By Molly Bentley



The invention is confined to paper so far

But the synthetic tree proposed by Dr Klaus Lackner does not much resemble the leafy variety.

"It looks like a goal post with Venetian blinds," said the Columbia University physicist, referring to his sketch at the annual meeting of the American Association for the Advancement of Science in Denver, Colorado.

But the synthetic tree would do the job of a real tree, he said. It would draw carbon dioxide out of the air, as plants do during photosynthesis, but retain the carbon and not release oxygen.

If built to scale, according to Dr Lackner, synthetic trees could help clean up an atmosphere grown heavy with carbon dioxide, the most abundant gas produced by humans and implicated in climate warming.

He predicts that one synthetic tree could remove 90,000 tonnes of CO2 in a year - the emissions equivalent of 15,000 cars.

"You can be a thousand times better than a living tree," he said.

Carbon sinks

For now, the synthetic tree is still a paper idea. But Dr Lackner is serious about developing a working model. His efforts suggest the wide net of ideas cast by scientists as they face the challenge of mitigating climate change.

Dr Lackner believes that carbon sequestration technology must be part of the long-term solution. Global reliance on fossil fuels would not decrease any time soon, he said, and developing countries cannot be expected to wait until alternatives are available.

The technology calls for two things: seizing carbon and then storing it. Direct capture of CO2, from power plants for example, is the simplest, according to Dr Lackner. But this doesn't work for all polluters. A car can't capture and store its carbon dioxide on-board; the storage tank would be too large.

"It's simply a question of weight," he said. "For every 14 grams of gasoline you use, you are going to have 44 grams of CO2."

The alternative is to capture emissions from the wind. In this case, a synthetic tree would act like a filter. An absorbent coating, such as limewater, on its slats or "leaves" would seize carbon dioxide and retain the carbon.

Dr Lackner predicts that the biggest expense would be in recycling the absorber material.

"We have to keep the absorbent surfaces refreshed because they will very rapidly fill up with carbon dioxide," he said. If an alkaline solution such as limewater were used, the resulting coat of limestone would need to be removed.

Dr Lackner is considering other less-alkaline solutions to prevent carbonate precipitation.

"There are a number of engineering issues which need to be worked out," he said.

Home use

A synthetic tree could be planted anywhere. A small one could sit like a TV on the lawn to balance out the CO2 emitted by one person or family.

But more practically, said Dr Lackner, a device the size of a barn would sit in the open air, near repositories for easy transportation and storage of carbon.

He estimated that 250,000 synthetic trees worldwide would be needed to soak up the 22 billion tonnes of CO2 produced annually.

But not everyone is rooted to the idea. Massachusetts Institute of Technology engineer Howard Herzog thinks Dr Lackner's design will not hold together on the scale he proposes.

He said you would expend more energy in capturing the CO2 - in keeping the slats coated in absorbent and disposing of it - than you would save.

"Once the solvent captures the CO2, it holds it on tight," said Dr Herzog, "and it's going to take a lot of energy to break those bonds."

He said that much more research was needed on the technology.

"The idea of air capture is seductive and would really be great to have," said Dr Herzog, "but it's important to separate out the concept from the technical details."

'Early days'

Meanwhile, Dr Lackner is pursuing his idea for carbon storage. While he was at the US Department of Energy's Los Alamos National Laboratory, his team worked on a storage method based on a natural chemical process known as rock weathering.

When CO2 binds with magnesium, it creates carbonate rocks which, according to Dr Lackner, retain carbon permanently and safely.

Currently, he said, the process is still too expensive to develop on a large scale.

But Dr Lackner is optimistic that the costs for carbon capture and storage will come down.

"This is still the early days of climate solutions," he said.