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Can Banking Carbon Cool the Greenhouse?

Stockpiling carbon dioxide in plants and soil may be effective only for the short term, if at all

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1:41pm, November 15, 2004
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If greenhouse gases are headaches for those who fear global warming, then carbon dioxide is the mother-of-all-migraines. Although other gases actually are better absorbers of Earth-warming infrared radiation, carbon dioxide has the greatest effect because it is by far the most prevalent greenhouse gas in the atmosphere.

Carbon dioxide is a natural by-product of practically everything—fossil fuel combustion, volcanoes, rotting vegetation, breathing—and its concentration in the atmosphere has been on the rise since the beginning of the Industrial Revolution. This has spurred many scientists and environmentalists to call for cuts to emissions—by reducing fossil-fuel use and developing more efficient technology.

Many scientists concerned about global warming are also looking for ways to lock away carbon dioxide, such as by pumping the gas into saline aquifers, oil fields, or the ocean (SN: 6/19/99, p. 392: http://www.sciencenews.org/pages/sn_arc99/6_19_99/bob1.htm). Others are beginning to consider exploiting the world's oceans, trees, and soils as huge reservoirs for stowing great quantities of the gas for long periods in an attempt to avert a global-warming catastrophe.

Some politicians in large, industrialized nations have grabbed onto biological carbon sequestration as a way to help meet emissions limitations set under an international agreement approved in Kyoto, Japan, 3 years ago. However, such proposals didn't fare well during negotiations in The Hague last month and eventually were the sticking point that prompted the collapse of the talks.

There's discouraging news on the scientific front as well. A variety of recent scientific studies shows that several of the proposed biological-storage schemes may have only short-term benefits at best and that some may actually spawn gargantuan problems of their own.

Biological pump

Consider a marine solution. Microorganisms called phytoplankton, which live at or near the surface of the ocean, form the broad base of the ocean's food supply. Although they're small, these tiny plants serve as a mighty biological pump. They take in carbon dioxide as they grow and multiply, and the ones that don't get eaten carry the carbon they've absorbed to the bottom of the ocean when they die. There, if undisturbed, they form an ooze that eventually becomes locked away as limestone sediments.

History illustrates that these little microbes can have a worldwide effect. About 55 million years ago, Earth underwent an episode of intense global warming. Surface temperatures rose between 5°C and 7°C over about 30,000 years. This warming probably resulted from increases in natural greenhouse gases, including large amounts of carbon dioxide emitted during volcanic activity. Sediment cores from beneath the Atlantic Ocean indicate that the higher temperatures and increased carbon dioxide caused phytoplankton to kick into overdrive, triggering faster limestone formation.

The analysis of the cores, presented by an international team of researchers in the Sept. 14 Nature, suggests that phytoplankton saved the day. Global temperatures and the rates of limestone deposition eventually returned to normal. The only problem: That rescue took 60,000 years.

Those who look to ease the effects of the current increases in atmospheric carbon dioxide say we can't afford to be that patient. To study one technique that might speed up the defense, researchers have fertilized areas of the ocean where phytoplankton typically don't grow.

In February 1999, a group led by Phillip Boyd of the University of Otago in Dunedin, New Zealand, dumped large amounts of an iron compound into a 50-square-kilometer area near Antarctica. Naturally low concentrations of dissolved iron limit phytoplankton growth there.

In the late-summer sun, at least one kind of phytoplankton—algae—responded as the scientists had hoped. These microorganisms accelerated their reproduction and soaked up extra carbon dioxide. Stimulated by the 1.7 tons of iron that dissolved from the fertilizing compound, the algae bloom spread during 13 days to cover about 200 square kilometers. Boyd's team, which reported its results in the Oct. 12 Nature, estimates that the phytoplankton absorbed up to 800 tons of carbon, which is equivalent to more than 2,900 tons of carbon dioxide.

About 6 weeks after the start of the experiment, satellite observations showed the bloom had stretched into a 150-km-long ribbon of algae that contained up to 3,000 tons of carbon.

Although Boyd's team successfully stimulated phytoplankton to soak up carbon dioxide, it was unable to determine the ultimate fate of the gas. Because the bloom lasted longer than expected, the time the researchers spent in the area after the iron was dumped was too short to tell whether the carbon would remain locked out of the atmosphere.

Even if successful at sequestering carbon, fertilizing the ocean could have unintended consequences. A 1998 report from the Cheltenham, England–based International Energy Agency Greenhouse Gas Research and Development Program suggests that large-scale fertilization could have severe ecological effects, including depletion of oxygen in large ocean areas.

