"It won't be a mandatory component of the ticket price," Kella told Reuters. Qantas will join a handful of airlines around the world that offer such a program, but is keeping details of how the scheme will work under wraps until it is launched at the end of September.

Kella said that on the day of the launch, Qantas plans to buy credit for the entire emissions of the airline on that day.

From then on, Qantas will buy credits for its staff's air travel and for the use of ground equipment, but it will be up to passengers whether to buy offsets for their own travel.

"Our market research indicates there is a broader interest. Whether or not in a voluntary scheme, people will take that up, it's a personal decision on the day," Kella said.

British Airways introduced a carbon offset scheme in 2005, but a British parliamentary inquiry last month said that only 1,600 tonnes of offsets were purchased on average each year, the equivalent of four return flights from London to New York.

Kella said Qantas hopes to learn from the experiences of other carriers, including BA and Scandinavian Airlines.

"We are hopeful that over time our percentage participations will be greater than what some of the other carriers have experienced," he said. The scheme will cover travel on Qantas and on its discount Jetstar carrier.

UN studies say that projected global passenger growth of 5 percent a year will far outstrip efficiency gains from better fuel or plane design.

On a one-way flight from Sydney to London, each passenger is responsible for 1.9 tonnes of greenhouse gases, according to estimates by a UK offset company CarbonNeutral Co. Qantas disputes claims the airline industry is a heavy emitter of greenhouse gases, saying that aviation only contributes 2 percent of total carbon emissions in the world, despite contributing about 8 percent of world economic activity.

International flights are now excluded from the Kyoto Protocol, the main UN plan for curbing climate change to 2012.