Carbon goes deep
Studies show CO2 has reached the bottom of the ocean.
Nature.com, Feb. 12, 2007
Human-generated carbon dioxide has sunk down to a great depth in the North Atlantic Ocean, a new study has shown. The work, published in Proceedings of the National Academy of Sciences1, suggests that the oceans store CO2 for longer than expected -- good news for reducing the risk of climate change, but bad news for marine life in the deep sea.
About half of the atmospheric CO2 produced by human activities since the beginning of the industrial revolution has ended up in the ocean2. The gas, along with oxygen and other compounds in the air, dissolves into the surface waters and is mixed around by the currents. Because the ocean is so huge, it has an enormous capacity to suck up gas.
Scientists have long known that CO2 would eventually be transported to the deep sea. But previous studies have been unable to spot man-made carbon dioxide at depths of greater than 4,000 metres. How much was down deep was a great unknown.
Now Douglas Wallace, at the Leibniz Institute of Marine Sciences at the University of Kiel, Germany, and colleagues say that the concentration of man-made CO2 in the western basin of the North Atlantic Ocean is at least 10% of that at the ocean surface. Wallace says that if the same holds true around the globe, then that would indicate a very large reservoir of man-made carbon in the ocean.
This study really emphasizes the "extraordinary invasion of what is now some 500 billion tons of fossil CO2 into the ocean", says Peter Brewer, an ocean chemist at Monterey Bay Research Aquarium Institute in Moss Landing, California.
Until now, scientists have grappled with detecting exactly where in the ocean human-generated carbon dioxide is stored. Most studies have used tracers of elements such as chlorofluorocarbons (CFCs) or 14C from nuclear bombs. But while anthropogenic carbon has been entering the atmosphere since the late 1800s, many of these tracers have only been around since the end of World War II.
Wallace and colleagues set themselves the challenge of developing a different way to look at older anthropogenic carbon in the oceans. They measured dissolved inorganic carbon and watched how its concentration changed over time in relation to other factors, including temperature, pH, dissolved nutrients and oxygen. Several decades worth of data let them work out the relationship between these factors and anthropogenic CO2 for different depths, down to 4000 metres.
Once CO2 is carried downwards, the surface waters become free to take up and store more CO2. And the deeper it goes, the longer it is expected to stay down. Although this buffers against global warming, it makes the deep sea more acidic -- corroding and dissolving the armour and skeletons of deep-water corals and other marine creatures. "This is a sign that we are radically changing the pH of the deep ocean," says Wallace.
The wildlife at risk includes organisms that construct hard parts from one of two forms of calcium carbonate: calcite or aragonite, both of which become soluble at a certain pH, temperature and depth. Deep-sea corals, for example, build their skeletons from calcium carbonate.
Wallace and colleagues show that aragonite is already unstable in the depths of the western basin of the North Atlantic Ocean. But in the eastern basin, the introduction of carbon is shifiting the horizon at which ocean life begins to dissolve. According to their calculations, this has shifted upwards some 400 metres since before industrial times, and is projected to rise up a total of 700 metres by 2050.
It is unclear what, exactly, this will do to the corals. Scott Doney, Senior Scientist at the Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution in Massachusetts, says that the rising of this horizon "will put in danger the deep-sea corals that are living at these depths". Brewer, on the other hand, says that "Pacific deep-sea corals appear to cope quite well. There are many examples from Pacific sea mounts where such corals are already well below the aragonite saturation horizon and thrive."
Wallace says that the potential effects of such "large chemical changes" on corals needs to be further looked into.
CO2 being pushed deep into the oceans
NewScientist.com, Feb, 12, 2007
Atmospheric carbon dioxide is being pushed deeper into the oceans than previously thought, according to researchers.
The findings mean the oceans may continue to absorb human emissions of the greenhouse gas more rapidly and for longer, they say, reducing their impact on global warming. But the research is bad news for the marine organisms that are already suffering from ocean acidification.
Higher levels of CO2 in the atmosphere, caused largely by industrial activities, push the greenhouse gas into ocean waters. Although this process is fairly well understood, scientists have only estimates of the depth at which CO2 from human activities is stored in the oceans.
"Previous estimates, based on educated assumptions about what the pre-industrial oceans looked like, suggested that in the high latitudes of the North Atlantic, anthropogenic CO2 was not found below 2500 metres," says Douglas Wallace of the University of Kiel, Germany.
Wallace and colleagues have now published the first measurements showing the location of CO2 from human activities in the North Atlantic. They used data collected during a research cruise in 1981 as a baseline, and then returned to exactly the same sampling locations in 2004.
"This revealed quite large changes in the CO2 in very deep water, between 3000 and 5000 meters," Wallace told New Scientist.
If their findings are replicated in the much bigger southern oceans, it could mean that the oceans' capacity to take up CO2 is greater than previously thought.
While this may soak up some of the CO2 that would otherwise warm the atmosphere, the flipside is that the new findings give further evidence that human activities are rapidly changing the chemistry of the deep oceans.
"There is a depth in the ocean above which calcium carbonate shells don't dissolve, and below which they do," says Wallace. The findings suggest that the CO2 pumped into the oceans has pushed up this boundary by 400 m, compared to its level before the industrial age. And the researchers predict that it will be 700 m shallower by 2050 if CO2 emissions continue their fast growth.
Wallace says that whether the findings are replicated in the southern oceans remains to be seen, and he is encouraging colleagues to replicate his study there. There may be differences. For example, much of the southern ocean's water sinks to the bottom off the coast of Antarctica. There, sea ice may prevent CO2 entering the water from the atmosphere to the same extent as in the north.
The scientist who first coined the phrase ocean acidification, Ken Caldeira, at the Carnegie Institution, California, US, says the extent to which the rising boundary will affect deep-sea corals and shelled organisms remains uncertain. But when human activities start impacting remote parts of planet, it's a wake-up call that we are interfering in our planets functioning on a very large scale, he says.
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0606574104)