Besides having detrimental effects on ocean life, anoxic areas could exacerbate the greenhouse effect. A team of Indian researchers reported in the Nov. 16 Nature that algae blooms off the western coast of India, which make the waters oxygen-poor in the late summer, generate nitrous oxide in amounts that the scientists say could have a serious impact on atmospheric warming. The gas probably results from microbes metabolizing nitrogen-containing fertilizer and other soil nutrients that wash out to sea from the Indian subcontinent.

Largest sinks

Why not tie up carbon dioxide in the land? Trees and soil are two of the largest sinks in which carbon can be sequestered, at least for a while. As with the ocean, however, these sinks may not be as effective as their advocates have argued. And, in some cases, trying to manipulate these systems to sequester more carbon—as some scientists propose—may actually aggravate the greenhouse effect when all factors are taken into account.

Forests, for example, provide some of North America's most effective carbon sinks. Increased atmospheric concentrations of carbon dioxide tend to spur growth of vegetation, says William H. Schlesinger, a botanist at Duke University in Durham, N.C.

In experiments that bathed young trees in up to 50 percent more carbon dioxide than normal, the trees grew 25 percent faster for 3 years. However, in the fourth year, the trees' growth rate has declined and no longer differs from that of their neighbors breathing ambient air.

Although the atmospheric concentration of carbon dioxide has been rising for at least 150 years, any extra growth it has spurred in North America's trees appears to be minor. Only about 2 percent of the carbon sequestration in U.S. forests in the past century has resulted from increased carbon dioxide, says John P. Caspersen, an ecologist at Princeton University.

His research, published in the Nov. 10 Science, indicates that the other 98 percent of the sequestration comes from reforestation of land that had been cleared for agriculture or timber in the 1800s. A comparison of historical and recent growth rates in five forest areas between Minnesota and Florida shows that the forests have been growing at nearly the same rate for much of the past century.

"American forests currently act as a carbon sink because there had been such a deforestation and because the trees are still maturing," Caspersen told Science News. The implication of this is sobering, he notes: The rate of carbon sequestration will probably slow as the forests mature.

And although forestation can help sequester carbon, computer modeling described in the Nov. 9 Nature suggests that in some cases, adding trees to the landscape could actually rev up the greenhouse effect. Richard A. Betts of the Hadley Center for Climate Prediction and Research in Bracknell, England, suggests that planting trees at high latitudes to increase carbon sequestration could lead to an increase in local temperature. The foliage absorbs more radiation than snow-covered ground would, thus wiping out any cooling that might have resulted from decreased carbon dioxide in the atmosphere, he explains.

Farmed away

Perhaps carbon dioxide could be farmed away. Agriculture, if done right, can help stockpile carbon in the soil, researchers suggest.

Scientists are still in the early stages of understanding the principles that underlie sequestering carbon in soil. Decreasing the depth and intensity of tillage, planting crops in rotation, and converting from annual to perennial crops all help increase the amount of soil carbon, says Wilfred M. Post, an ecosystem ecologist at Oak Ridge (Tenn.) National Laboratory. Post warns, however, that carbon-rich soil could become a source of carbon dioxide.

"It's a lot easier to lose carbon than to get it back," he laments. When native vegetation is converted to agriculture, he notes, the top meter of soil loses up to 30 percent of its carbon within 20 to 40 years.

Schlesinger notes that if farmers began to use more conservation-minded practices, including no-till techniques, they could reverse these losses. Even more, he reported in the June 25, 1999 Science, they could turn their fields into net sinks of carbon that could sequester up to 1 percent of the United States' current fossil fuel emissions. He notes that other farming techniques, however, have what he terms hidden carbon costs.

For example, the use of fertilizers to enhance plant growth with the goal of boosting soil carbon actually could result in increased atmospheric carbon dioxide emissions from other processes, such as fertilizer manufacture and transport. Furthermore, when ammonium nitrate fertilizers are used too liberally, soil bacteria can convert excess nitrogen in the soil into the greenhouse gas nitrous oxide.

In a genetic approach to increasing soil carbon, crops might be engineered to absorb more carbon or to decompose more slowly, Post suggests. Compared with the types of corn planted in the 1940s, newer strains have more nitrogen and less of the chemically tough carbohydrates in their stalks, he says. So, they decompose more easily and don't sequester carbon in the soil as effectively.

