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| Enlarge ImageIn the chilly South Atlantic aboard the NOAA ship Ronald H. Brown, WHOI graduate student Naomi Levine and colleagues worked around the clock for weeks, taking samples of ocean water from many depths, to learn how much dissolved carbon dioxide has been absorbed by the sea, where it accumulates, and how much of the carbon in the ocean is human-generated. (Photo courtesy of Naomi Levine, Woods Hole Oceanographic Institution) |
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| Enlarge ImageA graph of carbon dioxide levels in the atmosphere from the year 1000 to the present shows that CO2 has risen steadily since the 1800s due to human activities of fossil fuel burning and land use changes, and the upward trend is accelerating. The measurements are from two sources: air trapped inside ice cores, and direct measurements of the atmosphere (taken from the Hawaiian peak Mauna Loa) since the late 1950s. (Figure courtesy of Scott Doney, Woods Hole Oceanographic Institution) |
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| Enlarge ImageAs carbon dioxide builds up in the atmosphere, a large fraction has dissolved into the ocean, increasing the total amount of dissolved inorganic carbon and shifting seawater chemistry toward more acidic conditions. Since the end of the last century, the amount dissolved CO2 gas ([CO2 (aq)], shown as the red line) has increased because of both the rise in inorganic carbon levels and acidification. Simultaneously there is a decrease in the water’s pH (shown as the blue line), indicating rising acidity, and a decrease in the carbonate ion ([CO3 2- ], shown as the green line), the substance that many marine animals use to build their shells. (Figure courtesy of Scott Doney, Woods Hole Oceanographic Institution) |
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| Enlarge ImageOrganisms critical to ocean food webs, including plant-like coccolithophores (left), deep-sea corals (center), and small swimming snails called pteropods (right) depend on the ocean’s abundant dissolved inorganic carbonate for making their protective calcium carbonate shells. Declining carbonate and increasing acidity will likely make their shell formation difficult to impossible in large parts of the world ocean by the end of the 21st century, resulting in severe impacts on these organisms. (left photo by Richard D. Norris, Scripps Institution of Oceanography (formerly of WHOI); middle, courtesy NOAA; right, courtesy Laurence Madin, WHOI) |
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| Enlarge ImageColor-coded maps of the ocean (in 2005 and projected in 2099) show areas where it is/will be easy for animals to make shells and skeletons (in colors of purple, red, orange, and yellow) and areas where making shells is/will be chemically difficult to impossible (dark and light blues). The maps are generated from current seawater measurements and projected values for the saturation state, a measure of solubility of calcium carbonate (aragonite). If projections hold, in much of the 2099 ocean, these animals, from plankton to corals, could be gone. (Figures courtesy Scott Doney, Woods Hole Oceanographic Institution) |
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Scott C. Doney, Senior Scientist
and Naomi M. Levine, MIT/WHOI Joint Program graduate student
Marine Chemistry and Geochemistry Department
Woods Hole Oceanographic Institution
It is 4:30 a.m., far from land. A group of scientists clad in
bright yellow foul-weather gear gathers in the open bay of a research
ship. They wait in the chill air while the ship’s crew brings their
instrument back on board after a 6-mile roundtrip to the ocean floor.
As
the sun begins to rise, the scientists carefully remove seawater
collected at various depths from the 36 bottles on the rosette-shaped
sampler. Meanwhile, the ship steams 30 miles to another part of the
ocean, and the process starts all over again—seven days a week, for
seven weeks—over 3,600 miles from south to north in the South Atlantic
Ocean.
The year was 2005, and the research vessel Ronald H. Brown,
operated by the National Oceanic and Atmospheric Administration, was
following the same track of a similar seawater-collecting expedition in
1989. The 2005 researchers analyzed thousands of seawater samples for
chemical compounds including salt, dissolved inorganic carbon, and
chlorofluorocarbons (the now-banned chemicals formerly used in
aerosols). The scientists were there to document how the ocean has
changed over 16 years—as a result of increasing carbon dioxide levels
in the atmosphere and our changing climate.
