Natural short-lived halogen species (SLH) have a profound impact on the chemistry and composition of the atmosphere, destroying greenhouse gases and altering aerosol production, which together can change the Earth´s radiative balance. Therefore, natural halogens influence climate, although their contribution to climate change is not well established and most climate models have yet to consider their effects. Also, there is increasing evidence that natural halogens i) impact on the air quality of coastal cities, ii) accelerates the atmospheric deposition of mercury (a toxic heavy metal) and iii) that their natural ocean and ice emissions are controlled by biological and photochemical mechanisms that may respond to climate changes. Motivated by the above, this project aims to quantify the so far unrecognized natural halogen-climate feedbacks and the impact of these feedbacks on global atmospheric oxidizing capacity (AOC) and radiative forcing (RF) across pre-industrial, present and future climates. Answering these questions is essential to predict if these climate-mediated feedbacks can reduce or amplify future climate change. To this end we have developed a multidisciplinary research approach using laboratory and field observations and models interactively that allows us to peel apart the detailed physical processes behind the contribution of natural halogens to global climate change. Furthermore, the work also involves examining past-future climate impacts of natural halogens within a holistic Earth System model, where we develop the multidirectional halogen interactions in the land-ocean-ice-biosphere-atmosphere coupled system. This provides a breakthrough in our understanding of the importance of these natural processes for the composition and oxidation capacity of the Earth´s atmosphere and climate, both in the presence and absence of human influence.
The main objective of CLIMAHAL is to study the so far unrecognized interplay between natural emissions of climate active halogens and their impact on AOC and RF in pre-industrial, present and future climates, using an interdisciplinary approach. To fulfil this overall objective the project consists of three main subprojects or objectives as follows:
A) Laboratory work on photochemistry and kinetics of higher iodine oxides monomers, and collection of available observations on SLH from previous and on-going campaigns.
B) Development of an integrated model of the multidirectional halogen interactions within the land-ocean-ice-biosphere-atmosphere coupled system
C) Earth System modelling of natural halogen-climate interactions across past/present/future climates.
Our state-of-the-art integrated ESM has allowed us to quantify, for the first time, the contribution of SLH to the global energy balance in pre-industrial, present and future climates. This study (Saiz-Lopez et al., 2023) showed that SLH emitted by the ocean exert an indirect cooling effect on the climate system, arising from complex chemical reactions that modify the energy balance in the atmosphere. The work highlighted the net indirect cooling caused by SLH as the result of a trade-off between various cooling and warming effects of halogens, mainly on ozone and methane, with a minor contribution from aerosols and stratospheric water vapor. The study demonstrated that ocean-initiated atmospheric chemistry plays a role in partially mitigating anthropogenic warming. This cooling mechanism has been amplified since the beginning of the industrial age, as a result of human emissions which, in turn, have increased ocean emissions of halogens. This hitherto unrecognized interaction between SLH and Earth's radiative budget is nonlinear across past, present, and future climates, and is determined by a combination of natural and anthropogenic emissions, climate variability, and atmospheric chemistry. This work also showed that this natural cooling effect does not compensate for the global warming induced by human action since pre-industrial times, although it must be included in climate models to more accurately reproduce the observed increase in global temperature and improve projections of future scenarios.
