Contrasting trends of marine bromoform emissions in a future climate

Bromoform (CHBr₃) from the ocean is the most important organic compound for atmospheric bromine with an atmospheric lifetime of ~2-4 weeks. Natural production, being the main source of oceanic CHBr₃, is high at the coasts and in open ocean upwelling regions due to production by macroalgae and phytoplankton. Although highly relevant for the future halogen burden and ozone layer in the stratosphere, the global bromoform production in the ocean and its emissions are still poorly constrained in observations and are mostly neglected in Earth System Model (ESM) climate projections. Anthropogenically forced climate change may lead to considerable changes in ocean temperature and ocean acidification, and will also influence primary productivity. Especially biogeochemical processes in the Arctic will be strongly influenced by climate change in the near future.  However, the future trend of the marine emissions of bromoform and other very short-lived substances (VSLS) remains unclear. Two studies projected an increase of the relative importance of brominated VSLS for stratospheric ozone loss in contrast to other ozone depleting substances, due to increasing oceanic emissions of the brominated VSLS. Both studies applied constant (observation based) oceanic concentrations for the emission calculations in a future warming ocean, which assumes a production increase. Thus, a consistent way of addressing the bromoform production and concentration in the global ocean, its air-sea gas exchange and concentration in the atmosphere with high spatial and temporal resolution is ultimately needed to further progress with our understanding of potential future climate trends.

Here, we show first model results of fully coupled ocean-atmosphere bromoform interactions in the Norwegian ESM (NorESM) with the ocean model BLOM and the ocean biogeochemistry component iHAMOCC for the period from 2015 to 2100 (SSP585 scenario). Model data for the historical period until 2014 is validated with oceanic and atmospheric observations listed in the HalOcAt (Halocarbons in the Ocean and Atmosphere) data base.

On global average, our model results indicate decreasing oceanic CHBr₃ concentrations and emissions until the end of this century. In contrast, atmospheric CHBr₃ mixing ratios are projected to increase during the same period. The results indicate that the lifetime of atmospheric CHBr₃ increases until 2100 compared to current days as atmospheric loss due to photolysis and reaction with hydroxyl radicals is projected to decrease.

In contrast, bromoform in the Arctic shows an increasing trend of marine CHBr₃ concentrations, their emissions and atmospheric mixing ratios. Moreover, annual mean Arctic marine bromoform concentrations in 2100 (5.2 pmol L^-1) are projected to exceed global values (4.5 pmol L^-1). Increasing sea surface temperature and sea ice retreat in the Arctic drives the higher CHBr₃ productivity. The resulting emissions in the Arctic are projected to increase by 67% until 2100 indicating this region to be a significant source of brominated VSLS in a future climate. The relevance and uncertainties of the model results are discussed.