The oceans have
slowed greenhouse warming by absorbing excess heat-trapping carbon
dioxide from the atmosphere. But how much we can depend upon the ocean
to continue to act as a brake on ever-accumulating CO2 in the future? And will the buildup of CO2 in the ocean change its chemistry, making it more acidic and threatening marine life?
Pouring CO2 into the sky
Carbon
dioxide gas traps long-wave radiation (heat) leaving Earth’s surface,
thus raising temperatures. Without the warming caused by natural levels
of CO2 and water vapor in our atmosphere, the average surface temperature of our planet would be well below freezing.
Atmospheric CO2
levels have varied greatly over Earth’s history, but human activity is
significantly altering the global carbon cycle, and not in a good way.
Carbon dioxide is rising because of the burning of fossil fuels (oil,
coal, natural gas) and because we alter the land through increased
farming and the destruction of tropical forests and plants that take up
CO2 during photosynthesis.
As a result, CO2
levels are increasing faster than at almost any other time in planetary
history. Atmospheric carbon dioxide concentrations are already 30
percent higher than just a couple of centuries ago. Most climate models
project that they will reach 2 to 3.5 times pre-industrial levels by
the end of this century unless dramatic steps are taken to reduce CO2 emissions.
This higher CO2
will bring warmer temperatures. Climate models predict that global
temperatures will increase by 3.5°F to 8°F (1.9o C to 4.4o C) by the
year 2100—and even more in the Arctic and Alaska. Beyond the
temperature rise, a warmer climate is expected to shift rainfall and
drought patterns, which will have even greater consequences for people,
wildlife, and ecosystems.
The sea sink
Not all of the excess CO2
we humans emit stays in the atmosphere. The ocean, and to some extent
the land, act as large “carbon sinks” that significantly slow the
accumulation of atmospheric CO2 and the resulting climate change.
To date, about one-third of all human-generated carbon emissions have dissolved into the ocean. How fast the ocean can remove CO2 from the air depends on both atmospheric CO2
levels and ocean circulation and mixing—in the same way that the sugar
dissolving in iced tea depends on how much you put in and how fast you
stir. More CO2 in the air leads to more in the ocean; faster circulation increases the volume of water exposed to higher CO2 levels in the air and thus increases uptake by the ocean.
The sleep-deprived scientists collecting water in the South Atlantic were working to learn just how much of the extra CO2
has dissolved in the ocean in the past, and how much is entering right
now. This information can help us to better predict how fast
atmospheric CO2 levels may rise in the future⎯and what our future climate may look like.
What happens to carbon in the ocean
The hard work of collecting and analyzing thousands of water samples on ships such as the R/V Ronald H. Brown
is just the first step toward knowing what happens to carbon in the
ocean. Measuring the ocean’s uptake of the added “anthropogenic” CO2 is no easy task, for several reasons.
First,
there are large amounts of carbon normally in the ocean—about 50 times
the amount in the atmosphere. The increase up to now has been a small,
hard-to-distinguish percentage of the total CO2 dissolved in the ocean—about 3 percent in surface waters and only 0.25 percent over the full ocean depth.
So scientists must use advanced analytical techniques to tease out the small anthropogenic CO2
signal from the data. They also measure the ocean’s uptake of other
gases that humans put into the atmosphere, such as chlorofluorocarbons
(which, unlike carbon dioxide, is chemically non-reactive and remains
intact).
The ocean is also a dynamic environment, much like
the atmosphere. Currents and storms mix the water, making it hard to
track the flow of CO2. The picture is further complicated by
plants, animals, and bacteria, which continually take up, release, and
transport carbon in the ocean. These create sources and sinks of
dissolved carbon that vary across and through the oceans, and during
different seasons.
We do know, from several different research
approaches, generally where anthropogenic carbon is highest: in the
upper 1,000 feet of ocean—near the surface where it enters from the
atmosphere—and in cold water that forms near the poles and, being
denser, sinks in plumes to intermediate and deep depths.
What’s in store for the ocean?
