Facing Up to a Carbon Dioxide Rich Future

SEATTLE, Washington, February 16, 2004 (ENS) - An atmosphere richer in carbon dioxide (CO2) than today's atmosphere is forecast by 2050 as increasing amounts of the greenhouse gas are emitted through combustion of fossil fuels. Some scientists have predicted that the trees alive in the CO2 rich air of the future will grow faster than today's trees, absorbing more CO2 in the process.

But a group of studies released at the annual meeting of the American Association for the Advancement of Science (AAAS) indicates that planners seeking the mitigation of climate warming cannot count on the increased ability of trees to absorb CO2, known as the ability of a forest to act as a carbon sink.

A symposium today on "CO2 Fertilization: Boon or Bust?" will feature a futuristic Duke University simulation of forest growth under the CO2 enriched atmosphere expected by 2050 that does not reinforce the optimism of those who believe trees can absorb that extra CO2 by growing faster, said a spokesman for the experiment.

During seven years of exposure to CO2 concentrations 1.5 times higher than today's, test plots of loblolly pines boosted their annual growth rates by between 10 and 25 percent, found the researchers. But "the highest responses have been in the driest years, and the effect of CO2 has been much less in normal and wet years," said William Schlesinger, a professor of biogeochemistry and dean of Duke's Nicholas School of the Environment and Earth Sciences.

These findings suggest that nitrogen deficiencies common to forest soils in the Southeastern United States may limit the abilities of loblolly pine forests to use the extra CO2 to produce more tissues as they take in more of the gas, he said.

Funded by the U.S. Energy Department as one of 10 Free-Air CO2 Enrichment (FACE) projects in the United States, Duke's experiment is set up as an open air test of how higher CO2 outputs produced by fossil fuel emissions and other human activities could change a Southeastern forest ecosystem about 50 years from now.

The FACE experiments blow CO2 from a tank through vertical standing vent pipes into the exposure area. Sensors measure wind speed, wind direction, and CO2 concentration, and a computer control system regulates and monitors the CO2 releases. (Photo courtesy ORNL)

Trees absorb more carbon dioxide when the amount in the atmosphere is higher, but the increase is unlikely to offset the higher levels of CO2, according to results from another FACE experiment conducted at the Oak Ridge National Laboratory (ORNL) in Tennessee.

"Some people have used carbon dioxide fertilization to argue that this is a boon of the fossil fuel era and that it will lead to greater agricultural yields and carbon sinks," said Richard Norby of the Department of Energy's ORNL who is participating in the AAAS symposium. "Some recent experiments, however, have suggested that there will be no lasting effect of carbon dioxide fertilization. As is often the case, the truth may lie in between."

For the last six years, Norby and colleagues at ORNL have examined the responses to elevated carbon dioxide levels in a stand of sweetgum trees a few miles from the lab.

The research team pumped tons of CO2 into the sweetgum plots, raising the concentration of carbon dioxide in the tree stand from the ambient level of about 370 parts per million to 550 ppm, and studying the effects.

Among the findings is that young trees and other green plants respond favorably to elevated concentrations of atmospheric carbon dioxide. The relevance to the role of the terrestrial biosphere in the global carbon cycle, however, has long been subject to debate.

In every year since the FACE project began, net primary productivity, which is the total amount of carbon dioxide fixed into organic matter such as leaves, stems and roots, has been higher in plots given extra carbon dioxide. The average increase has been 24 percent, and there is no indication that the increase will not continue.

But, Norby notes, while his colleagues have observed a sustained increase in leaf photosynthesis, the response to carbon dioxide fertilization would not be apparent if only above ground growth were measured. Wood production increased significantly during only the first year of treatment.

While Norby and colleagues have learned a great deal about above-ground allocation of carbon dioxide, in recent years they have focused their efforts on impacts on fine roots and soil sequestration of carbon dioxide. Fine root production has increased in response to elevated carbon dioxide.

Fine roots are important for water and nutrient uptake, but they have a short life and their carbon returns to the soil within a year. Initial results suggest that the increase in carbon supply to fine roots has increased the carbon content of the soil. But Norby cautions that the positive effect of carbon dioxide fertilization is insufficient to halt the rising level of atmospheric carbon dioxide.

