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Atmospheric iron (Fe) deposition to the open ocean affects net primary productivity, nitrogen fixation, and carbon uptake. We investigate changes in soluble Fe (SFe) deposition from the pre-industrial period to the late 21st century using the EC-Earth3-Iron Earth System model. EC-Earth3-Iron considers various sources of Fe, including dust, fossil fuel combustion, and biomass burning, and features comprehensive atmospheric chemistry, representing atmospheric oxalate, sulfate, and Fe cycles. We show that anthropogenic activity has changed the magnitude and spatial distribution of SFe deposition by increasing combustion Fe emissions and atmospheric acidity and oxalate levels. We report that SFe deposition has doubled since the early industrial era, using the Coupled Model Intercomparison Project Phase 6 emission inventory. We highlight acidity as the main solubilization pathway for dust-Fe and oxalate-promoted processing for 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. Conversely, SSPs with stronger mitigation pathways (1–2.6 and 2–4.5) result in 35% and 10% global decreases, respectively. Despite these differences, SFe deposition increases over the equatorial Pacific and decreases in the Southern Ocean (SO) for all SSPs. We further observe that deposition over the equatorial Pacific and SO are highly sensitive to future changes in dust emissions from Australia and South America, as well as from North Africa. Future studies should focus on the potential impact of climate- and human-induced changes in dust and wildfires combined. This work was funded by the European Research Council under the Horizon 2020 research and innovation programme through the ERC Consolidator Grant FRAGMENT (grant agreement no. 773051), the AXA Research Fund through the AXA Chair on Sand and Dust Storms at BSC, the Spanish Ministerio de Economía y Competitividad through the NUTRIENT project (CGL2017-88911-R), the European Union's Horizon 2020 research and innovation programme under grant agreement no 821205 (FORCeS), and ESA through the DOMOS project (ESA AO/1-10546/20/I-NB). We acknowledge the EMIT project, which is supported by the National Aeronautics and Space Administration Earth Venture Instrument program, under the Earth Science Division of the Science Mission Directorate. RLM received additional support from the NASA Modeling, Analysis and Prediction Program (NNG14HH42I). We also acknowledge the resources obtained on the Marenostrum4 supercomputer at BSC, granted through the PRACE project eFRAGMENT2 and RES project AECT-2020-3-0020, along with the technical support provided by BSC and the Computational Earth Sciences team of the BSC Earth Sciences Department.

Global climate models poorly represent mixed-phase clouds, which leads to uncertainties in cloud radiative forcing and precipitation. In the FORCeS ice experiment (FOR-ICE) we compare three global climate models (ECHAM-HAM, NorESM, EC-Earth) and show which processes are crucial for a realistic representation of cloud ice and supercooled water in each global climate model framework using the factorial method as a statistical approach. A specific focus of the experiments is on secondary ice production (SIP) - which apart from one mechanism (rime splintering) is typically not represented in models, even if observations of ice crystal concentrations of ice crystal number in warm mixed-phase clouds often exceed available ice nuclei by orders of magnitude. We evaluate the importance of three SIP mechanisms combined (rime splintering, ice-ice collisions, and droplet shattering) compared to all other processes that can modulate ice mass and number in mixed-phase clouds: ice nucleation, sedimentation, and transport of ice crystals, and the Wegener-Bergeron-Findeisen process. To describe SIP we adopt two approaches: an explicit microphysical representation of the processes, and a parameterization based on a random forest regression of high-resolution two-year simulations in the Arctic using the polar Weather Research and Forecast model (polar-WRF). Satellite observations are used to evaluate if including descriptions of SIP leads to a more realistic representation of mixed phase clouds.

The ocean plays a key role in climate change mitigation by absorbing heat and CO2 accumulating in the atmosphere due to anthropogenic greenhouse gases emissions. While it is commonly accepted that anthropogenic CO2 is absorbed by the ocean primarily through physical and chemical mechanisms, biological cycling of carbon is mainly responsible for maintaining the vertical gradient of dissolved inorganic carbon in the ocean. Thanks to a series of processes known as the biological carbon pump, oceanic phytoplankton uptakes carbon and promotes sinking of particulate organic matter to the deeper ocean. Uncertainty about the strength of these processes and about their response to ongoing and projected climate change is large enough to confound predictions of future ocean carbon uptake. One of the most important limiting factors for phytoplankton growth over large oceanic areas is the availability of iron. While there is no doubt regarding the importance of dust transport and deposition over open oceans as the most important iron source, questions remain still open regarding closing the relationship between dust sources, long-range transport, iron solubilisation during transport and deposition processes. Overall, better understanding the mechanisms behind the spatial and temporal variability of atmospheric dust-ocean interactions is key to interpret the response of ocean biogeochemistry and to increase confidence in future climate projections. In this work, we present a reconstruction of global ocean biogeochemistry for the last 30 years, where we evaluate the impact on ocean biogeochemical cycles of newly produced iron deposition fields. These were derived with EC-Earth3-Iron, a version of the Earth System Model EC-Earth3 with a state-of-the-art description of the atmospheric iron cycle. EC-Earth3-Iron estimates iron emissions considering explicitly the mineralogy of dust sources, and accounts for the contribution of anthropogenic combustion and biomass burning sources. The model includes an advanced representation of the iron solubilization in the atmosphere, which occurs through acidic attack, organic-ligand mediated dissolution and photo-reductive processes. Furthermore, it allows distinguishing the contribution to the soluble (and total) iron deposition in the ocean of the natural and anthropogenic dust sources, fossil fuels combustion and biomass burning, and characterizing thus separately their impact on ocean biogeochemistry. We compare the ocean biogeochemistry reconstruction against a control where commonly used climatologies for iron deposition fields are prescribed.