The question for policy-makers and society is “Will the ocean continue to take up anthropogenic CO2?” Our best evidence is that it will—but less effectively because of interactions between the ocean and the evolving climate.
Several
factors come into play. Global warming will inevitably cause seawater
temperatures to rise. Warmer water holds less dissolved gas than colder
water, so the ocean will not be able to store as much anthropogenic CO2.
A
warmer climate will also melt ice and increase rainfall near the poles,
adding fresh water to the ocean. Fresh water is more buoyant than
saltier water and “floats” on top of it, stratifying the ocean and
slowing the mixing and circulation that transports anthropogenic CO2 away from the surface and into reservoirs in the deep ocean. The net effect will be even higher atmospheric CO2 concentrations and a further acceleration of global warming.
Warmer
temperatures, weaker circulation, and different stratification of the
ocean will have impacts on marine life and ecosystems, which in turn
could affect the ocean’s ability to store carbon. How these changes may
occur is not clear at this point, however, and may vary from region to
region.
A more acidic ocean
The
increasing amount of carbon in the ocean will cause another problem for
marine life: ocean acidification. The 3-percent increase in dissolved
carbon in surface water may seem small, but it is enough to
significantly alter the chemistry of seawater and threaten whole groups
of marine life.
The reason involves some basic chemistry. When CO2 gas dissolves in seawater, it combines with water molecules (H2O) to form carbonic acid (H2CO3).
The acid releases hydrogen ions into the water. The more hydrogen ions
in a solution, the more acidic it becomes. Hydrogen ions in ocean
surface waters are now 25 percent higher than in the pre-industrial
era, with an additional 75-percent increase projected by 2100.
A carbon-containing mineral, calcium carbonate (CaCO3),
is a vital component in the ocean, used by many marine creatures to
build protective shells and hard structures. Coral reefs, for example,
are the accumulation of calcium carbonate skeletons secreted by small
coral polyps.
Calcium carbonate shells are also used by several
groups of planktonic organisms, microscopic floating plants and animals
that are critical and abundant components of the marine food web. The
white chalk cliffs of Dover, for example, are made out of empty shells
that sank to the bottom of the sea when these organisms died.
The
problem is, acidic conditions are corrosive to already formed calcium
carbonate, and they also make it harder for organisms to build such
hard parts in the first place.
Consequences for marine life
Will corals and shell-forming plankton be able to adapt to a high-CO2 world? We do not know for certain, but preliminary evidence from laboratory and field experiments is not encouraging.
Higher
acidity has a negative impact on almost every species examined. In some
experiments, you can actually watch the shells of living organisms
dissolve away with time.
Especially vulnerable are small
marine snails called pteropods and deep-water corals that live in high
latitudes, where colder waters have already become more acidic. These
species play critical roles in their ecosystems—as food or habitat for
other creatures—so the impact of ocean acidification may soon extend to
other marine life, including fish and marine mammals.
If you
mention “climate change” to people, it often conjures up images of heat
waves, melting glaciers, hurricanes, droughts, and monsoon
rains—certainly not changes in the ocean, its chemistry, and tiny
plankton inhabitants. But we know that future climate change will
largely depend on the chemical composition of the atmosphere and the
sea—and how vulnerable they are to human perturbation. Understanding
how carbon cycles through the Earth system is key to unraveling vital
questions about our climate.
Some policy-makers and
entrepreneurs have even proposed injecting carbon dioxide into the deep
ocean to sequester it from the atmosphere. Ocean carbon-monitoring
projects such as the work on the NOAA ship Ronald H. Brown
contribute vital data to learn about the ocean’s changing chemistry.
Other methods, including experiments that use numerical modeling to
form predictions and studies on how ocean acidification affects ocean
life, must inform our decisions on how tightly we may want to regulate
carbon emissions.
Funding for Scott Doney's and Naomi Levine's work was provided by the
Woods Hole Oceanographic Institution Ocean and Climate Change
Institute, the National Science Foundation, and the National Oceanic
and Atmospheric Administration.
Posted: November 29, 2006 [top] |
