Pre-industrial, present and future atmospheric soluble iron deposition
and the role of aerosol acidity and oxalate under CMIP6 emissions
Abstract
Changes in atmospheric iron (Fe) deposition to the open ocean affect net
primary productivity, nitrogen fixation, and carbon uptake rates. We
investigate the changes in soluble Fe (SFe) deposition from the
pre-industrial period to the late 21st century using the EC-Earth3-Iron
Earth System model, which stands out for its comprehensive
representation of the atmospheric oxalate, sulfate, and Fe cycles. We
show how anthropogenic activity has modified the magnitude and spatial
distribution of SFe deposition by increasing combustion Fe emissions
along with atmospheric acidity and oxalate levels. We find that SFe
deposition has doubled since the early Industrial Era using the Coupled
Model Intercomparison Project Phase 6 (CMIP6) emission inventory, with
acidity being the main solubilization pathway for dust Fe, and
ligand-promoted (oxalate) processing dominating the solubilization of
combustion Fe. We project a global SFe deposition increase of 40% by
the late 21st century relative to present day under Shared Socioeconomic
Pathway (SSP) 3-7.0, which assumes weak climate change mitigation
policies. In contrast, sustainable and business-as-usual SSPs (1-2.6 and
2-4.5) result in 35% and 10% global decreases, respectively. Despite
these differences, SFe deposition consistently increases and decreases
across SSPs over the (high nutrient low chlorophyl) equatorial Pacific
and Southern Ocean (SO), respectively. Future changes in dust and
wildfires with climate remains a key challenge for constraining SFe
projections. We show that the equatorial Pacific and the SO would be
sensitive not only to changes in Australian or South American dust
emissions, but also to those in North Africa.