Steve Sawyer, a leader of the international environmental group Greenpeace and one of the first activists on the global stage to sound the alarm over the rising threat of climate change, died on July 31 in a hospital in Amsterdam, where he lived. He was 63.

The cause was complications of pneumonia stemming from lung cancer, which was diagnosed in April, his wife, Kelly Rigg, said.

Over nearly three decades, starting in the late 1970s, the American-born Mr. Sawyer rose from being a low-paid canvasser for Greenpeace, going door to door to solicit donations and memberships, to running the organization, whose global headquarters is in Amsterdam.

He came of age with the group, embodying its radical spirit in his early activist days, then helping to lead it to the forefront of the global environmental movement.