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Leaves of gas,
Scientists envision trees extracting excess CO_² from air.

August 8, 2008

The poet Joyce Kilmer was right: “Only God can make a tree.”

Klaus Lackner just wants to make it better.

Lackner, a geophysicist at Columbia University in New York, doesn't actually want to make trees. Rather, he wants to build devices that mimic one of the things real trees do: extract carbon dioxide (CO_² ) from air.

Living trees (along with other plants and some kinds of algae and bacteria) take in atmospheric CO_² as part of photosynthesis – the process of converting light energy into food. Lackner's “synthetic trees” don't photosynthesize anything, but they are very good at sucking up carbon dioxide.

“About 1,000 times better than a real tree of comparable size,” he said.

Rising levels of atmospheric carbon dioxide – a greenhouse gas produced both naturally and, more problematically, by the burning of fossil fuels – is a major contributor to climate change.

Researchers at the U.S. National Oceanic and Atmospheric Administration say global levels of atmospheric CO_² have reached 387 parts per million (ppm), up almost 40 percent since the start of the Industrial Revolution, and the highest in at least 650,000 years.

Accumulating levels of atmospheric CO_² pose a significant environmental danger to life on Earth, scientists say, from global warming to ocean acidification. Lackner believes CO_² -sucking synthetic trees can reduce the threat and provide more time for scientists and society to develop and expand the use of cleaner, renewable forms of energy.

Global Research Technologies Allen Wright, president of Global Research Technologies, stands beside a rack of coated resin membranes that extract carbon dioxide from the air.

A few hundred miles to the south of Columbia University as well as several thousand miles to the east, researchers at the University of Maryland and the University of Greifswald in Germany propose a different solution to the same CO_² problem.

These scientists suggest the answer is using real trees, lots of them.

“If it was possible to manage all of the world's forests, natural and plantation, the potential is there to offset all of the world's fossil fuel emissions,” said Ning Zeng, an associate professor of atmospheric and oceanic science at the University of Maryland.

But unlike, say, giant mechanical CO_² vacuums, biological trees age and die. And when they do, the carbon dioxide they have captured in their tissues over a lifetime is released. To keep it contained, Zeng and the German scientists both suggest burying the wood deep enough that it would not decompose, remaining essentially unchanged for centuries.

“The idea to store carbon as wood for infinite times buried underground means a paradigmatic change of human activity,” wrote Fritz Scholz and Ulrich Hasse in an essay published earlier this year in the journal ChemSusChem. “For the first time, humans would give back to nature what they have used before.”

Chemical bonds

Neither idea is ready yet for a worldwide rollout.

After a decade of work, Lackner's synthetic tree idea has reached the testing phase, however, with a demonstration project under construction in Arizona by Tucson-based Global Research Technologies. (Lackner is the chairman and chief technology officer.)

Atmospheric carbon dioxide levels have risen 40 percent since the Industrial Revolution, mostly from the burning of fossil fuels. Factories are a major source.

The working premise is basic chemistry: When air containing carbon dioxide passes over certain chemical compounds, such as sodium hydroxide, some of the airborne carbon atoms stick, creating a salt called sodium carbonate, or soda ash.

Lackner's original idea described a device that resembled a football goal post with Venetian blindlike louvers stretched horizontally between the uprights. The blinds behaved like filters, each coated in a solution of sodium hydroxide.

After a period of time, when the filter-blinds had absorbed as much CO_² as they could, the sodium carbonate would be washed off and processed. The sodium hydroxide would be recycled back onto the filter-blinds; the carbon dioxide would be liquefied and stored.

Not surprisingly, the original design has evolved. The carbon collectors now look more like tree trunks without branches or leaves. Inside are sheets of plastic resin that absorb even more CO_² than the earlier filter-blinds.

“The resin was being used for other industrial purposes,” said Lackner. “We discovered that it also absorbs CO_² .”

In the early plans, significant amounts of energy were required to separate the carbon dioxide from the sodium hydroxide, which made the idea less cost-effective. The newer resin membranes release their captured carbon simply by being exposed to moisture. “You don't even have to make the membranes wet, just humid,” Lackner said. “That takes very little energy to do.”

The technology is also scalable, according to Lackner. In one year, a 20-square-inch resin membrane can take up the annual carbon emissions equivalent of one American, he said. A synthetic tree with a collection surface 55 feet by 65 feet (but compact enough to be transported by truck) could capture an estimated 90,000 tons of CO_² each year, which is roughly the annual emissions output of 15,000 cars.

Lackner said such carbon-capturing devices could be used almost anywhere. Devices could be erected near major sources of CO_² emissions, such as power plants, factories or in urban areas. Or they could be arrayed, like windmill farms, in more remote regions.

A major issue is what to do with the collected carbon, which must be effectively and permanently sequestered if CO_² levels are to be reduced. Ideas for permanent CO_² sequestration range from injecting the carbon dioxide into depleted oil fields or underground mines to burying it in deep ocean trenches. (CO_² is denser than water and would be trapped on the sea bottom.)

Lackner favors injection as an immediate recourse – the technology is already being used – but he says a better long-term solution may be to develop a method of converting captured carbon dioxide into a stable, inert, harmless solid. Nature already performs this feat in the weathering of rocks, but the process works on a time scale far too lengthy to remedy pressing human problems.

