Scientists may have found an upside to air pollution.
A new study found that two pollutants coming out of the smokestacks at coal-fired power plants interact to make a kind of fertilizer for ocean-dwelling plankton. That may help increase how much planet-warming carbon dioxide the plankton absorb.
The authors didn't study the size of the fertilizer effect. But given the amount of coal burning going on, especially in Asia, they say it could be modest but significant.
While coal pollution is a major contributor to climate change and harms human health, "in this case it is doing us a good thing," said University of Birmingham environmental scientist Zongbo Shi, senior author of the study.
"Earth systems are sometimes very complicated," he added. "They are doing things that we don't really expect."
Iron is essential for plant growth. But the mineral is often in short supply in ocean ecosystems.
Burning coal releases iron particles. Some estimates say the amount of iron falling into the oceans may have doubled or tripled since the Industrial Revolution.
But when that iron leaves the smokestack, it is locked away in a chemical form that's not useful to plants.
Shi and colleagues found that as airborne particles drift along with other smokestack pollutants, they acquire a coating of sulfate, a chemical responsible for acid rain. That acidic coating triggers a reaction that generates iron sulfate, a soluble form of the mineral that plants can use.
The research was published in the journal Science Advances.
Amount of absorption
The study "provides a demonstration of something that we thought was occurring, but couldn't necessarily demonstrate," said University of California-Davis engineering professor Chris Cappa, who was not involved with the research. But it's still not clear how much is occurring, he said.
Burning less coal will still mean fewer greenhouse gas emissions. But Shi said the reduction might be less than anticipated.
"If we control air pollution, then all this potential uptake of carbon dioxide by the ocean will become less and less," he said. "Then we will have to cut more and more greenhouse gas emissions."
But outside experts cautioned that it would take more work to figure out what effect these air pollution particles actually have on how much carbon dioxide plankton absorbs.
"The link between iron fertilization and carbon dioxide storage is more nuanced," said marine chemist Scott Doney at the Woods Hole Oceanographic Institution in Woods Hole, Massachusetts. "There was a period of time when people talked about deliberately fertilizing the ocean with iron as a route to carbon storage. And it turns out not to be so simple."
Before people get too excited about the benefits of air pollution, they should know this: The same form of iron that plants find useful also triggers damaging chemical reactions in human lungs, Shi said. That may be responsible for some of air pollution's impact on health.
"It is not a simple good or bad," he said.