When Helen and Michael Allen got married, the wedding bill included an unusual extra item - about £120 ($270) to clean up gases blamed for causing global warming. And when the Rolling Stones toured Britain last year, they added about 15 British pence to the price of each ticket to plant 3000 trees to soak up the concerts' pollution and demonstrate, as they said, that "rock'n'roll is not a gas". One Dutch car rental company adds about 1 per cent to its prices to help plant forests to absorb the vehicles' emissions of carbon dioxide, a non-toxic gas widely blamed...