Karpf, one of the youngest leaders ever of an organization whose members tend to be baby-boomer age and older, is an old hand at mobilizing the "green" troops.
As a high-schooler in Maryland, he led swarms of fellow students in campaigns against highway construction and a weak vehicle-emissions program. At 17, he dropped a stack of signatures before a panel of bemused state legislators and testified fervently in favor of saving an old-growth forest.
Though familiar with such traditional environmental struggles, Karpf joins Sierra's 15-person board as controversy is tearing at the group from within.
Karpf was one of eight candidates named by the club's official nominating committee for the five available spots on the board. They were challenged by nine candidates who got on the ballot via petition, five of whom said population growth is a key U.S. environmental problem.
In various mailings, members of the club's old guard warned that some of those petition candidates were doing the work of racist, anti-immigrant groups.
Karpf and four of the other nominated candidates won overwhelmingly, each garnering more than seven times the votes of any petition candidate.
Former Colorado Gov. Dick Lamm, a petition candidate who urged the club to tackle overpopulation here and abroad, said he bore no ill will toward the winners. But he accused the club establishment of "environmental McCarthyism" by playing the race card.
Karpf agrees that overpopulation is an environmental problem, straining the world's resources to the breaking point. But he stressed it is a global problem, not an issue of people moving from one country to another.
"We need to get away from saying we'll just deal with it in our country," the new board member said.
Karpf became interested in the environment the summer before his sophomore year in high school, while at a camp for Jewish youths in Starlight, Pa., in scenic Wayne County.
That fall, he joined his school's environmental group in Rockville, Md., but soon became dissatisfied with just recycling cans. He founded a countywide group of student environmentalists, and soon they were lobbying the state legislature.
By the time he went off to Oberlin College, Karpf was active in the national Sierra Student Coalition, the club's student arm. He even took a year off from college to serve as the coalition's national director.
"He's always been setting his sights a notch higher," said Larry Bohlen, a Maryland activist who guided Karpf early in his green awakening. "Next it'll be the United Nations Environment Program."
In his candidate questionnaire, Karpf was open about his desire to defeat Bush in November, answering "Yes, yes, yes" to a question on whether that should be the club's chief mission in 2004.
Long-term, he is grappling with a subtler issue.
Karpf is fascinated with the Robert D. Putnam book Bowling Alone, which contends that America's social fabric is deteriorating as we spend more time in cars and less time interacting with each other.
Karpf sees the book as a call to reinvent the Sierra Club, which has an old-fashioned hierarchy with local chapters, meetings, and elected officials. Rather than focus on meetings, he said, the group must give members ways to take immediate action: affixing a stamp, attending a rally, calling a lawmaker.
One model is the Internet-fueled presidential campaign of Howard Dean, Karpf said.
"You ask me to go take action now, I'm going to go take action now," he said. "If you ask me to go to a meeting every month, I'm going to say I don't have time."
With such a full agenda, it's not clear that Karpf has much time himself.
He saves some on his studies by reading as he makes the half-hour walk to the Penn campus, and jokes that he has learned to cap and uncap a highlighter with one hand.
Karpf's father, Ronald, a statistician who has scant experience with the environmental movement, said he is "extraordinarily proud" of his son. He just hopes that the 25-year-old isn't stretched too thin.
Once, when Karpf was on a school break from Oberlin, the family went on vacation in the Shenandoah Mountains. His parents both brought novels.
Not so their serious-minded son. His reading material: 300 pages of grant applications.
Contact staff writer Tom Avril at 215-854-2430 or firstname.lastname@example.org.