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Will Ocean Iron Fertilization Work?Getting carbon into the ocean is one thing. Keeping it there is another. |
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(Second in a six-part series)
Part 1: Fertilizing the Ocean with Iron
Part 3: What Are the Possible Side Effects?
Part 4: Lessons from Nature, Models, and the Past
Part 5: Dumping Iron and Trading Carbon
Part 6: Proposals Emerge to Transfer Excess Carbon into the Ocean
In this age of satellites, it’s fairly easy to answer the basic
question of whether adding iron to the ocean can stimulate a plankton
bloom. When storms over land blow iron-rich dust into the sea,
satellite images show marbled swaths of green phytoplankton spinning
across waters previously blue and barren. Satellites also show plankton
blooms near the Galápagos and other islands where iron-rich deep waters
naturally well up to surface. Even blooms spurred by experimental
additions of iron to the ocean can be detected by satellite, and
shipboard scientists conducting the experiments reported an almost
instantaneous change in the color and even the smell of the water.
Twelve
experiments so far have not looked so closely at the trickier questions
of how much carbon dioxide taken up by a bloom is drawn out of the air
and transferred into the deep sea, and how long it remains sequestered
there. As yet, scientists have turned up only partial answers.
Philip
Boyd of the New Zealand National Institute for Water and Atmospheric
Research summarized the 12 experiments at an ocean iron fertilization
conference convened at Woods Hole Oceanographic Institution (WHOI) in
September 2007 and in an article in Science
magazine earlier last year. Four took place in the northwest Pacific,
two were in the equatorial Pacific, and six were in the Southern Ocean.
All 12 reported up to 15-fold increases in the chlorophyll content of
surface waters. (Chlorophyll is the sunlight-capturing molecule in
photosynthesis and is often measured in lieu of actual plankton
counts.)
Only a tiny fraction of the carbon drawn down by
blooms sinks from the surface into deeper waters, where it is
sequestered from the atmosphere. Estimates of the tonnage of carbon
sequestered (measured at 200 meters depth) per ton of iron added hover
around 200 to 1, a far cry from early experiments in laboratory beakers
that yielded estimates around 100,000 to 1, Boyd said.
But
those may be underestimates. Although scientists have spent up to
several weeks monitoring blooms after iron addition, ship schedules and
budgets have usually prevented them from monitoring long enough, or
deep enough, to obtain good measurements of “export efficiency”—the
proportion of carbon that sinks from the surface into deeper waters.
The
2002 United States-funded SOFeX experiment did show that more carbon
was exported into deeper waters below the fertilized ocean patch, WHOI
marine biochemist Ken Buesseler and colleagues reported. And
unpublished results from the 2004 European EIFeX experiment showed
levels of carbon sequestration that were far higher and far deeper (all
the way to the seafloor) than previously observed—but this occurred
only in the final days of monitoring, Victor Smetacek of the Alfred
Wegener Institute in Germany told participants at the WHOI conference.
The
emerging picture is that iron fertilization does in principle work well
enough to squirrel away carbon for at least a few decades—possibly
useful in the world’s efforts to solve its carbon emissions problem.
Although present yields seem low, improved methods could boost that
number in two ways: by refining logistics to make blooms larger, and by
increasing “export efficiency,” or the proportion of carbon that sinks
from the surface into deeper waters, where it is less easily returned
to the atmosphere.
Logistics and luck
Iron
addition is simple in principle, but once a ship is loaded up and
heading for open waters, even small experiments become a tangle of
logistics. The SOFeX experiment employed three research ships,
helicopter scouts, and 76 scientists to monitor the results of adding
one to two tons of iron to the ocean.
The typical method
involves drizzling acidified iron sulfate into the ocean as a thin
slurry, to reduce the amount that immediately sinks out of the sunlit
surface waters where photosynthesis happens. Adding the iron requires a
12-hour zigzagging cruise across a theoretical square of water whose
boundaries shift constantly in the ocean currents. In the weeks of
monitoring that follow, a ship typically spends 12 hours out of every
day just mapping out the boundaries of the bloom.
