It sure doesn't sound very much like a hidden benefit, but the large volumes of dissolved iron currently being released into the oceans from melting ice sheets might help take some of the hit out of global warming. This is according to a study out this week in the journal Nature Communications describing a feedback mechanism in which melting ice releases bioavailable iron, promoting the growth of phytoplankton (that are way into iron as a nutrient), which in turn act to capture atmospheric carbon while serving as a food source for seagoing animals.

Needless to say, capturing atmospheric carbon is a highly desirable thing within the general goal of keeping climate change in check. The researchers, concentrated around the UK's National Oceanography Centre, note that this particular iron source has been so far unconsidered. "The Greenland and Antarctic Ice Sheets cover around 10 percent of global land surface," says the study's lead author, Jon Hawkins, in a statement. "Iron exported in icebergs from these ice sheets have been recognised as a source of iron to the oceans for some time. Our finding that there is also significant iron discharged in runoff from large ice sheet catchments is new."

Note that there's a difference between the iron you might commonly think of and the stuff that's available as a nutrient. While iron is the fourth most-common element found in Earth's crust, it's usually found as one of the iron oxide minerals, which are unreactive and so limited in their usefulness for biological processes. They're just stupid rocks, in other words.

The good stuff, the highly bioavailable iron, is typically in short supply in Earth's oceans, at least from the perspective of phytoplankton. It's poorly soluble in water, and its main sources for ocean life include upwellings from deep, nutrient-rich water or places where deep water suddenly runs into a beach or shoal, thus eroding away solid material and the nutrients contained within. Places like these are where you find the oceans' largest marine habitats. Iron also winds up in our oceans via dust blown offshore from land masses and from icebergs, glaciers, and, indeed, melting ice sheets.

Where this gets interesting is in light of a human-directed scheme called iron fertilization. This is where humans intentionally release a whole bunch of iron into the planet's upper oceans in the hope of spurring phytoplankton blooms, which should then soak up some of our carbon. At least this is what's supposed to happen; experiments have been mixed, to put it kindly. It seems some algae have a thing for hoarding iron, storing it up rather than allowing it to go toward the development of more phytoplankton. Then there was the case of Russ George, the wealthy geoengineering enthusiast who dumped 100 tons of iron dust off the Gulf of Alaska in what has been described as a dangerous, unauthorized experiment.

Part of the problem is that we really only have a rough sketch of how phytoplankton communities work, what makes them grow and what makes them stash. Iron is, after all, only one of several nutrients that algae are way into; phosphorus and nitrogen are recognized as the two leaders, with iron in third. A 2012 report suggested that we need to pay more attention to regular old vitamins as we map out the species networks in these algae communities, citing one study finding evidence that vitamin B12 is one missing piece and whose introduction might more properly spur on phytoplankton photosynthesis and, thus, carbon retrieval. Mostly though, we just don't know enough.

One thing about geoengineering is that it's difficult to really get good data on its proposed methods. The goal of things like iron fertilization is to make simple (relatively), single (or limited) changes in a vast and complex system, hoping for an aggregate effect. So, how do you test geoengineering without, you know, doing geoengineering?

It seems that in the case of our melting ice sheets, we don't have much of a choice. The iron is being introduced and there's not much we can do about it, at least within the sort of timescale that might prevent it. In that case, we have an experiment on our hands, like it or not. And that experiment puts George's to shame: the UK team estimates a flux of 400,000 and 2,500,000 tons of iron per year being released from Greenland's glaciers and between 60,000 and 100,000 ton per year from Antarctica's.

"Of course, the iron release from ice sheet will be localised to the polar regions around the ice sheets," said Hawkins, "so the importance of glacial iron there will be significantly higher. Researchers have already noted that glacial meltwater run-off is associated with large phytoplankton blooms … this may help to explain why."

Hawkins and his team gathered data from Greenland's Leverett Glacier, spending the summer of 2012 gathering its meltwater and sending it back to England for analysis. The next tasks are manifold: quantifying the global climate significance, determining the precise bioavailability of the iron species being released, collating the Greenland results with observations from other glaciers, and, finally, figuring out what effect this is likely to have on phytoplankton community growth. We know there will be blooms, but with just iron being added to the sea, will those blooms be nearly as big (and helpful) as we might imagine?