Such trees 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. A large tree, the size of a barn, could sit in the open air, near repositories for easy transportation and storage of carbon. Klaus Lackner estimates that some 250,000 such trees worldwide would be needed to soak up the CO2 produced by human activity annually.
BBC News, 21 February 2003
Earth Institute at Columbia University
More recently, Klaus Lackner's trees featured prominently in the BBC documentary
Five Ways To Save The World