More signs that the movement to stop global warming has run out of gas emerged yesterday as Bill McKibben, the founder and Keystone XL pipeline opponent, announced that he was hanging up his hat.

The surprise retirement capped a hectic week for the environmental movement, during which one leader after another declared they were giving up on the cause.

“I’m bone-tired and written out. It’s time for the planet to take care of itself,” McKibben told a crowd of supporters who were sweating in the freakish early-spring heat wave that has rolled across much of the U.S. “I’m going skiing. Uh, make that water-skiing.”