Scientists now understand how the carbon and methane emissions from our cars, livestock and electricity use are helping drive dramatic shifts in our climate through their contribution to the greenhouse effect. But they’re just beginning to untangle the effects of some of the other pollutants we produce. For instance, iron emissions from coal burning and steel smelting could actually be helping the oceans thrive and suck up more atmospheric carbon, according to new research.

If that sounds like a good thing, it isn’t. When we reduce our levels of iron oxide emissions—which we ultimately have to, to protect human and animals from inflammation and other adverse health effects—it will necessitate an even more drastic reduction in pollution to avoid the effects of climate change, the researchers warn.

Iron is a vital nutrient for nearly all living things. Humans need it to make new blood cells, while many plants need it to perform photosynthesis. However, iron is relatively rare in the open ocean, since it mainly comes in the form of soil particles blown from the land. For the trillions of phytoplankton in Earth's oceans, iron is a "limiting nutrient," meaning the available amount of it is a natural check on these creatures' population size. (To prove this, scientists in the early 1990s dumped iron across a 64 square kilometer region of the open ocean and quickly observed a doubling in the amount of phytoplankton biomass.)

Some scientists have proposed taking advantage of this fact through geoengineering, or deliberately intervening in the climate system using technology. Much like forests on land, phytoplankton in the ocean serve as "carbon sinks" because they take up carbon dioxide and then take that carbon with them into the deep ocean when they die. Therefore, adding more iron to the seas could potentially make these sinks even more potent at sucking up the carbon humans have dumped into the atmosphere, these proponents reason.

But the new research suggests that humans are already—albeit inadvertently—geoengineering this process, according to a study published today in the journal Science Advances.

Despite its promises to halt the growth of its carbon emissions by 2030, China remains the world's largest producer and burner of coal and the largest manufacturer of steel. Along with carbon, steel smelting and coal burning release particles of iron that can easily be carried away by the wind. Scientists have speculated for years that all those emissions could be fertilizing the oceans with extra iron, thus driving phytoplankton population growth, says Zongbo Shi, an environmental scientist at England's University of Birmingham.

These iron particles come in the form of iron oxides produced by burning, and are thus insoluble and unable to be consumed by the plankton on their own. However, emitted along with those iron oxide particles are acidic gases like sulfur dioxide and nitrous oxide, Shi says. These gases could react with the iron oxide molecules as they're carried through the atmosphere to form soluble forms of iron.

"No one could prove this definitively," Shi says. He and his collaborators set out to fix that. In 2013, the researchers carefully collected aerosol particle samples from the air from a boat in the Yellow Sea between China and South Korea. Then, they used sophisticated electron microscopes and other detection techniques to parse out the composition of these particles.

The researchers found that the particles included sulfates that contained soluble iron. Since there is no natural source of iron sulfates in the atmosphere, Shi says, they concluded that these particles must have derived from human emissions. "We have proved that this process indeed exists," Shi says.

Phillip Boyd, a marine biogeochemist at the University of Tasmania who was not involved in the research, says the study provides "compelling evidence" that these atmospheric interactions can make emitted iron available to ocean life. However, the scientists are "sort of halfway there" when it comes to seeing how much impact manmade iron fertilization actually has, says Boyd, who is a leading researcher on ocean-climate interactions and geoengineering.

Eastern China has iron-rich soil and is close to the iron-rich Gobi Desert, Boyd says, meaning that there is plentiful natural iron potentially seeding the oceans there. Determining how much of the iron in the air is from natural versus industrial sources will be the "acid test" for how much effect human emissions are actually having on ocean life, according to Boyd.

Shi agrees that it is vital to understand the human contribution to this process. Next, he plans on working to collect more atmospheric and oceanic data to build a thorough model of human iron fertilization of the oceans going back a century. This model would also be able to predict how much impact our 150 years of human industry have had on the levels of carbon in the atmosphere.

It may turn out, Shi says, that our emitted iron has helped tamp down atmospheric carbon levels. "If the amount of soluble iron is being doubled [in the oceans]," says Shi, referencing a 2011 study, "then you'd expect to have something like 30 [extra] gigatons of carbon dioxide being absorbed by the ocean in a century."

Reducing the amount of iron being deposited into the oceans through reducing emissions could make efforts to reduce the greenhouse effect even harder, he says. "There will be less phytoplankton, less carbon dioxide absorbed by the ocean," Shi says.

However, Shi is wary of proposals to dump iron into the oceans to geoengineer away the greenhouse effect. "Geoengineering is a very controversial subject," he notes, referencing the fierce debate over this kidn of large-scale human intervention and its many potentially unintended effects. With respect to artificial iron fertilization, biologists fear that it could lead to widespread algal blooms which could choke out oxygen from the water for other ocean creatures and lead to yet unknown effects.

What’s certain is that we cannot continue spewing iron emissions at our current rate, says Shi, because they have been shown to cause inflammation in people who inhale them and could harm other living things. People may think that “by releasing iron, it could potentially do us a favor,” he says. But while they may help the planet, at least in the short term, these “particles are always not very good” for human health, he adds.