A researcher at the Duke University FACE project checks a tree for signs of CO2 enrichment. (Photo courtesy Duke University Photography)

Meanwhile, some other species in Duke's experimental forest plots have grown at faster rates than the loblolly pines, scientists report. Still unpublished data shows 70 percent growth increases for poison ivy, Schlesinger says.

The latest information about carbon dioxide fertilization, by which plants soak up carbon from the atmosphere, "really paints a different picture of the way the world works," said panelist Chris Field of the Carnegie Institution of Washington.

In a book edited by Field and scheduled for publication in late February, researchers concluded that the land contains many large pools of carbon that are likely to shrink in the coming century.

Field and his colleagues also have discovered in a previous study that there may not be enough biologically available nitrogen to support certain optimistic estimates of the land's capacity for carbon fertilization.

"If you put together these two lines of evidence, we're looking at a future in which we may see less carbon being removed from the atmosphere," Field said.

"The fact that carbon dioxide fertilization is likely to be more modest does not imply that carbon management through planting trees is a bad idea," Field explained. "Planting trees is a great idea. It's just that the trees will grow at their normal rates or slightly faster, rather than at supercharged rates."

Field co-organized the AAAS symposium with Stephen Schneider of Stanford University, pulling together speakers studying a variety of different landscapes.

Jeff Dukes of the University of Massachusetts Boston has been monitoring changes in a California grassland, over five years of exposure to various types of environmental change. Presenting a new analysis covering five years of data, Dukes reported that their response to elevated atmospheric carbon dioxide was minimal.

"Carbon dioxide may boost or suppress grassland productivity in some years, but over the longer term it's pretty much a wash," Dukes said.

Schlesinger stressed that planting trees is an effective way to sequester carbon, but "shouldn't expect those trees to grow much faster in the high CO2 world of the future."

Based on available evidence from the Duke experiment, "I'd be surprised if the forests of the world will take up more than one-third of the carbon dioxide from fossil fuel emissions in the year 2050, which is what our experiment simulates," Schlesinger predicted.

Ultimately, the Earth's ability to take up carbon will depend on the oceans. The oceans have already absorbed some 400 billion tons of fossil fuel carbon dioxide, and this trend will continue; ocean uptake now is more than 20 million tons of carbon dioxide per day, according to Peter Brewer of the Monterey Bay Aquarium Research Institute. "But is this a blessing or a problem?" he asked.

Some researchers have raised questions about the impact of changing ocean acidity, known as the pH, on marine life.

Brewer reported on the first small scale ocean experiments, in which his research team added carbon dioxide to the deep sea off California, raising the acidity of the surrounding ocean, exposing animals to waters that may simulate the ocean of the late 21st century.

He described some new experimental techniques that should make it possible to extend these types of experiments, making them both spatially larger and longer lasting.

"It's the only way to find out how coral reefs, deep sea fisheries and other marine environments will react to a change in ocean pH; you have to do the experiment," he said.

Developed countries can earn Kyoto Protocol credits by replanting forests that have been logged, such as this one in Brazil. (Photo courtesy Dominican University)

One of the most difficult issues protocol negotiators had to resolve was how much credit the 37 developed countries governed by the protocol could receive toward their targets through the use of sinks.

In Bonn in July 2001, the 180 governments that are UNFCCC Parties reached a broad political agreement to allow credits for revegetation and management of forests, croplands and grazing lands as carbon sinks. Individual country quotas were set; the result is that sinks will account for only a fraction of the emissions reductions that can be counted towards the Kyoto targets.

The meeting also adopted the rules governing the Clean Development Mechanism (CDM), through which developed countries can invest in climate friendly projects in developing countries and receive credit for the emissions avoided by these projects. The rules specify that energy efficiency, renewable energy, and forest sink projects can qualify for the CDM.

But in view of the results of the FACE experiments reported at the AAAS symposium, the value of forest sink credits may have to be revised.

Find out more about FACE online at: http://cdiac.esd.ornl.gov/programs/FACE/face.html

FACE projects around the world are mapped at: http://cdiac.esd.ornl.gov/programs/FACE/whereisface.html