OSLO, March 14 (Reuters) - Scandinavian airline SAS <SAS.ST> said on Wednesday it would let clients offset greenhouse emissions from their flights by investing in renewable energy.

SAS was the second airline after British Airways <BAY.L> to introduce the scheme.

Norway's Prime Minister Jens Stoltenberg, who has said his centre-left government will offset all emissions from officials' flights from 2007, encouraged other airlines to follow suit.

"Global warming is the most serious environmental challenge we face," he told a news conference.

"I encourage all airlines ... to do the same," he said. He also urged people and businesses to offset emissions.

Under the deal, the SAS website offers customers the option of investing in wind power projects in China, India and New Zealand to compensate for their share of carbon dioxide emissions from burning jet fuel on flights.

Sitting at Stoltenberg's computer, Transport Minister Liv Signe Navarsete bought a return ticket from Oslo to Brussels and paid an extra 4 euros ($5.27) to offset projected emissions.

"We are the second airline to introduce this," said Ola Strand, the head of the Norwegian unit of SAS. Airline officials said that British Airways already had a similar scheme.

Some environmentalists are critical of such projects, saying the world should work to cut emissions rather than offset them.

But Stoltenberg said: "I don't believe in a climate policy that says it's wrong to move, it's wrong to travel on holiday and that people should sit at home."

"I believe that people have a lot of joy from travel but at the same time we should do it in an environmentally friendly way," he said. He declined to estimate how much it would cost the state to offset travel emissions.

Navarsete said Norway, the only country in Europe with a tax on carbon dioxide emitted on domestic flights, hoped to join any European Union scheme to add an environmental charge for international flights.

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