will enable scientists to more accurately assess how close humanity is to reaching and surpassing various planetary boundaries. Our article introduces the concept of AANE (Anthropogenically Amplified Natural Emissions), which suggests that the rise in anthropogenic pollutants since the pre-industrial era has altered the natural emissions of short-lived halogens (SLH) through efficient chemical coupling. This means that the role of SLH in the natural climate baseline has not remained constant from the past to the present, and it is expected to change further in the future. As a result, planetary boundary thresholds must be continuously reassessed, incorporating new atmospheric components, like SLH, that were previously overlooked. Most notably, our research shows that natural SLH directly affects at least three of the major planetary boundaries currently defined by the scientific community, including “stratospheric ozone depletion”, “climate change”, and “atmospheric aerosol loading”.
In conclusion, our work shows that the so far unrecognized interplay between natural SLH and Earth's radiative balance is nonlinear across pre-industrial, present-day and future climates. Therefore, chemistry-climate models that do not include the direct and indirect radiative effects induced by SLH may overestimate the predicted global warming and/or underestimate the stratospheric ozone recovery dates addressed in current scientific assessments. Consequently, we call the whole chemistry-climate community to implement a complete representation of SLH in their models, as they are a key component of the natural climate system, and including them will reduce uncertainties regarding the contribution of reactive gases and aerosols to the evolution of Earth's radiative balance during the Anthropocene.
Finally, note that many climate intervention techniques proposed to counteract global warming are based on the release of halogens into the atmosphere. None of these projects account for the direct and indirect effects induced by SLHs highlighted in our research, and therefore, before advancing any environmental agreement or strategy, we call the environmental agencies to first advance with our pioneering research and improve the representation of the complex coupling between SLH, methane, ozone, aerosols and the complete tropospheric oxidation capacity, as well as the bio-geo-chemical interconnections behind the anthropogenic amplification of natural emissions.
Figure 2: Group photo of research team, from left to right: Dr. Rafael P. Fernandez, Alfonso Saiz-Lopez, and Carlos A. Cuevas in front of the Institute of Physical Chemistry Blas Cabrera in Madrid, Spain, which has led research on short-lived halogens over the last 15 years.
References:
IPCC (2023), Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II, and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, IPCC, Geneva, Switzerland, pp. 35-115.
Saiz-Lopez, A., Fernandez, R.P., et al. (2023), 'Natural short-lived halogens exert an indirect cooling effect on climate', Nature, vol. 618, pp. 967–973.
Saiz-Lopez, A. & von Glasow, R. (2012), 'Reactive halogen chemistry in the troposphere', Chemical Society Reviews, vol. 41, pp. 6448–6472.
World Meteorological Organization (WMO) (2022), Scientific Assessment of Ozone Depletion: 2022, GAW Report No. 278, WMO, Geneva, Switzerland.