To draw down carbon and cool off the planet, ocean fertilization gets another look | Science

In January 2009, a German research ship set out for the Southern Ocean carrying 6 tons of iron and a boatload of controversy. The iron was meant to trigger a massive phytoplankton bloom that would suck carbon dioxide (CO2) from the air, but environmentalists objected, viewing the trial as a reckless form of geoengineering. The German government briefly suspended the work, before letting it go ahead. It would be the last iron fertilization experiment for more than a decade.

But that could soon change, after a panel of leading ocean scientists last week said such experiments were a priority and called for the United States to spend up to $290 million on even larger ones that would spread 100 tons of iron across 1000 square kilometers of ocean. Already, researchers next year plan to pour iron across a patch of the Arabian Sea.

Rigorous tests of the strategy are critical, says Ken Buesseler, a biogeochemist at the Woods Hole Oceanographic Institution and a co-author of the National Academies of Sciences, Engineering, and Medicine (NASEM) panel report. “I think it is going to happen with or without the science,” Buesseler says. “My fear is we see this commercialized before we know some of the fundamentals about the ocean response.”

Even if nations make steep cuts to greenhouse gas emissions, many scientists believe that to prevent severe climate change, the world also needs to pursue “negative emissions technologies” that would pull CO2 and other warming gases from the air. Billions of dollars have gone into land-based schemes that, for instance, promote reforestation or agricultural practices that store more carbon in the soil. But Scott Doney, a University of Virginia oceanographer who chaired the NASEM report, says when it comes to carbon sequestration research, “The ocean is a relatively new space.”

The ocean has already absorbed nearly one-third of the carbon emissions from human activities, and scientists hope it can shoulder even more of the burden. Besides iron fertilization, the panel looked at rehabilitating coastal ecosystems; growing vast plantations of seaweed; and spurring plankton production by forcing nutrients up from deep in the ocean. Higher cost options included using electricity to strip CO2 from seawater and inject it underground; and spreading pulverized rocks across the ocean to make it more alkaline, increasing the amount of CO2 it can absorb.

Iron fertilization is among the cheapest options. Photosynthetic plankton act like tropical rainforests, sucking CO2 from the atmosphere. Their populations are often limited by a scarcity of iron, which sifts into the ocean in windblown dust from deserts, in volcanic ash, and even from underwater hydrothermal vents. Extra iron would stimulate a bloom, the thinking goes, causing plankton to take up extra carbon. The carbon would sink into the depths in the form of dead plankton, or the feces or bodies of organisms that eat them. In theory, the carbon would be entombed for centuries.

Tests have shown the iron does stimulate plankton growth. But key questions remain, says Dave Siegel, a marine scientist at the University of California, Santa Barbara, who served on the NASEM panel. How much of the absorbed carbon makes it to the deep ocean is uncertain, he says: Other organisms might consume the sinking material and re-emit the carbon as CO2. Another question on Siegel’s mind: How would companies or governments track these carbon flows well enough to claim they are countering greenhouse gas pollution?

Buesseler is encouraged by recent computer modeling, published by Doney, Siegel, and colleagues in Environmental Research Letters, showing nearly one-third of the carbon captured near the ocean surface by events such as plankton blooms should sink to the deep ocean. Ocean-fertilization strategies could be viable “if we can get even 10% down deep enough,” he says.

But skeptics note that a recent survey of 13 past fertilization experiments found only one that increased carbon levels deep in the ocean. That track record is one reason why making iron fertilization a research priority is “barking mad,” says Wil Burns, an ocean law expert at Northwestern University.

Stephanie Henson, a marine biogeochemist at the United Kingdom’s National Oceanography Centre, also worries about surprise consequences of the approach, likening it to the catastrophic introduction of rabbits to Australia ecology. “You could just imagine something like that happening in the oceans completely by accident.” But Buesseler thinks gauging the potential risks is one reason to go ahead with the research.

David King, head of the Centre for Climate Repair at the University of Cambridge, is ready to test these politically charged waters. Next summer, working with scientists at India’s Institute of Maritime Studies in Goa, he plans to spread iron-coated rice husks across a swath of the Arabian Sea, to learn whether suspending the nutrient for longer can spark a bloom with less iron.

To head off environmental concerns, King plans to confine the work within a giant plastic bag running from the surface to the sea floor several kilometers below. “There’s an enormous amount of naysaying going on,” King says. “There are many, many people saying let’s leave the oceans alone, as if we haven’t already interfered with them.”