As carbon emissions keep rising each year, with no end in sight, scientists have begun dreaming up all sorts of zany geoengineering schemes for slowing down the rate at which the planet's heating up. Artificial volcanoes to cool the air! Giant mirrors in space to deflect sunlight! Fertilizing the ocean with iron  to mop up that carbon!

Our heroes! (credit: Alfred Wegener Institute for Polar and Marine Research)

As it turns out, that last idea might actually work. A team of researchers has published a new study in Nature showing that, under the right conditions, it's possible to lace the ocean with iron in order to stimulate the growth of phytoplankton. The tiny algae absorb carbon from the air during the course of photosynthesis, and when they die, the carbon gets buried deep down on the ocean floor. It's a potentially promising technique. Yet other scientists warn that even if this plankton scheme worked, it would likely only play a small part of any effort to tackle climate change.

Here's the basic idea: Phytoplankton in the ocean require iron, phosphorus and nitrogen to grow. Yet there are vast ocean regions in the South and around the Equator that lack sufficient iron. So, the theory goes, adding iron to these areas would stimulate the growth of extra phytoplankton, which would scoop up additional carbon dioxide from the air. If many of those organisms, called diatoms, then died and sank down to the ocean floor, the carbon would be sequestered, possibly for centuries.

Trouble is, past experiments to prove this haven't worked out well. In 2009, a team of German and Indian researchers fertilized 115 square miles in the South Atlantic with iron. It triggered a bloom of phytoplankton. But some of the algae got swept away by ocean currents, so it was difficult to know what happened to the carbon. Others attracted a herd of hungry crustaceans, which ate the plankton and (eventually) recycled the carbon back into the atmosphere. That defeated the whole purpose.

So, this time around, a team of researchers, led by Victor Smetacek and Christine Klaas of the Alfred Wegener Institute for Polar and Marine Research in Germany, tried something different. They essentially created a swirling eddy in the ocean and dissolved iron into the seawater. The confined area saw a surge of microscopic life — several species of diatoms that blossomed as they absorbed carbon dioxide from the air. And, when the researchers stopped adding iron, the phytoplankton died, with half of them sinking 1,000 meters below surface.

Outside experts say that Smetacek's experiment, written up in Nature this week, appears quite promising. But, they say, it's still unlikely that iron fertilization could ever play a decisive role in limiting human greenhouse-gas emissions.

Back in 2010, Ken Caldeira, a scientist at the Carnegie Institution, tried to figure out how much potential iron fertilization had. Ultimately, the extent of phytoplankton growth is limited by the amount of phosphorus and nitrogen in the ocean. So, in the extreme case, if we tried to fertilize as much of the ocean as possible, Caldeira and his co-author Long Cao calculated that we could only sequester about 1 gigaton of carbon each year into the deep ocean. That's about 10 percent of our present-day annual emissions from fossil fuels and deforestation.

But that's a best-case scenario. "It's probably high by a factor of ten over what's feasible," Caldeira told me. More realistically, iron fertilization likely has the potential to sequester about 1 percent of our current annual emissions. To be sure, every little bit helps. But humans couldn't keep emitting greenhouse gases at current levels and expect phytoplankton to mop up the mess.

What's more, some scientists are still worried about the consequences of artificially mucking with ocean ecology in this way. In 2009, an op-ed in Nature by four ocean researchers warned about unexpected side effects from large-scale iron fertilization — for instance, the dead plankton would pull key nutrients out of the ecosystem. What's more, sending more carbon dioxide into the ocean's depths could make the deeper layers of the ocean more acidic, and it's not clear what the impacts would be. For a variety of reasons, Smetacek has said he doesn't favor large-scale fertilization without further testing.

"It does look like some small additional percentage of emissions could be taken up by the ocean through iron fertilization," says Caldeira. "But it also raises questions about whether we want to continue to use the ocean in this way as a dumping ground for fossil-fuel emissions."