Changes in climate

Biological sequestration of carbon dioxide, even if effective, can be erratic. The rate of carbon uptake by nonagricultural soils depends on a number of factors, including changes in climate.

Arctic environments are particularly sensitive to such variations, says Rommel C. Zulueta, a research assistant at San Diego State University. Zulueta and his colleagues, led by biologist Walter C. Oechel, studied two tundra ecosystems in Alaska and found that some areas of permafrost lost substantial amounts of sequestered carbon when the climate there became warmer and drier in the early 1980s, but that loss has since diminished. The scientists reported their findings in the Aug. 31 Nature.

During the warmer summers, the researchers noted that a thicker layer of surface soil thawed. Better drainage within that layer allowed more air to infiltrate the soil, which in turn provided more oxygen to the microbes there. The result, Zulueta says, was an increased emission of carbon dioxide from the soil.

In recent years, the scientists have noted that the tundra sites have begun to adjust to the climate change. During the summer months, more carbon is sequestered by the growing plants than is lost by the soil. Low microbial activity in winter, however, continues to emit more carbon dioxide from soil than is sequestered by plant growth in the summer. So, on a year-round basis, the ecosystems still are a net source of carbon dioxide.

Last year, the study sites were both cooler and wetter than in the two previous years, Zulueta notes. Although the plants had a decreased uptake of carbon, he says, it's too early to tell whether the soil microbes were less active and therefore lowered the overall emission of carbon dioxide.

Another study, reported by an international team of scientists in the Nov. 17 Science, quantifies the fickleness of biological sequestration. Its analysis of 20 years of air-sampling data from the National Oceanic and Atmospheric Administration suggests that the changes in the rate of carbon sequestration on land are about twice those observed over the oceans. These rates can also vary wildly from place to place and from year to year.

In the 1980s, the team says, tropical ecosystems accounted for most of the year-to-year changes in terrestrial carbon flux. However, mid- and high-latitude ecosystems in the Northern Hemisphere were responsible for most of the changes between 1990 and 1995. Also, North America showed much higher rates of carbon sequestration in 1992 and 1993 than it did during 1989 and 1990. The reasons for these variations aren't clear, the researchers say, because climate models that accurately predict carbon fluxes in tropical regions don't do a good job of calculating the variations seen in the Northern Hemisphere.

Carbon sequestration isn't just an abstract topic of scientific scrutiny: It's a political hot potato, as well. Last month, disagreement about carbon sinks led to the collapse of negotiations for implementing the Kyoto Protocol. Under that agreement, developed nations would have to trim their annual emissions of carbon dioxide between 2008 and 2012 to a rate more than 5 percent below their emissions in 1990.

Nations such as the United States and Canada, which have plenty of wide-open spaces that can be devoted to forestry and agriculture, wanted to take credit for these carbon sinks to offset some of their considerable industrial emissions. The European Union, many developing nations, and some environmental groups disagreed with this proposal. Among their reasons was a concern that these carbon sinks may not remain effective in the long term.

"We're looking for the most certain, risk-free way to halt global warming and to protect wildlife," says Jennifer Morgan, the Washington, D.C.–based director of the World Wildlife Fund's climate-change campaign. "We already know how to reduce carbon dioxide emissions, and these large, impermanent sinks are a big risk," Morgan says.

One big risk to forests is wildfire, which can quickly release massive amounts of carbon dioxide into the atmosphere. North American forests acted as a sink for most of the 20th century because fire-suppression efforts decreased the frequency of forest fires. Caspersen says this sink could soon become a source of atmospheric carbon dioxide if fire fighting policies change.

Even with current practices, a particularly bad fire season could be disastrous. Wei Min Hao of the U.S. Forest Service in Missoula, Mont., estimates that this year's U. S. fires, which scorched 8.1 million acres, added the equivalent of 100 million tons of carbon dioxide to the atmosphere. This is more than one-third the amount of the gas soaked up by U.S. woodlands each year.

Many scientists have concerns that biological sequestration techniques won't be up to the task of counteracting carbon dioxide emissions. "What I'd like to see is more attention to cutting back [human-made] emissions," Schlesinger declares. "That's the name of the game."

"Eventually, we're going to run out of places to put carbon dioxide if we keep burning fossil fuels," agrees Post. "Nevertheless, some of these [carbon-sequestration techniques] could buy valuable time for us to develop alternative technologies to the inefficient ways we now have to sequester carbon."

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