Iron is one of the ocean’s most critical and scarce nutrients, limiting phytoplankton growth across vast stretches of the Pacific, the Southern Ocean, and beyond. Scientists have long pointed to windblown desert dust as the dominant atmospheric source — mineral particles traveling thousands of kilometers before settling on the sea surface.
But a new study suggests they may have been overlooking something far more familiar. Across the world, billions of residential coal stoves burn daily, each releasing fine particles into the air. New research finds that the iron in those particles reaches the ocean in a form marine life can actually absorb — and that this source has gone largely unaccounted for in global models.
Iron: the nutrient the ocean is starving for
Phytoplankton form the base of marine food webs and drive a significant share of the ocean’s carbon sequestration. Across wide stretches of the North Pacific and the Southern Ocean — areas known as “high nutrient, low chlorophyll” (HNLC) zones — phytoplankton can’t fully use available nutrients because one key ingredient is missing: iron.
A micronutrient, needed only in trace amounts, yet its absence limits biological productivity across a large fraction of the global ocean. Mineral dust has long been considered the main atmospheric source, but not all iron is equally useful. Marine organisms can only absorb the soluble fraction, and mineral dust tends to deliver iron in relatively insoluble forms.
A combustion-by-combustion measurement: not all fires are equal
To figure out which human activities matter most, researchers measured iron content and solubility in aerosols from six distinct combustion sources: power plant coal fly ash, steelwork fly ash, municipal waste fly ash, oil fly ash, residential coal, and biofuel burning. The differences were striking. Iron solubility varied by more than three orders of magnitude across sources — power plant coal fly ash came in at a median of just 0.03%, while biofuel burning aerosol reached nearly 56%.
Residential coal combustion landed at about 28%, nearly 1,000 times more soluble than its industrial counterpart. The mechanism comes down to temperature: industrial furnaces operate between 1,200 and 1,700°C, converting iron into poorly soluble oxides, while home stoves burn far cooler, keeping iron in a highly soluble sulfate form.
Why residential coal had been hiding in plain sight
Despite that dramatic solubility difference, residential coal had never been directly measured for iron solubility before this study. Previous global models treated residential and industrial coal as a single category, assigning both the same iron solubility parameter of 0.2% — effectively erasing any residential signal.
The oversight matters geographically. Residential coal burning is concentrated in China, India, South Africa, and parts of Eastern Europe, regions that sit upwind of iron-limited ocean areas. Without direct measurements, modelers had no reliable way to separate the residential contribution from industrial noise.
Updating the global model: how much iron reaches the sea?
Armed with new solubility values, the research team built three new emission inventories separating residential from industrial coal sources, then fed them into the Community Earth System Model (MIMI). Across all combustion sources combined, anthropogenic activity may contribute up to 20% of the global soluble iron flux to the ocean — a share previously underestimated.
Residential coal burning alone could account for up to 21% of soluble iron flux in certain regional ocean basins, particularly around Southeast Asia, the Bay of Bengal, and the eastern North Pacific. Global residential coal iron emissions across the three inventory scenarios still span three orders of magnitude, reflecting how poorly constrained the total mass of residential iron emissions remains.
Consequences for ocean life and future projections
The ecological stakes are already observable. Recent isotopic work has confirmed that anthropogenic iron has shifted spring phytoplankton bloom dynamics in the North Pacific transition zone, accelerating seasonal uptake of upwelled nitrogen in a historically iron-limited region. During the COVID-19 pandemic, when East Asian emissions dropped sharply in early 2020, the chlorophyll-a response to iron deposition off China’s coast weakened by a factor of four compared to previous years.
Under the high-pollution SSP3-7.0 scenario, anthropogenic iron fluxes are projected to peak around 2050 before declining. Projected declines in residential coal use could reduce a key iron fertilization signal in HNLC regions, with uncertain but potentially significant consequences for marine carbon uptake.
What scientists still need to find out
The study points clearly to where the largest knowledge gaps remain. Ship-based aerosol observations are sparse across the Southern Hemisphere and in ocean regions directly downwind of residential burning. Expanding field campaigns to the Bay of Bengal, eastern North Pacific, and South China Sea would most directly reduce uncertainty in global iron cycle models. Isotopic fingerprinting offers another avenue, though no signature has yet been identified that separates residential coal combustion from other anthropogenic sources.
Most urgently, the team emphasizes that constraining the total mass of iron emitted from residential sources matters more than refining solubility estimates. The solubility is now measured — what remains unknown, within three orders of magnitude, is simply how much iron residential stoves are putting into the air.