Blooms are
hard to track because the added iron rapidly dilutes, sinks, and reacts
with seawater, becoming virtually undetectable after a few days, Boyd
said. Researchers add minute amounts of an inert tracer, sulfur
hexafluoride (SF6), itself a potent greenhouse gas. (One kilogram of SF6
added in an experiment is equivalent to releasing 7 tons of carbon
dioxide, estimated WHOI oceanographer Jim Ledwell, but even that is
insignificant compared with the amount of CO2 drawn down by a bloom.)
“It’s
really our safety net,” Boyd said. “We’ve learned a great deal about
how upper ocean physics can rapidly redistribute the added iron and SF6,” which would have been hard to detect without the tracer.
Depending
on local currents, blooms can wind up strewn across the ocean like a
ball of string or confined within swirling loops of water known as
eddies. The eventual size of blooms from small iron additions can span
1,000 square kilometers or more, extend to depths of up to 100 meters,
and drift hundreds of kilometers from their starting positions. Plain
bad luck can get in the way, too, Boyd said. The first experiment shut
down after only five days when a mass of less dense water moved over
the iron-fertilized water, pushing it far below the sunlit surface and
ending the iron-induced bloom prematurely.
Location, location, location
Boyd
likened a successful bloom to the real estate market. “ ‘Location,
location, location’ applies equally well to these iron addition
experiments,” he said. “Put it in the wrong place, and you’ll be
chasing your tail across the ocean.” Blooms depend crucially on
location at two levels: the appropriate ocean region as well as the
particular patch of water in that region chosen to receive the iron
slurry.
So far, iron fertilization has been contemplated
mainly for ocean waters known as high-nutrient, low-chlorophyll (HNLC)
regions. These areas have high levels of other nutrients that plankton
need to grow, including nitrate, phosphate, and silicic acid. Only iron
is missing. HNLC waters occur in the northern and equatorial Pacific
and in the Southern Ocean, so iron addition ought to work in any of
them.
Logistically, equatorial waters would be easiest to work
in: It’s warm and sunny all year, the seas tend to be fairly calm, and
the warm waters encourage rapid growth. But there is already enough
plankton growth in equatorial waters to eventually use up their
nutrient supply anyway; adding iron there just creates a faster,
concentrated bloom in a specific location, but the net effect on
atmospheric carbon dioxide levels is arguably negligible.
Other
factors make the Southern Ocean a better choice. It has a much larger
area with much higher nutrient levels, which should increase the total
size of blooms that could be stimulated. It has such an abundance of
nutrients that they actually sink before they can be utilized—unless
more iron is supplied.
Other possible locations are the
low-nutrient, low-chlorophyll (LNLC) waters at middle latitudes, where
both iron and nitrate are missing. Research is less far along in LNLC
waters, said Anthony Michaels of the University of Southern California,
but one three-week experiment in the North Atlantic showed that adding
iron and phosphorus can stimulate the photosynthetic bacteria Trichodesmium.
Once armed with iron, this species can convert dissolved nitrogen gas
into a usable form and possibly set off blooms as large as the ones
seen in HNLC waters, Michaels said. The advantage is that such blooms
add their own nutrients, rather than stripping them from surface
waters—one of the key criticisms of iron addition in HNLC regions. But
the blooms could quickly die out once another limiting nutrient,
phosphate, is exhausted.
Once an ocean region is chosen, it
pays to carefully choose a location with respect to its prevailing
currents. Confining a bloom within a large, slowly rotating ocean eddy
makes it easier to study, but the teeming activity may soon deplete the
eddy’s waters of other nutrients, particularly silicic acid, according
to Boyd, and prematurely end the bloom. Linear currents that stretch
fertilized water patches into winding ribbons are harder to track, but
they also tend to draw in new water, replenishing nutrients and
prolonging the bloom.
Engineering a better bottom line
Logistical
improvements may make it possible to spur larger blooms in the future,
but how might engineering improvements make those blooms more efficient
at sending carbon into the deep ocean?
