The climatic importance of the gases exchange between the ocean and the atmosphere has focused mainly on the exchange of greenhouse gases, such as carbon dioxide, methane and nitrous oxide. Less attention has been paid to ocean emissions of reactive gases into the atmosphere, where they initiate chemical reactions that can indirectly affect Earth's radiative balance and climate.
One such group of reactive gases are the so-called short-lived halogen compounds (SLHs), compounds of chlorine, bromine and iodine which last for no longer than six months in the atmosphere. These molecules are naturally emitted from oceans, polar ice, and the biosphere. Measurements over the past two decades measurements have shown their ubiquitous presence in the global atmosphere.
The study used a state-of-the-art Earth system model developed at the IQF-CSIC to quantify the contribution of short-lived halogens to the global energy balance in pre-industrial, present and future climates. The results show that the SLH emitted by the ocean exerts an indirect cooling effect on the climate system, arising from complex chemical reactions that modify the energy balance in the atmosphere. The paper highlights 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 demonstrates 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.
Finally, the work shows that this natural cooling effect does not compensate for 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.
Alfonso Saiz-Lopez, Rafael P. Fernandez, Qinyi Li, Carlos A. Cuevas, Xiao Fu, Douglas E. Kinnison, Simone Tilmes, Anoop S. Mahajan, Juan Carlos Gómez Martín, Fernando Iglesias-Suarez, Ryan Hossaini, John M. C. Plane , Gunnar Myhre and Jean-Francois Lamarque. Natural short-lived halogens exert an indirect cooling effect on climate. Nature, DOI: 10.1038/s41586-023-06119-z