The Swedish head of state now drives an eco-friendly hybrid car despite a life-long reputation as a "petrolhead".
"When I drive into town it's on electricity," he told Swedish newspaper Svenska Dagbladet.
Like the Prince of Wales, who famously fitted Highgrove House with "eco" toilets, the king is taking steps to make his residence greener by switching to more environmentally-friendly heating and to low-energy lightbulbs.
"We do what we can here at the palace. It's an ongoing project trying to save energy, but it isn't easy in an old property," he said.
King Carl Gustaf plans to attend the UN climate change conference in Paris starting in just over a week. "We have to think that we will do our best," he said. "And I think there is a general feeling that we're going to achieve it."
He took part in the first UN environmental conference, in Stockholm in 1972, and says that talking about global climate change is "an opportunity and a duty I have."
- The Sunday Telegraph