Science - USA (2021-12-17)

PHOTO: JOSHUA STEVENS AND LAUREN DAUPHIN/NASA EARTH OBSERVATORY

1424 17 DECEMBER 2021 • VOL 374 ISSUE 6574 science.org SCIENCE

I

n 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 trig-

ger a massive phytoplankton bloom that

would suck carbon dioxide (CO 2 ) from

the air, but environmentalists objected,

viewing the trial as a reckless form of geo-

engineering. The German government

briefly suspended the work, before letting

it go ahead. It would be the last iron fertil-

ization 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 criti-

cal, 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 com-

mercialized before we know some of the

fundamentals about the ocean response.”

Even if nations make steep cuts to green-

house gas emissions, many scientists be-

lieve that to prevent severe climate change,

the world also needs to pursue “negative

emissions technologies” that would pull

CO 2 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 oceanogra-

pher who chaired the NASEM report, says

when it comes to carbon sequestration re-

search, “The ocean is a relatively new space.”

The ocean has already absorbed nearly

one-third of the carbon emissions from hu-

man activities, and scientists hope it can

shoulder even more of the burden. Besides

iron fertilization, the panel looked at re-

habilitating coastal ecosystems; growing

vast plantations of seaweed; and spurring

plankton production by forcing nutrients

up from deep in the ocean. Higher cost op-

tions included using electricity to strip CO 2

from seawater and inject it underground;

and spreading pulverized rocks across the

ocean to make it more alkaline, increasing

the amount of CO 2 it can absorb.

Iron fertilization is among the cheapest

options. Photosynthetic plankton act like

tropical rainforests, sucking CO 2 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 stim-

ulate a bloom, the thinking goes, causing

plankton to take up extra carbon. The car-

bon would sink into the depths in the form

of dead plankton, or the feces or bodies of

organisms that eat them. In theory, the car-

bon 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 organ-

isms might consume the sinking material

and re-emit the carbon as CO 2. Another

question on Siegel’s mind: How would com-

panies or governments track these carbon

flows well enough to claim they are coun-

tering greenhouse gas pollution?

Buesseler is encouraged by recent com-

puter 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 strat-

egies 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 biogeo-

chemist 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 Cli-

mate Repair at the University of Cambridge,

is ready to test these politically charged wa-

ters. 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 peo-

ple saying let’s leave the oceans alone, as if

we haven’t already interfered with them.” j

Panel urges large tests of iron-triggered plankton blooms

GEOENGINEERING

In 2018, phytoplankton swirled in the Gulf of Finland.

Some researchers want to trigger blooms with iron.

By Warren Cornwall

To draw down carbon, ocean

fertilization gets another look