More promising is the possibility of making CO_² a highly coveted commodity. Carbon dioxide already has some commercial uses. In its solid form, it's “dry ice.” Carbon dioxide is also used in fire extinguishers, to process foods and in greenhouse agriculture, where higher CO_² levels can boost plant growth.

Lackner, though, sees an even more intriguing market for CO_² . He said it is possible to convert it to a liquid hydrocarbon, not unlike gasoline or diesel.

“The idea of synthetic gas isn't new,” Lackner said. “The technology exists, but it's always too expensive. But if oil prices continue to rise, if electricity becomes a renewable energy, synthetic gas could become economically viable. Carbon capture from the air might eventually replace fossil fuels.”

Growth industry

Synthetic carbon-capturing trees are not without their skeptics. Lackner estimates the devices could eventually do the job at a cost of $30 per ton of captured carbon, which works out to roughly 25 cents per gallon of gas.

Actual real-world costs remain to be seen. They won't be known until the technology has been tested longer and on a larger scale.

Miles Silman, an associate professor of biology at Wake Forest University, offers a more basic objection: “Why would you build machines to do what trees are supremely adapted to do? And do it using solar power?”

That is the notion behind the independent proposals by Zeng at the University of Maryland and Scholz and Hasse in Germany. They contend that expanded natural and planted forests could soak up the 22 billion tons of CO_² produced annually from fossil fuels.

At least in theory.

“The physical potential is there,” said Zeng.

In a paper published in the journal Carbon Balance and Management, Zeng posits the idea of regularly thinning natural forests, maybe once every five years, then burying the wood in deep trenches between the remaining trees.

Alternatively, he said, the wood could be stored in airtight containers on-site.

“The disturbance can be kept quite small,” Zeng said. “If you collected wood over a 1-kilometer square, you could bury it all in a single hole that would be roughly 1,000th the size of the collection area.”

Scholz and Hasse's proposal goes a step further. They envision tree farms expressly planted for future burial. Whenever possible, the farms would be located near obvious burial sites, such as open-pit mines or very deep, specially chosen lakes, to reduce costs.

“It is well known that wood covered by a sufficiently thick layer of soil will keep for an infinite time,” the authors wrote in ChemSusChem earlier this year. “The only process that will change the wood is extremely slow carbonization, a process that requires millions of years. Fresh wood that sinks to the ground of freshwater lakes is equally stable and will remain there for an infinite time.”

As trees were removed and buried, they said, new trees would be planted and the process repeated.

But huge questions loom. Scholz and Hasse estimate it would require roughly 2.47 billion acres of trees to remove global carbon dioxide emissions each year. That's an enormous amount of space, greater than the entire surface area of the United States. It's about one-fourth of all the Earth's land surface currently covered by forest.

“Of course, we would achieve a lot if we could compensate 20, 30 or whatever percent of CO_² release,” said Scholz. “There is plenty of fallow land in Europe, and in countries like Brazil and Indonesia, all of the deforested areas. Further, trees could be planted in so many places, along roadsides, really everywhere.”

Available suitable space isn't the only issue. The ability of trees to absorb atmospheric carbon dioxide varies by species and location. Tropical forests are far more effective CO_² absorbers than their temperate or boreal counterparts. Yet the latter forest types are much more common in the developed countries that are most likely to pursue global CO_² -reduction strategies.

Then there's the question of whether growing lots of trees hurts more than it helps in the fight against global warming, said Ken Caldeira, a research scientist at the Carnegie Institution.

Some kinds of forest are heat sinks, the dark foliage collecting and retaining warmth from the sun. Tropical forests, however, generate lots of evaporative moisture, which fuels clouds that cool the Earth's atmosphere by reflecting sunlight back into space. Temperate forests do not give off as much moisture. Indeed, studies indicate their heat-absorbing capacity appears to raise temperatures, at least regionally.

Scholz, however, notes that growing more trees would simply put the world closer to its arboreal past. “We should not forget that the natural situation was that most of the land in Europe, South and North America and Asia, especially China, was once covered by trees and shrubs. It cannot be wrong if we go to some small extent back to that situation.”

Zeng voices greater uncertainty. Or at least the existence of more questions than answers: Will vast plantations of trees adversely affect groundwater supplies or seriously deplete soil nutrients? Will they harm other plant life? How will natural habitats and ecosystems be affected? Can large-scale decomposition of buried wood actually be prevented?

Only more research can answer these questions, Zeng said. He and students have launched a modest experiment to begin looking for answers, burying relatively small amounts of wood underground and in a pond at a Maryland site. Zeng is also organizing a session on the topic for the American Geophysical Union conference in December in San Francisco.

Whether either idea – synthetic trees or real trees – will bear fruit remains to be seen. More important, say researchers, is the need to act.

“Carbon dioxide emissions are growing every year,” said Lackner. “Even if the world were to somehow hold emissions at their current rate, the global level will continue to go up by two parts per million each year. Five hundred and fifty ppm is probably inevitable within this century.

“This is not a scientific question. It's a question of our pain threshold. How long can we go before we all agree that something must be done?”

Put another way: How long before we find ourselves up a tree we can't get down from?

 

Your humble Ace Reporter

Bob

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