One route might lie in
understanding bloom ecology on a microscopic level. For example, though
phytoplankton are often lumped together as “marine algae,” the term
actually refers to a loose assortment of plants and single-celled
organisms called protists, bacteria, and archaea, each with different
sizes and biochemical requirements. Much of what ensues after adding
iron depends on what species were initially in the waters. Most blooms
end up dominated by phytoplankton called diatoms, whose silica shells
are hard to eat. When the diatoms exhaust the water’s silica reserves,
the blooms wind down, Boyd said.
The interplay among nutrients,
phytoplankton, and their zooplankton predators suggests ways to improve
export efficiency. Adding iron in discrete pulses may allow
phytoplankton to get a head start on their predators. On the other
hand, long-term additions may promote sustained blooms that sequester
more carbon over time, as has been seen with studies of naturally
fertilized waters, according to Stéphane Blain of CNRS/Université de la
Méditerranée in France. The type of iron added is another key factor:
It must be in a chemical form that plankton can easily use, not one
that oxidizes to an unusable form or sinks quickly before it can be
used.
Another way to improve bloom efficiency might be by
targeting plankton-eating organisms known as salps. These gelatinous,
colony-forming animals can act like marine vacuum cleaners, ingesting
huge quantities of plankton, especially diatoms. They convert their
food into large, heavy, carbon-rich fecal pellets that sink much faster
than the feces or dead bodies of other zooplankton. Encouraging the
growth of an indigenous species already present in the oceans would be
essentially “low-intensity, free-range aquaculture,” said one
proponent, Brian Von Herzen, of the Climate Foundation. Even if it were
possible to cultivate phytoplankton and salps selectively, the effects
of such an ecosystem manipulation remain unknown. Salps have few
predators and so are something of a dead end in the food chain.
Early
predictions from models of HNLC regions for iron fertilization’s
potential earned attention by suggesting the technique could remove
around one billion tons of carbon per year from the atmosphere at a low
cost. Other estimates are lower, but none considers the LNLC areas that
may also be important. But realizing such a number would require major
achievements: fertilizing the entire Southern Ocean and increasing the
efficiency of transferring and sequestering carbon in the deep,
according to Jorge Sarmiento of Princeton University. It also ignores
the likely environmental problems from such a large-scale alteration of
the oceans.
As scientists and commercial outfits prepare to
move ahead with experiments, questions about the realistic upper limit
for carbon sequestration remain open. If iron fertilization achieves
only 10 percent of the one-billion-ton-per-year potential for carbon
removal, that would represent 1.4 percent of the world’s current annual
carbon emissions—perhaps still a large enough number to be of use in
mitigating climate change. Whether such a project would also be
profitable depends on improving techniques for creating blooms in the
hostile Southern Ocean. At present, predictions about what will
actually happen range over “about two orders of magnitude,” Boyd said.
“And that’s [a difference of] six to 600 bucks, if you want to put it
on a balance sheet.”
—Hugh Powell
The
Ocean Iron Fertilization Symposium: Some 80 natural and social
scientists from several countries—along with environmental advocates,
business representatives, policymakers, legal experts, economists, and
journalists—gathered at Woods Hole Oceanographic Institution (WHOI) on
Sept. 26-27, 2007, to discuss the pros and cons of ocean iron
fertilization as a means to mediate global warming. This series of Oceanus
articles summarize the wide range of issues raised at the conference,
convened by WHOI scientists Ken Buesseler, Scott Doney, and Hauke
Kite-Powell. They reviewed and edited these articles, with input from
many conference participants. All the articles in this series will be
published next week in a print edition of Oceanus (Vol.
46, No. 1). Videos and PDF versions of presentations at the conference
are available at http://www.whoi.edu/conference/OceanIronFertilization.
The symposium was sponsored by the Elisabeth and Henry Morss Jr.
Colloquia Fund, the Cooperative Institute for Climate Research at WHOI,
the WHOI Marine Policy Center, the WHOI Ocean and Climate Change
Institute, the WHOI Ocean Life Institute, and Woods Hole Sea Grant.
Posted: January 7, 2008 [top] |
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